Tag Archives: exposure

Alignment (Part 2 of 2)

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In a previous column: Alignment Part 1 of 2, I touched upon the many elements that complicate capturing the moon near an object on the horizon. Here they are again for consideration:

  • The amount of moon illumination changes daily.
  • The moon’s rising and setting location must be accurately calculated – and it changes daily.
  • Exposures to capture moon detail require the right amount of foreground illumination.
  • The site chosen must have an unobstructed view of the sky in the desired direction.
  • To get a “big moon” it is necessary to get far enough away from the foreground.  If too close, depth of field problems may arise.
  • A well supported telephoto lens is required.
  • Capturing a shot of the moon near the horizon means the atmosphere must be relatively clear of clouds, dust and haze and when very low in the horizon there is more atmospheric distortion.

Figuring out how to tackle the moon location is computationally challenging. Fortunately with the internet there are many free resources to aid in this endeavor. And more fortunately, there is one tool which is almost ideal for the task: The Photographer’s Ephemeris.

We will address the problems step by step.

  1. Obtain the appropriate camera gear.
  2. Identify a suitable target.
  3. Calculate how far away we want to be from the target.
  4. Identify possible vantage points to shoot that target.
  5. Verify (visually, if possible), that the target is viewable from the vantage point and that there is sky behind our target.
  6. Verify that the moon will pass near our target and at an opportune time of day.
  7. Determine how high in the sky the moon should be.
  8. Fine tune the location to be sure the geometry is correct.
  9. Pray for good weather!

The camera gear element of the puzzle is easy: get the longest telephoto lens available. 2,400 mm will work great with a 35 mm (full frame) camera. I do not have anything that big (or expensive), so I use a 70-200 mm lens with a 1.4x extender on a 1.6 crop factor camera.  That effectively gets me 200 \times 1.4 \times 1.6 = 448 mm focal length. The “short” focal length of 448 millimeters means I can not fill my frame with the moon – it would take 32 moons laid out in a grid. Getting more foreground in the shot creates more opportunity for an arresting image however. Besides, those really big lenses are not only expensive, but unwieldy. In fact, they call them telescopes! Working with a crop camera in this scenario is a benefit.

No telephoto? Well then I probably would not bother – at least I would not bother trying to capture moon DETAIL.

Picking a Target

The moon is obviously one of our targets, but we want something interesting in the foreground to pair the moon with. Ideally we want a target that clearly stands above the surroundings and preferably one that allows us to get the proper distance away to maximize the “big moon phenomenon”. How far away?  Here is an easy formula: multiply the height of the object by 114.6.  If the object is 100 feet tall, the proper distance is 1,114.6 feet away.  If the object is 20 meters tall, the distance is 2,292 meters.  If 6 inches, then a distance of 687.6 inches is about right.

For the curious, the number 114.6 corresponds to 1 \div {\tan{(0.5)}}, where 0.5 is the number of degrees of the angular size of the moon from anywhere on earth. If shooting from somewhere else in space more advanced trigonometry may be needed.

It might be tempting to start with something short and nearby, like a golf ball. But getting a good depth of field is going to be difficult.

Let’s get started on the target, shall we? Fire up The Photographer’s Ephemeris (TPE) and follow along with me.  Switch to Ephemeris Mode (it is the first selection in the upper left). In the search bar (lower left), enter “Pioneer Park, San Francisco, CA“.

Now would be a good time to make the TPE window as large as possible, and select the “Satellite” mode in the map.

Figure 1: Pioneer Park Coordinates and Elevation according to TPE

Right above the upper right corner of the map you should notice two things: an elevation (here shown as +190 ft), and the GPS coordinates (37.8…blahblahblah).  If you prefer metric (or it shows metric and your prefer feet, you can change that using “Configure”).

Looking at the zoomed in map, put the cursor over the map near the bottom and click and drag upward. The map should move and soon you should see a conical shape casting a long shadow. Hooray. You found the Coit Tower. Double click in the center of the structure and it should look about like this.

Figure 2: Coit Tower in Pioneer Square, San Francisco, CA

Here I cheated and moved the elevation (+266 ft) and the GPS coordinates on to the image from above the map from the bar above.  I also zoomed out a bit so you can see the parking lot that you first landed on.

Did you notice that the elevation moved up from 190 to 266 feet?  You gained 76 feet in just a few parking spaces! It is steep there, but that number is NOT a measurement of the height of the tower, my friend. That is the elevation of the BASE of the tower. Don’t believe me… click a few spots near, but not on the tower or the building.  Click things farther away if you like, I’ll wait.  As you can see from the image at left taken from the parking lot, there is clearly not a gain of 76 feet between the two places.  The elevation information comes from a variety of sources, mostly the United States Geological Services (USGS) data.

What you hopefully learned is not to COMPLETELY trust the elevation shown. The elevation does not include buildings or trees and is not that precise, but it will probably be good enough.

