Reversing Order For Focus-Stacked Images
This article discusses techniques for capturing and combining multiple frames for focus-stacking photography. As the article title suggests, we will take a particular look at the order in which you import and stack your images into your focus-stacking software. I discovered a bit of this information a couple days ago when working on my latest macro image. I've been using Zerene Stacker for the last several months for focus-stacking and when I first obtained the program I had noticed an option for reversing the order of the image stack (where the ‘normal’ stacking order follows file names progressively) but I hadn't experimented with the option until assembling this project.
For a little background: one of the issues with focus stacking is the final composite image typically has to be cropped on all four sides to eliminate pixels that show overlapping of the frames being stacked. Figure A to the right shows a comparison between composites with the normal stacking order on left and the reversed stacking order on the right. Please ignore the heavy vignette in the comparisons - I'll come back to that in a moment.
Both images above were stacked from the same 124 progressively-focused frames but the two stacking orders show a big difference in results. Look at the top and bottom edges of the image stacked in the normal order. The streaks you see on both edges (most apparent on the bottom) show where all of the overlapping frames occur. The same overlaps also occur on the left and right edges but are somewhat hidden in the black vignette.
Figure B at left shows a 1:1 view of the bottom edge where I measure approximately 480 pixels that were the overlapping frames. Clearly those overlapped edges get cropped away for the final image, so the point I'm making here is that we lose a lot of image resolution when having to crop away those edges. The number of pixels you have in overlapped edges may differ from project to project depending on your focusing technique and how many frames you use for the focus stack. For this particular project, the total difference is approximately 960 pixels side-to-side and approximately 640 pixels top-to-bottom. That's a fair amount of resolution to sacrifice right from the beginning.
Notice the overlapped edges are not present in the reversed-order stack. And you will also notice the reversed-order stack shows an apparent larger image, as if there is greater magnification in the second image. The apparent difference in magnification is also a result of the stacking order. So by looking at these comparisons it seems that reversed-order stacking has a clear advantage so why not reverse the order of images for all focus-stacking projects? The answer depends on one very important element when capturing the frames to begin with, and that involves the technique you use for focusing your lens.
For anyone who doesn’t understand focusing prime (non-telephoto) lenses like a macro lens: The main lens used in this project is an 85mm macro lens. The focal length is 85mm, period. The focal length never changes. However, when the focus is adjusted, there are glass elements that move inside the lens. The glass elements move in one direction to focus on things further away from the camera, and move in the opposite direction to focus on things closer to the camera. So while the focal length is always 85mm, there is a perceptible difference in image magnification when the glass elements move in one direction or the other during focusing.
Figure C at left shows the extremes in focusing that I used in this project. In this comparison the image on the left is the first frame captured which is focused on a point that I decided would be the deepest zone of sharp focus. The image on the right is focused on the closest foreground element. You can see the same apparent difference in magnification as with Figure A above.
Back to the topic at hand which is choosing the order to stack your images. As I mentioned before, it just depends on the technique you use for focusing your lens to begin with. During capture you have to adjust lens focus in one of two ways: Either front-to-back, which means you focus on the foreground first and progressively refocus as you work toward the background. Or, back-to-front, which means you focus on the background first and progressively refocus as you work toward the foreground. As a matter of habit I typically focus front-to-back. There's no particular reason for me doing that - it's just the habit I got into when I started dabbling with focus stacking in late 2012. But for this project I decided to work from back-to-front. For whatever reason I could see things better in that order for this particular subject. When I first imported the files into Zerene Stacker I used the default stack order. After seeing all of the overlapping frame edges I remembered that I captured the frames in a 'reversed order' than I typically would, so I reversed the order and allowed Zerene to re-stack the images.
After seeing the results of the reversed-stack composite I then understood why Zerene would offer the option for reversing the order of the image stack. The option is definitely accommodating to whichever direction you choose to focus your lens. If you're using a different program for focus stacking I recommend looking into a way to achieve similar results within that program.
About that heavy vignette: The main lens used here is a Nikon DX lens shot on an Nikon FX body (In Nikon-ease, DX lenses are designed to be used with 1.5x ‘crop’ image sensors and FX denotes a full-frame image sensor). The vignetted area is the difference between the sizes of the full-frame image sensor versus the footprint of the DX lens being used. The camera can auto-crop the full-frame area to fit the DX lens footprint if the Auto-DX mode is enabled and produces 4800 x 3200-pixel images. For this project I disabled the Auto-DX mode to see if I could increase the resolution a bit with my own crop, which I was successful in doing. I was able to scrape out 5823 x 3734 pixels for this one so it does show some nice detail at full size!
And finally, here's the full-size final image from this project. Hover over the image for all the juicy details.
I hope you enjoy the image and until next time, Happy Stacking!
Good day, Richard. Where the edges of the overlapping frames are concerned, the behavior is the same when stacking the frames regardless if the camera remains stationary and you only adjust the focus ring, or if you leave the focus ring alone and slide the entire camera/lens. All of my macro work since April, 2014 (except a couple recent spiders in the outdoors) comes from sliding the camera/lens together. There are a lot of fancy and expensive focus rails out there but I've adapted a DIY table-top sliding device that allows me to adjust X and Y linearly to as little as 0.02mm increments.
There is actually a third and fourth way to focus if I understood you correctly, and those are to use a focusing rail rather than changing the focus the lens from a fixed position, move the camera and lens so the plane of focus moves through the object. Obviously this can be done in both directions.
Do you know what the impact of that approach would be?
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