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. 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.