In a while you will need to know the height of the tower above the base. Guess where you can find that? Yep, Google. Did you find it yet? It’s 210 feet (65.4 meters) tall.

So doing the math: ideally we’d like to be 210 x 114.6 feet away (24,066 feet or 4.5 miles) to have the moon’s apparent size be as big as the tower. Unfortunately going to the east, our choices are mostly in the San Francisco Bay, farther away on the Oakland Shore (near the Bay Bridge), or closer. Treasure Island looks like a good spot. It’s 2.11 miles and there is a lot of flat, publicly accessible shoreline to move along to align the moon behind the Coit Tower.  And besides even though the Coit Tower sits up on a high hill, only about the top half of the tower is above the sky line. So 2.11 miles might work out very nicely.

Since we have chosen a site to the east of the Coit Tower when can the moon appear behind it?  Near moon SET of course.

If you want your diagram to look exactly like mine, change the calendar to June 15, 2011. And change the Ephemeris mode to “Detail” (use the D key, or click the box down near the calendar).

When you switch to Detail mode, a hollow little gray marker will appear. Usually to the right of the red marker near the right edge of the map. Don’t lose it – you’ll need it in a minute.

Calculate the Moon Location Near Moon Set

You may have noticed all those colored lines extending from the Coit Tower in Figure 2. Here is what they mean: the light yellow line is the direction of sunrise, the orange line is the direction of sunset. The light blue line is the direction of moon rise and the dark blue is the direction of moon set.  All by itself that won’t help much. To see the moon setting in the west behind the Coit Tower, you obviously must stand to the EAST. But where?

Zoom out your map until you can see the Coit Tower on the left, and Treasure Island on the right. Make sure you are in Detail Ephemeris mode (you’ll know when you see a graph like this:

Figure 3: Sun/Moon graph and time slider.

Your map will look something like this:

Figure 4: SF Bay Map with Coit (lower left) and Treasure Island (upper right)

I have stripped off all the stuff around it to focus your attention. You’re focused, right?

Now would be a good time to play with the time slider. Click and drag it. Whoa! Did you see the lines moving? The skinny ones, that is.  There is a lot going on here, but the one thing you’re not yet seeing is where you need to stand to see the moon behind the Coit.

Stephen Trainor, the author of TPE put a cool feature in this tool. He did so because I asked politely and I support him with donations – I urge you to do so too. Buy his iPhone/iPad version of the tool (or Android if that’s available) or make a donation if you’re using the desktop (free) version of the Ephemeris. It’s the right thing to do!

Move your time slider to 5:13 as in Figure 3.  Now hold down the shift key. Did you see the thin blue line jump out? That blue line traces roughly where the shadow of the moon would appear. It can’t be completely accurate, however since the exact location would have to take into account topography, trees and man-made structures. We helped ourselves around that worry by choosing a flat shoreline where not much can get in our way.

Now would be a good time to find that hollow gray marker. Lost it? Click “D” then “D” again. It will appear near the right side of your map connected by a dim gray line to the red marker.

Hold down the shift key again, and drag and drop the gray marker on the Treasure Island shore DIRECTLY over the dark thin blue line.  Zoom in if you have to and get the marker EXACTLY on the line. And try not to stand behind a building or a palm tree.

You probably didn’t notice, but three things appeared at the bottom of your Ephemeris Graph in the box labeled Geodetics.  Those are: Apparent Altitude (which here will be negative), Change in Elevation (also negative), and Distance and Bearing.  Each time you move the gray or red marker it will recalculate the distance, altitudes and angle between gray and red.

One last little coup for now… notice next to the word Geodetics it has a little red and gray dot with an arrow over the top? Yeah, click that. The gray and red locations magically flip. Now all of your altitude and elevations will be positive. The calculations are FROM red TO gray. Since red is at sea level now, and gray up 266 feet on the top of Pioneer Hill the angle above the horizon toward the hill is  positive: specifically the base of the Coit tower is 1.1 degrees above the horizon. So can we conclude that the moon must be 1.1 degrees high in the sky?

NOPE. Sorry, we can’t. So close and yet SO far!

Q: What is wrong? Did you figure it out?
A: TPE has no idea how tall the Coit Tower is! (Stephen tells me one day he’s going to add the ability to specify the height at the red or the gray marker), but for now, YOU have to make that adjustment yourself. I’m afraid it’s going to involve some math. Trigonometry, actually.

What is the CORRECT Angle?

If you can answer this question, you’ll get the solution. “If an object at 2.12 miles away is 210 feet taller than the current 1.1 degree elevation, how many more degrees will that be?”

\tan^{-1}(Height / Distance) = altitude\ in\ degrees

Or in this case  InverseTangent( 210ft / 11311ft ) = 1.06 degrees.

So the CORRECT altitude is 1.06 + 1.1 or 2.16 degrees.

Hint Use the built in calculator in MS Windows in Scientific mode (Alt+2). Set the units to degrees. To get to the inverse tangent function (also called tan-1) use the “i” (inverse) key.

