Tag Archives: Star Trails

The Revenge of Lens Correction

There are plenty of ways to make your images look weird.  Some of the perturbations are due to sneaky little things that Adobe Photoshop, Adobe Lightroom and/or Adobe Camera RAW might be doing to the data.  We already talked about the “cooking” that is applied by default to RAW images and why letting that cooking stand unchallenged may be a bad thing. We’ve even warned you about Blur and Jaggies that may NOT in fact be in your images.

Recently Dan Barr asked us what we thought was causing a problem in his stacked star trails. If you read the title you’ve probably already figured out the culprit… Lens Correction!  You may not notice anything weird if you process only a single image, but what if Star Trails, or image stacking are what floats your boat?

Notice the strange pattern in the upper left. This image is cropped from a larger image.

Notice the strange pattern in the upper left. This image is cropped from a larger image. Image by Timbo2013

Look in the upper left of the image above. That cross hatching is one possible artifact.

Why does this happen? The lens correction is a mathematical model that moves pixels around. Not surprisingly, since the images change – even if slightly, the results vary slightly, too.

How do you fix the problem?  Don’t mess with your images before you stack them.  Save the lens correction, contrast adjustments and other tweaks for after you’ve finished stacking.

Here is a before and after comparison:


Notice the odd “Moire” like pattern above and to the right of the mountain? (Image courtesy of Dan Barr)

It’s a little subtle. Here is the weird part close up – notice the vertical undulations? The oddness somewhat resembles sensor banding noise except when you look at a larger scale, the lines are concentric.



By redoing the stacking operation without performing lens correction, Dan was able to get an image without the waves:

Stacked first, then adjusted – no moire!

With the strangeness vanquished, Dan was able to improve the brightness and contrast as well.

Intervalometer Tricks

Red Rockin' Spiffed Up

Scratching your head and wondering what an Intervalometer is?  We’ve covered that in this article, and talked about some super fancy Intervalometers in this article.

Before we launch into the tricks, let’s first get some terminology straight.

  • Long Exposure – in my vernacular this is an exposure over 30 seconds – the limit of most DSLR cameras.
  • TimeLapse – a series of photos taken over time that compresses (or expands) the actual time when made into a movie. Usually all the exposures use the same settings.  An event that takes 3 hours can be distilled into a 30 second video.  An event that takes fractional seconds – like a balloon popping – can be shot at high speed and expanded into a movie that lasts much longer.  Usually expanding the time is called “Slow Motion”.
  • StarTrail – like its timelapse brethren, implies a series of shots taken over time and combined into one exposure to show the star motion OR a StarTrail can be created from a SINGLE very Long Exposure.
  • Bramping (aka Bulb Ramping) – a timelapse techinque in which the length of the exposure is changed over time to accommodate the setting sun, rising moon – anything that involves a gradual change in the ambient light.

How is a Timelapse different from a Star Trail?

The two are not different, except that by intention a Star Trail created from multiple exposures requires a minimal interval between one shot and the next or gaps result. For a timelapse – which can be taken at night or day – the key is having a regular interval between each shot.  Changing the interval between shots has the effect of warping time.

Ok, Got it. Tell Us About the Tricks

Sub-Second Intervals

Why would you want sub-second intervals?  For one, to catch as many meteors as possible. The second or so that the camera spends with it’s shutter closed is a second you might miss that brilliant fireball. Another reason to keep the interval REALLY short is to reduce or eliminate gaps in star trails.  But sub second intervals are the hardest trick of all.  There are almost no intervalometers that allow setting an interval shorter than one second, and even if it’s possible many cameras can not handle sub-second intervals. However, there are a few devices that can do sub-second intervals: Trigger Trap for one.  The best way to find the shortest possible interval is to set up the camera and try! Set the interval to say 700 ms and see if your camera can run off a sequence of 15 to twenty 30-second shots without missing a beat. If that works, set the interval to say 500ms.  Note that the minimum interval will depend on the camera, as well as the size of the image, and speed of the memory card. Once you find the minimum, leave a little extra time and use that. My Canon 5D Mk II was happy with 450 ms intervals between shots. That’s HALF of the waiting time of one-second intervals.

