Category Archives: Color Correction

Filters and the Night Sky

I wanted to buy a light pollution filter. I saw there are 2 kinds of filters UHC (Narrow band filter) and the Broadband Filters (e.g. CLS). Which one should I get?

I think the easy way to answer the question is as follows.

For Narrowband substitute the word Red and for Broadband substitute the word Polarizer.

A Red filter will definitely block out light that is not red. A polarizer lets all colors through roughly equally, but a polarizer does diminish the total amount of light. If you use the red filter you’ll still see, but you’ll see only the red component of the light. If you use a polarizer things will look normal, but dimmer.

The same is generally true for Narrow and broadband filters. Of course a Narrowband filter doesn’t have to be red, it could be green, blue, ultraviolet, etc and a broadband filter also doesn’t have to be neutral. Indeed, the idea behind a broadband filter is to pass as much light as possible in as many wavelengths as possible EXCEPT for the predominant wavelengths of the light pollution.

The only broadband filter I’ve used, the Astronmik CLS, is a light blue color and definitely gives things a blue tint that requires correction.  The filter I have drops into my camera just ahead of the sensor. That setup makes is more versatile, but it only works on Canon cropped cameras, not on my 5D II, for example. I purchased my filter from OPT for about $130 and frankly haven’t used it much because it got stuck in my Canon 40D.

One problem with light pollution is that it comes in many different wavelengths (colors) depending on the light source: tungsten, fluorescent, halogen, low or high pressure sodium vapor, etc. So the effectiveness of any broadband filter changes significantly with the lighting conditions around your photography/observing location.

On a more practical matter visual astronomy is usually done with a telescope or binoculars and the filter must match (one of) the standard eyepiece sizes or simply be hand-held. Visual observing is also different from photography in that color is easily recorded by a camera, but the dim light of stars, galaxies and nebula is almost always perceived in grayscale by we human observers. For photography the filter size depends on where the filter is placed in the optical system – thus you may need differently sized filters for different uses. I’m much more of an astrophotographer than an astronomer so I can’t really comment on what works best for visual astronomers.  Fortunately Shawn Grant does a great job explaining this in his article.

One filter that MIGHT have some significant application in landscape astrophotography is a graduated neutral density filter.  But I’m not really a big fan of these filters as I describe in a previous article.

Painting with Light

Originally Published: Sep 18, 2012
Last Updated: May 11, 2016

Cascade of Stars [C_049409+12]

A Flickrite asked me a question:

 I’d like to know how long do you shine the light or use flash when you are shooting

I was really tempted to give the answer “as long as I need” but I’d just seem cruel to answer the question that way.

The truth, however is “as long as it takes”. Eric Harness, one of my partners in the Star Circle Academy endeavor uses a quite different technique than I do. He prefers to reduce the ISO and paint with light for the entire length of the exposure.  I like to keep painting short and purposeful. Each strategy has its strengths and weaknesses.

How Long?

Voyage into Pointy Land [C_061012]Painting for best effect is a knack, not a science.  You just have to try it and refine your technique based on your results.  Remember to check not just the LCD, but also your histogram when deciding how well you’ve done.  Here are some important points that will influence both the method and how long you will paint with light:

  1. Check your ISO and f/stop.  The lower the ISO or higher the f/stop the longer you will have to paint and the brighter the light has to be.
  2. The distance to the things being painted (due to the inverse-square law). You may spend a fractional second on a granite boulder nearby and long slow seconds on dark, light-eating evergreens in the distance.
  3. Brighter lights require less painting but a more deft hand.
  4. Ocean foam or white water in a waterfall will reflect a lot more light than lava rock – you have to paint the rock much longer.  A still pond or lake require a LOT more light than you would expect. The reflectivity (texture) of the surface matters quite a lot.
  5. It is harder to be precise with wide beams, but easier to uniformly illuminate.
  6. Spotlight or narrow beams make it easier to highlight specific things – but also makes blow outs and hot spots harder to control.
  7. When trying to highlight a certain thing or reveal some shadow detail you will need more light.
  8. House in the Wood [C_049613-17]Often there is too much to paint in a single shot. Using stacking techniques you can paint the scene in sections – and even in different light knowing that they are easily combined later. Or you could expose longer and try to get it “all in” but it is hard to get it just right like that.

Power and Direction

I think most people overpaint – they make the subject stand out too much or blow out the details.  Neither of these are blown out, but they have obvious hot spots.
Beholders of the Galaxy [C_061001] vs Anticipation [C_050158]

  • Don’t paint “head on”. Paint at a 30-45 degree angle to the camera view – and even from behind the subject for rimlight.
  • Consider painting from both sides to fill in shadows – but paint less from one side than the other to keep the scene from looking flat.
  • Be mindful of the color you are painting with and the thing being painted.  Bluish LED lights produce a very different feel from warmer incandescent light
  • Be cognizant of the color “bias” due to existing light.
  • If you use a flash, set it on manual and don’t try to expose all at once. Tilting the flash upward helps to even out the exposure.
  • When painting with a bright light, quick movement is essential. Continuous circular motion helps prevent hot spots.
  • Sometimes you can get more even and pleasing light on what is in front of the camera by painting the ground or rocks BEHIND you – much like bouncing a flash off an interior wall.
  • For dimmer light slow methodical movement is better.
  • Moonbow with Character(s) [C_039385]Try throwing in some color!