NOTE: If you do not want to do the trigonometry, there is another way to find the angle: use your camera.  Go to the desired site, take a picture with your telephoto lens aimed level with the horizon and with the top of the object visible. Determine the angular field of view of your lens/camera combination. Then measure the height of the target on the image and use the ratio of the height of the target to the field of view.  That sounds complicated, but it’s actually pretty easy. Using a 200mm lens, my angular field of view is 4.3 degrees. My photo shows that the tower spans 1000 of 1800 possible pixels. So the tower is 4.3^{\circ} \times (1000 / 1800) = 2.388^{\circ}

Now that we know the moon altitude must be 2.16 degrees we do not have to start over. Let us make sure the red maker is back on the tower and adjust our slider until the moon height is 2.16 degrees, then follow the line of the direction of the moon set to get our new location.

Of course if we move significantly higher, lower, nearer or farther away we must recheck the angle calculations.  In a hilly or mountainous location it is extremely non-trivial to get all the heights and angles just right. Using the “Terrain” mode of the map may help, but changes of a few dozen feet may make a big difference in the alignment.

Just remember the following things:

  1. The satellite maps may be out of date. A tree, building, crater, fence or obstacle might be in the location you want – or directly in front of it.
  2. There is no substitute for prechecking the line-of-sight BEFORE the event (see 1 above)
  3. Terrain maps are not visible when zoomed in.
  4. Elevations of the terrain are ROUGH.
  5. Moving 10 feet to the left or right may make or break the shot.
  6. I am NOT available to solve your trigonometry problems! Ok, I am but there will be a fee!

But wait, there’s more!

Getting the Ideal Exposure

To get the ideal scenario for moon details AND foreground light, it helps that the sun is on the opposite side of the sky and sometime during Civil twilight. In Figure 3, above, notice how the time we arrived at (5:13 AM) has the moon 2 minutes before Civil twilight.

Wondering what Civil twilight is? It is the legal equivalent to either dusk or dawn. Dusk when the sun has set, dawn when the sun has not yet risen. Signs that say park hours are “Dawn to Dusk” mean something quite precise. But those times change daily. For more click on the word “Civil” in the Ephemeris and it will tell you! Or take a look here.

The ideal exposure for detail in a full moon is about 1/100 of a second at ISO 100 and f/9. But atmospheric conditions, and the moon’s altitude may significantly affect the settings to use.  The best choice of aperture is to stop down enough for a sharp shot that keeps the foreground through to infinity (the moon’s focal distance) in focus.  If your foreground is at or beyond your hyperfocal distance (as it most probably will be), you’re good to go.

The problem, of course, is that your foreground is probably not going to fare well unless it is also well lit – so bracketing your exposures is always a great idea. The darker the twilight, the wider the bracketing needs to be.

Verifying The Sight Lines

After all the calculations and planning, a group of Bay Area Night Photographers ran out at the crack of before dawn to capture the “Full Moon Set behind Coit Tower“. One of the bleary-eyed ambitious photographers was Phil McGrew. Phil get’s extra kudos for going the morning before the planned event (that’s two thermoses worth of coffee) and here is what he got:

Photo 5: Coit Tower? And the Moon by Phil McGrew

The moon is in the right spot, but, whoops, there is something else in the shot, too! A big square building blocking the view behind the tower.  A more thorough scouring of the map in Figure 4 might have revealed the problem (see Figure 5).  Behind the Coit, and set up on a hill are a series of apartment buildings.  From almost anywhere else on Treasure Island, or Fort Baker in Marin, the Coit tower sits all by itself on the skyline.

Figure 5: Oops! (Click to see it larger)

What are the takeaways here:

  • There is no substitute for direct observation from the planned location. Any number of things can be a problem from light posts, billboards, trees and shrubs to, well hulking square buildings in the line of sight.
  • Extra scrutiny of the sight lines in TPE *might* save one from a needless trip to get a direct observation.
  • Knowing the local topography helps as does picking a structure or formation that clearly stands above the surrounding area.

Phil also discovered that the lack of brightness on his foreground meant he had to choose between exposing for moon detail, or exposing for the foreground. In Photo 5 he nailed a great foreground exposure and might be able to tease some moon detail out of the RAW file.  Or he could resort to…

One Last Trick – HDR

First I am a hater of images that have been composed by dropping a well exposed (oversized) moon into a separately taken landscape. There are technical challenges to embrace here so why not embrace them! Besides my desire as a scientist and engineer is to maintain reality through honest acquisition.

I am not, opposed, however, to using technology to overcome the limits of technology. Namely a camera can not readily capture the range of exposure – brightest to darkest – that the human eye can so a trick called “High Dynamic Range” photography (also called tone compression, tone mapping or image fusion) is sometime a necessity.

In the morning of June 15th, moonset behind Coit Tower was the target as describe earlier. That evening, moon rise behind the Transamerica Building was the goal.

