Tag Archives: Moire

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.

The Blur and the Jaggies

In our last workshop in the Bristlecone Pines, several students were concerned that their star trail photos showed strange blurring and “jaggedness”.  Before I show you any of the photos, can you guess what the cause might be?  I can assure you that their photos were pleasingly sharp and there was no motion blur.

There are two primary causes of the jaggies, and both are easily rectified.  The artifact occurs due to Aliasing which produces a Moire-like pattern.  Say What?  Take a large image with parallel or concentric lines, try to fit it onto a smaller image area and strange patterns may occur.

See the image below for an example.  It looks like the star trails have turned to mush in the selected area. Meanwhile just below, it appears the star trails look like the edge of a saw blade. What is that about?


This is a big file.  It’s 5616 x 3744 pixels, and we are viewing it at only 440 x 330 pixels. To show the whole image on a small space many pixels have to be skipped or dropped.  When scaling down like this – for the DISPLAY – we got a Moire pattern due to aliasing – just like the trippy effect when the weatherman wears pinstripes.  The effect can also be caused by viewing the file at a different aspect ratio, and some programs show even more harsh moire pattern.

But before we panic, let’s zoom in to 50%. For this view the program has had to mash two pixels together to see one pixel on the screen. Even now we still see a little bit of the effect.



Only at 100% do we notice that there really is no smeary pattern after all! In fact, that’s when we notice that we have tiny gaps in our star trail.  Do we worry about those gaps… nah. Not only are they easy to fix, but as we’ve already seen by the example above, we might not see them in the finished image anyway!  If after zooming in you STILL see the blur or jaggies, you might have problem number two (see below). OR it could be that you’ve downsized the image so much that the aliasing artifacts have shown up.


The other cause of jaggies is not using your display at its native resolution.  So, for example if you have a 1920 x 1200  (1.6 aspect ratio) monitor but are displaying your photo at 1280 x 960 (1.42 aspect ratio) your *display* is likely to look distorted. The squashing or stretching of pixels on the display will increase the strange effect.  The fix is easy: Use the native resolution of your display!  Can’t speak for you Mac people, but on most windows machines, the “recommended” resolution is the best for proper display.

What if The Blur Or Jaggies are Visible at 100%?

If after looking at a 100% view on a properly configured display the problem is still noticeable, there is no hope except to start over with the full size image and try a different downsizing algorithm. E.g. Bicubic smoother instead of sharper, or “bilinear” or “nearest neighbor” – all of these options appear in the Image Size for resampling of Photoshop. The latest version of Photoshop (CC) seems to pick a good algorithm if left in the “Automatic” mode.

Here is one of the images that student, Susan Starr, noticed jaggies with. I told the browser to shrink the vertical size – but I did not change the actual image size, The effect is not very noticeable on my display.  Click the image for the correct aspect image size.

Jaggies or Not?

Jaggies or Not?