Star Man and Perseus [C_059960-1]

Tools

About now you may be wondering what flashlight(s) to get.  We can’t really answer that, however we do suggest you snag the following:

  1. A BRIGHT light (120 lumens or better)
  2. A SPOT light (one with a very narrow beam)
  3. An incandescent light
  4. A broad, dim light (like a keychain light)
  5. Colored lights (red, amber, blue, purple) or some cellophane or gel.

Sooner or later you’ll be like us and carry EACH of the above. Oh, and don’t spend a lot!  Get some cheap stuff – like you find at the checkout counter in a hardware store.

Nonetheless here is a list of our favorite illumination toys. Some or all of them may no longer be available.

Milky Way Post Processing: Color Correction

I’m sure you did not skip the first two parts of this series, right? Did you? If so, please see Finding the Milky Way and Capturing the Milky Way. I’ll wait until you get back.

Back so soon? Hope you had fun reading about the Milky Way and how to photograph it. Here is a confession: You really do not need to jack your ISO up as far as I stated in Capturing the Milky Way. What happens when you set the ISO high is that you lose some dynamic range, and you will get some clipping (loss of highlights), and of course you increase the noise – BUT your processing will be a little easier because you won’t have to push any settings more than just a smidgen.

Hear are the general steps I take to attack my Milky Way images.

  • Noise Reduce
  • Color Correct
  • Contrast and local enhancements
  • Foreground/background blending

There are dozens of ways to do each of these tasks.  If you love Lightroom (I don’t particularly like it because it is SO slow to load and doesn’t allow me to blend multiple images) you will find some great resources by Ben Canales. For a $20 donation he’ll walk you step by step through his processing regimen.  The only downside to his tutorial is you must have web-access to view it – you can’t save a copy.

Even though I would normally noise reduce first, I am deferring the explanation for now and attacking the color balance problem. Sometimes all you need to properly color correct is to open your image in Adobe Camera Raw and use the White Balance Tool.

Much of the area near the Milky Way is “white” so clicking that diffuse glowing part with the white balance tool will properly balance your sky… or not depending on how bad the light pollution is.  Where exactly should you click? Not on individual stars (though that may work too if you pick the right colored star and you do not have clipping).  Just about anywhere except the brightest areas of the Milky Way should work.  It will not hurt at all to “click around” a bit until you get a natural look.  Here is a Milky Way image color corrected using the ACR white balance technique:

Milky Way Rest [C_049455]

However if the light pollution is pretty bad, you don’t have a raw file or your sky is quite orange/brown, you will want to employ a more potent solution.  This solution comes from Sky at Night Magazine.  Below is a video we recorded during our Photo Manipulation 150 Webinar.  One giveaway that your sky is not naturally colored is if it is orange, brown, green or completely blue.  I am not going to tell you not to render your sky like that – after all it is your photo and your taste will dictate what you want, but if you want people who enjoy astronomy to take your photo seriously do not go too far from reality.

One of my favorite images of the Milky Way resulted from allowing the camera to select a white balance. I used a blue-white LED flashlight and that caused the night sky to go “sepia”.  I did do some local enhancements to bring out the Milky Way. How I did the enhancement will be discussed in the next installment covering “Local Enhancement”.

Famous III  [C_035478]

If, however you want to get your sky naturally colored despite the light pollution, hopefully you’ll find this video informative – there are a bunch of additional tips, too!

You may have to enter the password BrownSky to watch it.

RESOURCES:

Related Articles Include

  1. Local Enhancement (Bump up Those Stars)
  2. Image Blending (Foreground O MaticEasy HDR)

Are we getting this right? Got a question? A quibble? Please leave a comment! And if this is really resonating with you, please share.  We love it when you share.

Trouble with Long Exposures – Part 1 of 2.

I administer a group on Flickr called “Star Trails” and moderate a group called “Best of Star Trails“. The good news is there is a constant source of new exciting photography there… and a fair number of beginners facing some common problems. Some of the problems are due to limitations in the camera, and some are due to the selection of exposure time, ISO, f-stop or focus. Some are due to cockpit errors of the kind I described in my August 13th article: Many Paths to Failure regarding unattended shooting with an intervalometer. This list is in addition to those problems and in a way is a bit more fundamental.

Common problems are:

  1. Poor Focus
  2. Dim Stars (low contrast)
  3. Strange Colors
  4. Purple or Pink Glow
  5. Gaps in Star Trails – see part 2.
  6. Lots of Noise (Colored Speckles) – see part 2.

Let’s tackle those one at a time.

Focus is Poor

Poor focus is a topic unto itself which I covered in My Camera Can Not Focus in the Dark – And Neither Can I! But there are a few other causes besides having an incorrect focus. Additional problems that may create noticeable lack of sharpness:

  • An unsteady tripod (often noticeable when there is wind). And it may not just be the tripod. Check the quick mount plate and the tension on the knobs.
  • Condensation (that is dew) on the lens. Use a lens hood (helps), and if really bad a lens heater.