You can click the diagram to the left to see where we were. As kismet would have it, the very parking space that I had calculated at the correct spot was open and I pulled in!

The haze was heavy, contrast was low. But in the end, the moon peeked (and peaked) right on schedule and right where it was supposed to go. It is always satisfying when things work out like that. More satisfying if the weather is great.

Rising Moon Collage

Photo 6: Moon rising over San Francisco (through the haze)

The fifth shot in the panel above is like all of the others in that it is a three-shot bracketed exposure combined using Photomatix Pro. The three shots were:

Figure 7: Bracketed Exposures

A wider bracketing range may have helped, the haze was quite thick. Using Photomatix Pro, playing with the knobs a bit I got this result:

Yellow Moon [5_032356-8tm]

Photo 7: High Dynamic Range Composite of 3 Images

I can only imagine what having a clear day to shoot in might have accomplished.

Best of luck on your alignments!

Comments, questions, praise, quibbles over the math – we’ll listen. Find us on Facebook.  Or attend one of our workshops. Want to keep it cheap, hook up with me, Steven in the Bay Area Night Photography group.

 

Stacker’s Checklist

Created November 2, 2010
Last Updated April 19, 2019

Note: Items in RED are suggestions that apply in particular to star trail captures and may be changed based on circumstances at the scene and goals.

Site Selection

  • Sunrise, Sunset, Moonrise, Moonset and moon phase all known.
  • Safe area, travel paths known

Equipment

  • Camera, tripod, release plate, camera batteries, memory card, lens, intervalometer + batteries, lens hood, rain protection, headlamp, flashlight/torch, and items for light painting.

On Site

  • Tripod set up – no leaning (center column should be vertical) – leg locks tightened.
  • Camera aimed, leveled.
  • Camera locked onto tripod. Head tightened.
  • Tripod weighted/secure and everything is wobble free. Keep the tripod low and out of the wind for best stability. Do not extend the center column.
  • Neck strap removed or secured to prevent wind throw. Intervalometer and any other cord, or wiring also secure. Velcro on the intervalometer and the tripod leg is a handy trick.
  • Save GPS coordinates and/or mark site with glow stick / other?

Camera Settings

  • Manual Mode, Bulb exposure
  • ISO 200  (varies but from 100 to 800, and up to 6400 if capturing meteors or the Milky Way)
  • Single Exposure
  • LCD brightness down
  • Image review time off
  • Record in RAW
  • White Balance = daylight (Auto not recommended)
  • Aperture f/4 (f/1.4 to f/7.1)
  • Auto focus OFF
  • Image stabilizer (vibration reduction) OFF
  • Long Exposure Noise Reduction OFF
  • Mirror Lockup OFF
  • Auto Exposure Bracketing OFF
  • Focus Assist OFF (this often fires an infra-red beam/red beam and will annoy other photographers). On many cameras this feature is on the flash unit/speedlite. On Nikons, this resource may help.

Timer Setup & Test

  • No delay, length of exposure = 1:59 minutes (adjust based on conditions. A 2 minute total interval is a good starting point), interval = 1 second, Num exposures >= 120
  • Timer cabled to camera
  • Test sequence (lens cap on) – Verify that second shot starts before canceling.

Focus & Final Framing

  • Check image composition, field of view.
  • Set camera to Aperture priority mode (not needed if it is already dark)
  • Take several bracketed shots in daylight or twilight: if it is already dark take a high ISO “range finding” shot. E.g. 2000 ISO for 30 seconds.
  • Pixel peep and adjust focus until sharp.

Battery and Card Shuffle

  • Remove memory card and insert second card. Format new card in camera.
  • Take second set of bracketed shots.
  • Return camera to Manual/Bulb mode.
  • Turn off camera and remove battery.
  • Reinsert battery (or insert fresh battery).
  • Verify that all settings are correct (See Camera Settings, above)

Final Steps

  • Check for wobble. Start by lightly jostling the camera, tripod, center column and even walking around in the area to make sure no movement occurs.
  • Set DELAY on interval timer appropriately (at least 5 seconds).  Goal is to start and/or end in twilight.
  • Secure cables for timer, external batteries (and neck strap). Do not block battery or memory card access.
  • Switch to aperture priority mode (so that your manual settings do not change), take a single image and re-verify focus. If already dark, take a high-ISO range finding shot for this task.
  • Switch back to Manual/Bulb.
  • Verify all camera settings as described in Camera Settings
  • Start Timer and verify that the timer is running.
  • If practical wait for first two shots to complete.
  • NOTE: You can leave the lens cap on for the first few exposure to collect DARK frames.

My thanks to Mike W. for comments and improvements to this checklist.

Additional References

Exposing for Stars

Original Publication: October 20, 2010
Last Updated: November 2, 2017

In two previous articles I covered the most common problems that face anyone doing long exposures. In part 1 of 2 I discussed: Poor Focus, Dim Stars (low contrast), Strange Colors and Pink or Purple glow. In part 2 of 2, I tackled gaps in star trails, and noise.