Shake Reduction – Mirror Lock Up

Many people worry about mirror slap. Mirror slap occurs when the little mass of the mirror “wiggles” the camera enough to blur long-ish exposure shots. Mirror slap is particularly pernicious in the 1/4 to 2 second exposure length. It may also be a problem if you have your camera attached to a delicately balanced telescope at high magnification.  How do you solve the problem?  It depends on your camera, but there are several approaches to try:

  1. Leave live-view on (which will eat batteries and may result in excessive warming of the sensor)
  2. Use the camera self-timer in mirror lock-up shooting mode. Most cameras will behave properly if your exposure length is not bulb. That is, they will move the shutter, wait for the delay to expire and then take the shot. Remember to allow a delay that is at least one second longer than the shot length plus the self timer delay. For example, let’s say you want to take as many 24 second exposures as possible but you need at least 8 seconds for mirror slap to stabilize. Set the camera to 24 second exposures with a 10 second self-timer.  Then set the intervalometer to take a 1 second(!) exposure every 36 seconds. The reason for the 1 second exposure is to allow enough time for the shutter release to be recognized while the 36 second delay allows for 10 seconds of timer, 24 seconds of shot and a 2 second safety buffer.
  3. See the Maximum Shots, minimum interval trick. But instead of 1 second delays, change the length of the exposure to the amount of time you need for camera stability + 1 second.
  4. Want to do shake reduction in BULB mode and without a self-timer?  Set the exposure length to the desired amount of time and use a short interval. With mirror lock-up on, you’ll get every-other exposure at the desired length. Note: this is the most “iffy”mode as it depends on your camera behavior.
  5. Finally for shake reduction in BULB mode WITH a selftimer, set the intervalometer as normal, but set the length of exposure longer and include the self-timer interval. For example to take 60 second exposures with a 10-second self timer, set the exposure length on the intervalometer to 70 seconds.

Variable Length Shots

While this technique seldom works well, you can allow the camera to determine the exposure length via metering.  All you have to do to make this work is to have the camera take 1 second exposures (as before) no more frequently than the longest exposure you expect to take. Some fancier devices, like the Trigger Trap and the CamRanger can even be configured to change the exposure length over time. This feature is called “Bulb Ramping”.

Extended Self Timer

Got a big group shot and no wireless remote. Not a problem. Set the camera to a short (e.g. 2 second) self-timer delay, and set the shot delay to say 20 seconds – or as long as you need to safely climb on top of the human pyramid to get that perfect shot.  Since you can allow multiple shots, you’ve all got plenty of time to change your poses, or re-architect your human pyramid.  We use this trick all the time when we’re conducting workshops. It allows us to set up our camera and walk away while we instruct. We leave enough time to set up for the shot.  Most cameras will blink or flash to let you know they are about to take a picture so everyone can time that cheesy fake smile.  Note: If using a Flash, you can lengthen the interval between shots to give the flash extra time to recycle.

Maximum Shots, Minimum Interval

When not in Bulb mode, it can be maddening to not have the Intervalometer and the camera in sync. Set the camera to 20 seconds and the intervalometer to 19 and you’ll miss about every other shot. Bummer.  Here is a trick to maximize the number of shots and not care much about the actual exposure time.  Set the intervalometer to take one second shots with one-second intervals between each.  The maximum shot-to-shot delay will be two seconds that way and it doesn’t matter what your exposure length is on the camera if it’s NOT bulb.

What if you want BULB mode? How do you configure that?

Answer: (Select the text below to reveal)
That’s the normal intervalometer configuration mode. Camera is bulb, length of exposure is whatever you need and the interval should either be 1 second, or exposure length PLUS one second depending on the intervalometer.

Blobulous Revisited – Part 2

Last installment we covered the basic idea behind creating a star trail where a foreground element is moving. In this case the moving element is a radio telescope peering into the sky to discover planets and black holes.  A normal “lighten mode” stack produces the image at the left, below, while with just a little bit of work we can get the image on the right.


To recap, we use a single frame from the sequence and some careful masking to remove the blurred part of the image. For effect, we also don’t use the stack in 100% mode. See the prior article for details.

In this article we’ll show you step-by-step how we achieved the total look.

Of course we start with the stack.


Then we layer in a copy of the stack and a single frame (the last or nearly last frame).