Stars Are Not Very Bright

Often the lack of stars is due to an unnecessarily small aperture. Selecting a smaller aperture can help with your image, too. Here are some examples. First is an example from Miguel Leiva:

Photo 1: f/18, ISO 100 for 30 minutes.

???? Trails of Moon, Venus & Jupiter over the Nepean River 30/11/08

Photo 2:  f/20 ISO 400.

In Photo 1 a small aperture allows greater depth of field so that focus is sharp from the foreground to infinity but that small aperture also diminishes the contrast in the stars. Taken to an extreme a high f-stop (tiny aperture) with stars can produce an effect like that in Photo 2 by Vincent Miu which was a runner up in the 2009 Astronomy Photographer of the Year contest.  The very small aperture, f/20, eliminates all but the brightest elements from the night sky.

While a tiny aperture reduces the number of stars captured, a large aperture (small f-stop number) and/or a high ISO results in many more visible stars especially when the sky is dark. Compare these shots:

Cone Heads STILL in Awe [22012-2362]

Photo 3: f/3.5 at ISO 640: A lot of stars make for a pleasantly dizzying image.

(son of) Bristlecone Pine Star Circle

Photo 4:  f/4 ISO 100

Photo 3 was shot at f/3.5 ISO 640, while Photo 4 was f/4, ISO 100. Both  include about the the same star field but  many more stars are present in the higher ISO shot.  Even if you are not trying to reduce the number of stars in the field, you might be forced to use a smaller aperture to get more depth of field.  Another common problem that causes reduction in contrast is sky glow. When the sky itself begins to lighten you can be sure that the stars will not contrast well.  The best way to control this is to take shorter exposures and later at night – or on a clearer night (cold winter nights produce the clearest skies). The moon is also a huge source of glow. Treat the glowing moon just as you do artificial light glow – reduce your exposure length (and ISO) to take pictures when the moon is strong. But do not give up just because you can barely make out stars in your night sky – the camera can see them better than you can!  Photo 6 is a perfect example. The city glow made it impossible to see more than 8 or 9 stars toward the north and yet the star trails are quite present.

Colors are Strange

Many people are surprised to see that the stars in their photos are different colors: red, orange, yellow, blue and white. Those are the natural colors of the stars. People are also surprised to see a blue sky however even modest amounts of moonlight or a very long enough exposure will result in blue sky! Unfortunately sometimes the stars or the sky are unnaturally colored. Usually the culprit is one or more of these factors:

  1. Incorrect white balance setting (I recommend “Daylight”)
  2. The presence of artificial light.
Pleasanton Circular File [5_018700-20]

Photo 5: White balance problem due to different types of light. In this image I compromised to keep the colors on the land as natural as possible.

Getting the white balance right is not hard except when there is lot of artificial light – streetlights, city glow, etc. Unfortunately there are many different types of lights each with their own color characteristics. The popular low pressure sodium vapor lights are nearly monochromatic yellow-brown in color. There is really no way to get a naturally colored look when sodium lights predominate the scene. Florescent, tungsten, LED, and other light types all differ in their color profiles and when several different sources are in play for a scene it gets harder to keep a natural looking scene.

Sometimes when handed lemons you can make lemonade as in Photo 6. I could not correct for the predominate sodium vapor lights so instead of fighting I adjusted the color temperature to make the foreground elements look as natural as I could and did not worry that the stars became white – most people think of them as white anyway. It certainly helps that the image also includes a portion of twilight illumination to help keep the scene realistic looking.

A City and A Mountain. Part A [5_024371-434g]

Photo 6: When corrected for the sodium vapor lights the mountain looks almost natural, but the stars have lost their color.

And there is yet one more way to solve the color problem; but you will have to do some editing. To fix different color lights you can color balance each element separately and then combine the elements into one image. For example using “Daylight” white balance for the star trails and “Tungsten” for the street scene may produce a natural and pleasing looking photograph. Photo 6, above was manipulated in a similar way. Once it became completely dark the glow from the city lights caused flaring and ghosting. The solution was to choose one properly exposed frame from twilight and layer that on top. Layering like this is easier if you have an overexposed daylight shot that you can use as a mask. More on that in the Night Photography Workshop

There is Pink or Purple at the Edges

Some cameras, particularly older models may suffer from “amp noise”. The glow or noise is usually visible at the corners or edges of the photograph and usually only with longish exposures (over 8 minutes). Here is an example from Ethan Doerr of what “amp noise” may look like.

star trails

Photo 7: Amp noise is prominent in this photo taken on a Nikon D80 with a 572 minute exposure at 100 ISO. Nikon: D80, D90, D40, D200, D3000, and possibly other cameras may exhibit similar anomalies. Photo by Ethan Doerr – used with permission

If your camera is subject to amp glow there are some tactics you can try. The simplest is to keep your exposures short and stack them. Or perhaps allow the camera to cool down from time to time. Or only shoot in Antarctica ;-).

For more Trouble with Long Exposures see Part 2.