It seems like I have omitted a rather important element: how to choose your exposure settings in the first place.  An astute Flickr user asked What is the Best ISO for Stacking Startrails. Good question and I realize I have not approached the question from that point of view… starting from the beginning, that is. So pay a little attention here and I will hopefully demystify that question for you.

First, the answer will always be “it depends”.  Just as with daylight or indoor exposures the settings to pick depend on the conditions. Is there a bright moon? Is there a strong sky glow from light pollution? How well does your camera manage noise? How cold (or warm) is it? Are there any bright light sources that need to be managed?  What is the intended result?

Most people attempt to approach a star trail the way they would expose any low light scene. But that approach is flawed.  Let me illustrate the germaine elements with a story about my friend, the little star named “Drizzle”.

Drizzle Drizzle Little Star

The film in a film camera, or the sensor in a digital camera can be thought of as a piece of absorbent canvas. Imagine that the light (photons) of a single star are a stream of tiny little droplets of star juice. The brightest star – Sirius – can spurt 200 droplets per second. The faintest stars visible to the eye, like Drizzle, only manage a single drop per second.  Moreover to be able to notice anything at all, we require a minimum of 20 droplets. Doing a tiny bit of math we realize that we have to collect droplets from Drizzle for at least 20 seconds. But for Drizzle to stand out in any noticeable way we need to collect Drizzle juice for about a minute – 20 was only enough for Drizzle to be discernable.

But wait. We have a collector in front of our canvas that grabs the incoming droplets and shrinks them. That collector is our lens and the shrinkage is related to the size of the iris (aperture). Even when we make our collector opening as big as we can (f/2.8) it will take two collected droplets to equal up to one direct droplet. In fact, if we make our aperture even smaller (f/16) we shrink all the incoming  droplets so that they are only 1/100th of their original size. If we set our collector on f/16 we will have to collect two hundred minutes worth of Drizzle’s meager output to be able to see Drizzle’s shine.

But wait… we forgot about something else much more important! Drizzle and his companions are continuously moving across the sky and each droplet of their juice passes through our collector and falls on an ever-changing place on our canvas. After perhaps 15 seconds the juice of all of our stars will fall noticeably further away – on the next pixel. Poor Drizzle stands no chance of making an appearance because he cannot spew enough juice in his brief time over any part of our canvas to leave a noticeable mark.
How then can we help little Drizzle make an appearance? Well, we can collect more of Drizzle’s juice by using a bigger collector (lens). With a bigger lens instead of collecting one droplet at a time, we can collect two or with a really, really big lens perhaps 4 droplets at a time. After all, a bathtub in the rain collects more water in a minute than a thimble will! Sadly we already have the biggest collector we could afford, and opened it as wide as it will go: so how else can we help Drizzle? Answer: We will employ the last trick our camera can muster: we can change our canvas so that one droplet leaves a mark as noticeable as though it were 10 times its size. This is what is happening when we change our ISO from 100 to 1000.

Oh, I forgot to mention that while we are collecting juice from our starry friends, truly random events occur that cause droplets to appear out of nowhere and plop onto our canvas. We call this noise. The longer we allow our canvas to collect juice, the more random droplets there will be scattered all over our image – unwelcome intrusion by the sprinkler from hell. Unfortunately our sky is not completely black.  Dust and moisture in the air grab light pollution from distant city lights and make the sky rain droplets everywhere. The stronger the light pollution is the harder it will be for Drizzle to stand out. In fact, when the sky becomes as bright as Drizzle we will not be able to see Drizzle at all.

What did we learn from Drizzle, besides sympathy for his plight?

  1. If we narrow our aperture we will get fewer noticeable stars. And the stars we do get will stand out less (lower contrast).
  2. If we use a bigger (larger diameter) lens we can collect more light and thus see more stars.
  3. If we increase our ISO we get more stars, and unfortunately more noticeable noise.
  4. The longer we expose the more noise there will be.
  5. As the background sky glow increases the dimmer stars will be overwhelmed and there will be insufficient contrast to see them.
  6. Eventually an exposure that is too long will wash out the sky and stars.

And the most important take away:  The number of stars we can capture in an image is unrelated to the length of the exposure because the stars are moving.

That last one surprises most people. That is why I highlighted it. I see many folks trying hard to work out the right exposure based on the ambient light. But that is not the correct starting point.

And Now… The RIGHT Exposure

I hear you: “We just want to know what the correct exposure is! We really did not need to hear about your constipated little star.” True, perhaps I spilled more than you wanted to know but my little drizzle buddy hopefully made it clear that only the aperture and the ISO settings have a significant effect on how many stars will be observable in the image.  Now that we know that a really small aperture will eliminate most of the stars, that a long exposure will invite more noise, and that a higher ISO will both allow dimmer stars to be seen AND increase the effect of any noise we hopefully can use those parameters to narrow in on what we need to control. Here are our goals:

  • Use shorter exposures for less noise, better contrast, and less interference from background glow
  • Select moderately open apertures for more light and better contrast
  • If possible shoot in cooler ambient temperatures.
  • Where possible select locations with darker skies.