Here we have set the background (stack) to 36% Opacity, effectively darkening it. The single frame is 100% opacity and in Lighten blend mode. What we want to do is to remove the blurred part by replacing it with the unblurred single image. It’s easy to do. Select the STACK, create a “Reveal All” mask, and then go to town painting black on it (be sure to select the MASK, not the image). When done, the mask we create will look something like this:


Notice how we used a slightly soft brush to “blend” the background and the still frame. We could stop right there, but I notice that the ground and the telescopes are a bit too bright, and I’d like to make the stars pop out. So the next course of action is to apply a curve, select the “Increase Contrast” option. Here I’ve adjusted the result just slightly.


Next we want to tone down the bright stuff. We add another adjustment layer, and a “Hide All” mask and then paint white back on the mask to tone down what we want. You’ll notice that in the process, the colors intensify a bit.


The next step we’ll want to take is to reduce the saturation – our radio telescope is moving from white to yellow. So the next step is to add a Hue and Saturation layer. As before we mask off everything, and then paint in only the radio telescope. We could cheat and use the same mask from the Brighten stack layer – and just invert it, but it’s not a complicated thing like a tree, so it’s pretty easy to change the mask to only operate on the radio dish and pedestal.  At the left, you can see how strongly we moved the saturation – and we upped the brightness a bit, too.


If we didn’t mask off the telescope and instead applied the saturation adjustments globally, we’d see this – not what we want. (Shift-Click on the mask turns it on or off – in this case we see that the mask is off by the red X through it)


You won’t notice in the small size, but the large image has a number of Hot Pixels (red, and blue) that stand out. To solve this problem we use “Alt-Ctl-Shift-E” (Command-Option-Shift E for you Mac-o-philes) to make a copy of the layer. I named the layer “Heal” because I then used the spot healing tool to fix up those little problems.  I recommend making the healing tool diameter just slightly larger than the area to be healed.


To make the Radio dish pop just a little more, a little sharpening is in order. In fact, sharpening the ground will work, too.  Duplicate the Healed layer (Ctl-J or Command-J). Name it Sharpen then use Filter -> Sharpen -> Smart Sharpen.  However I don’t like sharpening my stars, they look harsh. As before we’ll create a Hide-All mask for our Sharpened layer and use a white brush to reveal the areas we want to have sharpened. This is called selective sharpening.  In the small image here, the effect is not as obvious as in the larger image.


To get just a bit more pop, a little more contrast is in order. I created a curve and pulled up the midtones a fair amount while making minor negative adjustments to the highlights and the darks.  But, that adjustment brightened some areas a bit too much so I created a reveal-all layer mask and painted black on the areas that were then too bright.


Et Voila, we’re done!


Warning: That RAW image is not really RAW – and why it matters

On the left is an “Auto” Adjustment while the same data on the right is unadjusted. See below and you will discover that there is some serious misinformation on the web about ACR adjustments.

You may know that Photoshop does not know how to open raw files like NEF, CR2. Every time you open a raw file, it is actually opened by Adobe Camera RAW (ACR) which is an internal component common to Photoshop, Photoshop Elements and Lightroom. And there is an Adobe Camera Raw Defaults setting that is automatically applied per each camera type unless the user chooses custom settings. What you may not know is that I highly recommend stacking your star trail images without making any adjustments. Once you make adjustments, especially changes to contrast, tone curve, brightness, shadows or exposure you increase the visibility of gaps and noise.  I explain why this is so in my Down with the Noise Webinar, but for now, just take my word for it!

Confusion Abounds

Unfortunately it is quite complicated to remove the default Camera Raw adjustments due to conflicting details on web sites, including on Adobe’s own FAQ. As my experiments show, the “default” settings for ACR apply adjustments. Adobe says that using CTRL-R (CMD-R on a Mac) resets to the defaults for a RAW file, but it doesn’t reset everything because the default settings do have adjustments!  Below are the choices for selecting, saving and resetting Camera Raw Defaults – you find this menu in the upper right of the ACR display – see more illustrations below.