I hear you oh impatient one. You wanted to know WHAT EXACT SETTINGS you should use. Try this:

f/4, ISO 200, 4 minutes.

That should be about right on most nights where there is half or less moon and not too much sky glow. But remember that exposure triplet is for the sky not the foreground. We can refine those settings by answering the following questions and adjusting the exposure, ISO and aperture as indicated.  Adjustments to larger ISO are optional. Adjustments to a lower ISO are not… except to try and see that failing to reduce ISO or exposure time produces lots o’ noise.

  • What is the air temperature (Fahrenheit)?
    • Greater than 80 degrees (ISO ÷ 4)
    • Between 60 and 80 (ISO ÷ 3)
    • Between between 38 and 60 (no change)
    • Below 32 (ISO × 2)
    • Below Zero (ISO × 4)
  • How bright is my sky (background glow)?
    • About what you would expect less than 20 miles from a large city:  ISO ÷ 2 and exposure ÷ 4, f-stop +1 (1 minute at ISO 100, f/5.6 or f/7.1).
    • It is quite dark but the moon is full (ISO ÷ 2) – exposure ÷ 2
    • It is dark, but the moon is 1/2 full (no change)
    • Some noticeable sky glow, but I can see the Milky Way (ISO x 2)
    • I sat in the dark for 30 minutes and I literally cannot see my hand in front of my face. I can only tell it is there because it blocks out the star light and I still have sensation in my fingers. (ISO × 3) or f/stop + 1.
  • How many stars do we want in the image?
    • As many as I see with my eyes – aperture at f/4 or f/5.6 or even f/7.1
    • Plenty (ISO × 2)  – an aperture at f/4 to f/5.6
    • A sky full (ISO × 4)  – aperture at f/2.8
  • How good is my camera at managing noise?
    • Really Terrible: ISO  ÷ 4 and exposure ÷ 2.
    • Terrible (ISO ÷ 2)
    • OK (no change)
    • Good to Great (ISO × 2) and increase exposure by double

Suppose we end up computing a value of f/5.6, ISO 25 and exposure 2 minutes. What does that mean?  It means something is far less than ideal and throwing in the towel is in order – especially if the minimum ISO on the camera is 200.  If on the other hand the series of multiplications and divisions nets an ISO greater 1600 it is better to dial the ISO down a bit (unless you have a newer generation, highly  capable camera).  The above is pseudo scientific and adapted from observations and exposures – both successes and failures. Actual results may vary.

Ok, now I am sure an explanation is in order. Let me start with the one thing I think causes the most variability – the ambient temperature. The hotter it is, the stronger the noise. That is simply a fact of the electronic world. Film shooters have an advantage here. Film does not become noisier as it is exposed longer – it does have a different problem though. The longer film is exposed, the less sensitive it becomes.  Scientists and astronomers are well aware of the temperature problem. In fact, the really serious image makers super-cool their sensors to remove as much noise as possible.  How much difference does temperature make?  Well, a guy named Gary Honis built a miniature refrigerator to cool his camera.  He is very happy if the mini-fridge gets the temperature of his camera down to 5 degrees! Why? Well one can take a look at his charts.  But I can summarize: a 5 minute exposure at 77 degrees fahrenheit had more than 2,500 noisy pixels while at 5 degrees, less than 100 pixels displayed noise! Twenty five times LESS noise!  When it is exceptionally cold outside I smile because I know that if my battery survives my image is going to be that much better!

The next issue is the camera itself. The smaller and denser the sensor (the higher the megapixels) the more prone it is to noise. Sure some cameras have sophisticated processing to reduce the noise but most of the algorithms also reduce the contrast and thus the sharpness as well. A big sensor with big pixels is better for lower noise performance.  Another issue has to do with the design of the sensor – the method used to collect and read out the count of photons in each of its buckets (pixels). Some methods are just better than others.

Look at the histogram. If the shots are even close to being over exposed reduce the exposure time. If things are too dark, and it is miserably cold out increase the ISO or open up the aperture – or both. If it is warm only open the aperture because a higher ISO or longer exposures will result in more noise.

To get a well lit foreground requires one of the following:

  • Shooting when there is more moon (and get fewer stars)
  • Painting the foreground with artificial light
  • Shooting for a much longer time (and live with the noise).
  • Shoot some shots at twilight to get a nice foreground and layer in the star trails.

Can I Reduce Noise by Averaging?

In a word, no. The normal method of stacking selects the brightest pixel from each shot… and hopefully those are the stars. But those bright pixels can also be noise. If there is noticeable random noise in 10 images then there will be 10 times as much noise in the finished image! It is important to keep the noise as low as possible in the first place!  Averaging can be done. Something undesirable then happens: the foreground image and the overall sky will look much smoother but the star trails will lose contrast. Why is that?  What is the average of 100 + 1 + 1 + 1 + 1?  Answer: twenty one. What did I just do… I took 5 shots with nearly black pixels  (1’s) and one shot with a bright star (100) and effectively I reduced the brightness of the star from 100 to 21.   Here are examples. First is stack which selects the brighest pixel from each of the 11 images. Next the same 11 shots are averaged. The red light on the foreground is from one of the frames. Averaging the star trails clearly reduce the contrast of the stars (and the Saguaro) very significantly.

11 Images stacked using “Brighten” (lighten) mode. One of the images has the Saguaro lit by a brake light of a nearby car.

11 images averaged. Notice the big difference in the contrast of the startrails and the muted light on the Saguaro. On the other hand the sky is “smooth” since averaging also averages out random noise.

If it is not possible to reduce noise by averaging, then what is LENR (Long Exposure Noise Reduction)? Many DSLRs can be configured to enable or disable LENR. Usually I leave it off because using LENR makes your shot take from 50% to 100% longer. Why? Well the camera is effectively doing what the photographer can do by putting a lens cap on. It is taking a dark frame. It closes the shutter and lets the pixel counts accumulate.  It then uses those accumulated pixels to subtract them from the image it just captured. If you have amp glow (multi-pixel hot spots that appear pink or purple) the subtraction will remove the glow.  If you have hot pixels – areas that always read out as bright, the subtraction will eliminate those as well since they will also be in the dark frame.  Since the performance of the sensor changes rather dramatically with increased or decreased heat taking the dark frame immediately after the exposure works best. Of course the camera imaging chips may also do things like sharpen or blur pixels that it thinks are out of range with surrounding pixels. Blurring may improve the appearance overall but it also has an effect very similar to the average stack we looked at earlier.  It is much better to do what we can to keep the noise low in the first place.

What about simply increasing the exposure with the exposure (or lightness) control? There is no magic in that slider. If you do nothing to “increase the exposure” the pixel values are left alone. If, however you increase the exposure by 1 stop, the effect is to double every value. A 10 becomes a 20, a 50 becomes a 100. This, of course, makes those pixels brighter – but it also makes every other pixel brighter, too, including the mild noise. A slightly noisy unnoticeable “3” value becomes a quite noticeable “24” when you increase the exposure by 3 f-stops – an eight-fold increase.

**NOTE: In the earlier Drizzle discussion the word droplet and photon appear to be used interchangeably. But photons are very tiny things and in reality Drizzle’s one droplet per second is really about 2,000 photons per second in case more scientific numbers are desired.

 

***Extra NOTE: There *is* a way to reduce the noise by averaging, even if you didn’t take Dark Frames.

Magic and Photography

Or perhaps this article could be titled The Magic of Photography. Magic (illusion, prestidigitation, sleight of hand) and photography have much more in common than might seem immediately evident.

I am an amateur magician, and a card carrying member of the International Brotherhood of Magicians. That means I know how some tricks (illusions, deceptions) are accomplished and can perform a few as well. Indeed I have even created some of my own illusions. Being a brother in magic also means I have agreed to not reveal secrets to those outside the brotherhood. Fortunately there is no such oath for photography.

In magic there are basic underlying principles – both in the presentation and in the methods used to create illusions. In photography there are several immutable principles that govern optics and exposure as well as principles of composition and human nature that effect how we perceive things. Both magic and photography use these principles of perception. Both are about what should be emphasized and what must be eliminated from the observer’s perception.

Magic uses the word exposure, for example. In magic exposure governs the allowable viewing angles. Some effects are astounding unless the audience happens to be in an exposure zone and is perceptive enough to notice a concealed object or apparatus. In a photograph the selection of strong viewing angle or vantage point can create a scene that draws the viewer in – while a poor angle creates visual chaos.

What you Appear To See

In both photography and magic it is what you appear to see that lead to a satisfying experience – and sometimes the experience is as much about what you do not see! For example in the Photo 1 below of the Buttermilk Mountains near Bishop, California, I moved low and to the right to remove the road and the pile of tractor tires from the scene. My intention, however, was not just to conceal the distractions but also to place the blooming rabbit brush into the foreground to create four distinct layers of color. I could have gone one step further and cropped out the tiny bit of road that remains but I’ll bet you didn’t notice it (hint, look in the lower left).  In magic this would be called hiding in the open. For example a magician might hold a coin behind a dollar bill. If you have no reason to suspect that a coin is concealed there you will never notice unless the magician clumsily handles the bill or flashes the coin.

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Photo 1: Unseen are a road and tires. Visible are 4 distinct regions of color in this photo of the Buttermilk Moutains near Bishop, California

Viewing Space

Close up magicians like to work within a visual space approximately the size of a computer monitor or small television. They call this framing. As long as the magician can maintain your focus in the viewing area you are very unlikely to notice a surreptitious snatch of an object from a pocket or table top. If  you have ever seen a good magician perform a classic cups and balls routine, it can be downright stupefying to see an object as large or larger than the cup appear beneath it! Magicians have a huge advantage over a photograph though. Magicians can engage and distract their viewers visually, audibly and with motion.