Fullscreen capture 2212013 85814 AM.bmp

In my tests with ACR 7.0  CTRL-R – which theoretically is the same operation as selecting Camera Raw Defaults – did not remove hand applied adjustments to clarity, tint, noise reduction, sharpening, vibration or saturation, tone curve, and other settings. What CTRL-R actually does is remove adjustments to Exposure, Contrast, Highlights, Shadows, Whites, and Blacks.  Using the Camera Raw Defaults (first highlighted choice in the list above) doesn’t get what you might expect!  So I went further. I set all the values to zero, then used Save Camera Raw Defaults, selected Camera Raw Defaults for the image and opened it using the Open Object button. When you use Open Object ACR creates a .XMP file – sometimes called a sidecar file – that I inspected to see what has been set.  The non-zero settings in the XMP file after saving my custom camera raw defaults and choosing Camera Raw Defaults included the following non-zero settings:

 crs:CameraProfile="Camera Faithful"

When I saved my own Camera Raw Defaults I turned on Chromatic Aberration and Lens Profile correction and overrode the white balance to “Camera Faithful” just to be sure that the new Defaults were actually using my saved default settings. But wait! There are still Brightness and Contrast adjustments listed even though I had set those values to zero.  It is also not clear whether it is applying a tone curve adjustment. The good news is that my saved defaults are NOT doing any sharpening or noise reduction whereas the ACR defaults (the default defaults?) do mess with those.

Further Experiments

Before I tried to set my own Camera Raw Defaults, I followed advice I found online. That is how I discovered that the default, Default Camera Raw settings include both sharpening and color noise reduction.

Camera RAW "Defaults"

Using the “Camera RAW Default” selection from the menu. Some changes are still being applied!

Notice how the settings file (XMP) contains adjustments for color noise reduction, a tone curve, and sharpening. The real head scratcher is that the side-car (XMP) file also shows adjustments to Shadows, Brightness and Contrast – which are NOT shown on the Basic (leftmost) settings panel for the image.  Not knowing the internals of Photoshop, I can not tell if the brightness, shadow and contrast adjustments are actually present or not.

The XMP file and the display do not agree.

Basic does not show adjustments that are in the XMP file!

Unfortunately there are sites that claim that using the CTRL-U (CMD-U) sets all the values to default. This is incorrect. CTRL-U toggles between automatic and not automatic  which is the clickable text Auto in the settings dialog. What I’ve called Default RAW Adjustments in my comparison photo at the top of this article is actually automatic adjustment – I was mislead! What is automatic? It is a roulette wheel whereby you let ACR take its best guess at what it thinks will look right.  Apparently it is pretty smart unless you let ACR do its automatic thing on a night image in which case the result will not be very pleasing.

Camera Raw 7.0  -  Canon EOS 40D 2212013 81342 AM.bmp

The stated Adobe method to reset to Camera Raw Defaults is to use CTRL-R (CMD-R on Mac). After using this magic sequence I see that there is still sharpening, a tone curve and much more.


Yeah, me too.

In fact, the default RAW setting can be per camera per ISO. The bottom line for me is that I do not trust ACR to not mess with my image unless I apply a Linear “Develop Settings” to all the images I’m going to load. And I am not even sure that some adjustments are not still being made despite my strong desire to have my images be unfooled around with.

But Why Do I Care?

A RAW file that has nether been sharpened nor had a tone curve applied looks flat and boringish. Why so boring? A digital camera records images in a linear fashion but our eyes don’t perceive things that way. To prevent people from squawking, ACR by default applies tonal adjustments to convert the raw data into something more adapted to what we see.

Of course you might ask why anyone would ever want to look at the un-adjusted image, and the answer is I wouldn’t want to either… but when stacking the fact that the pixels haven’t been diddled with beforehand makes the result better.

How do you get really RAW Raw Images?

For starters, you can set all of your Raw Defaults to Zero and save them as I noted in the Confusion Abounds section above. As a further belt-and-suspenders technique I also created a preset called “Linear” using the Save Settings menu. I apply the “Linear” preset to my images before I open them to force the sidecar files to be created. Whether ACR is still messing with some of the data is not clear.

But what about Cooked Images?

I don’t always go “really RAW” – I may tweak the settings in ACR for a more pleasing visual appeal. The literature indicates that ACR is a bit better at making adjustments than Photoshop is.  The good news is that you can have your cake and eat it too because no matter what you do in ACR it does not change the data – just the adjustments that are applied to that data.

Here is how I made adjustments to the same image shown earlier along with all the non-zero values from the .XMP (sidecar file).


So if RAW is so Complicated I Should Stick to JPEGS, right?

Heavens no!  If you shoot JPEGS rather than Raw you’re throwing away a lot of good data. The processing to convert the captured data into a JPEG involves lots of decisions made on your behalf, behind your back, and without the ability to change your mind later.  Yes, you can diddle with the image, but you will not get the results you might if you had not let that little conversion monster distort your pristine data. In other words, you’ll eventually regret what happened.