A photograph, however, is purely visual and can only imply motion or sound. Of course a photograph also has a border and thus a frame. The size and distribution of elements within and near the borders of an image can create pleasure or dissonance. I have often heard the mantra to fill the frame with the subject and with good reason. If the primary element of the image is small then viewers are likely to wander in the picture searching for meaning and connection – and less likely to find it. Photography can employ devices to aid with this problem. Framing devices such as tree branches, fences, and terrain features can bring attention to the main thing.  Leading lines and “S” curves are very pleasing, too when they draw the eye where the key element of the shot is.  A photograph that has too many elements makes it harder to understand what is important.  Selective focus is another tool in the photographer’s arsenal. What is bright, and what is in focus draws the viewers interest therefore whatever is important in the image should be well focused and whatever is a distraction should be removed from the frame or blurred in a pleasing way.  A magician has an advantage – he can easily distract (misdirect) you with a noise, a question, or a gesture. A photograph, however, is unchanging and must be well composed or its message is thwarted.

Contrast and Visibility

Did you ever wonder why magicians often perform coin magic with old silver dollars? It is certainly not because silver dollars are more magical than a US dime, but the old US silver dollars are much larger – and thus easier to see. In photography making your subject stand out from the background is the analog of the magician’s large coin. Indeed a classic of magic involves changing a large silver coin into a large copper coin. It is pretty astounding because the two coins are easily contrasted. I have changed a silver US quarter into a Canadian quarter and guess what… nobody notices! The coins are the same color and size so the viewer never catches on unless the coin is the sole focus.

In a similar way a photographer can employ negative space – a large empty area to set off their subjects, or strong color, light or tonal differences to emphasize the key element.

Implication and Scale

The truth about magic is that it is not what is seen that is amazing. It is what the observer believes they have seen. Surprise comes not when a pen is thrust through a dollar bill but when the pen removed and the bill appears unharmed.  Of course there is a rational explanation why there is no trace of damage: the pen did not really go through the bill (it just appeared to do so), or the actual damage done is hidden from view, or perhaps the pen is not what it appears to be. And guess what: all three methods of penetrating and restoring a bill are used! All three methods result in a similar experience for the observer.  In photography, like in magic, there are many adjustable variables in an exposure. One can vary the aperture, sensitivity, time, angle, light or direction and all can produce nearly identical results – albeit with some important and subtle differences.

I see lovely pictures of waterfalls all the time. But the experience of a waterfall is very different from a photo. Standing near a waterfall I hear the sound, feel the wind and coolness of the water and perceive its size. In a photo how do I get those connections? Unless the photo contains clues water flowing over pebbles and water flowing over enormous boulders look identical. Ambiguity in scale can be intriguing, but it can also be frustrating.  Where the scale of the scene is important to the impact of the image something of known size must be present – e.g. a person or a leaf. Some element in the image should also give the viewer a sense of orientation – a principle that Galen Rowell calls “visual daylight“. We literally get that visual daylight in the original, uncropped Photo 2 – complete with trees and warm sunlight. This image was taken at the end of Whitney Portal Road in the High Sierra west of Lone Pine, California.

... Life is But a Dream ... [C_009244ps]

Photo 2: A relatively small waterfall with plenty of clues about the scale - including trees and leaves. And the long exposure (1/2 second) gives a sense of motion.

Photo 2 could have a stronger impact. For starters, the branch across the top is rather distracting. Cropping the original photo (see Crop 1) to provide a vertical treatment gives a strong verticality, and a diagonal S curve. The water seems to flow in at the upper left and out at the right with plenty of clues about what it is and the size. The perspective feels like we see it while standing in the stream (which I was!) and from a low angle.

Crop 1: Shaped like this emphasizes the verticality and the water.

A traditional landscape view completely changes what we are seeing. The image is now about the boulder – or the fall we can not be sure which and it is less appealing. The boulder feels like a big stop sign telling us not to enter into the scene.

Crop 2: A more traditional landscape format. It lacks the interest of the original in part because we are not sure what the subject is.

Cropping off the distracting branches, but leaving in a rock at the lower right we now can appreciate the boulder and how it is part of the scene but not feel blocked by it. While the viewing angle has not changed this Crop 3 gives a sense that we are now looking downward onto the scene.

Crop 3: Here the branch is removed and the photo flows diagonally from the upper left to the lower right through a diagonal S curve. This treatment is less curvaceous than Crop 1.

We have now looked at 4 views identical in every respect except how they have been framed. Good magicians think about and structure their performance with framing in mind. They must present coherence in subjects and motion, and leave out extraneous and distracting elements. And all of these concepts are also true of photography!

Go out and create a magical photograph – but do not expect that Abracadabra will get it done alone. Invest some practice and thought.

I hope you always find your light magical and your subjects enchanting.