Focus Stacking in The Field Using Free Programs on Mobile

In this post I will describe my current method for focus stacking, which can be achieved entirely on a smartphone. I have found this method to work decently well, although I will likely improve on it in the future. Any input on how to improve this process (better free programs, other techniques you've discovered, or problems with my current method) would be appreciated. I would also like to suggest setting adblockers to turn off for Focus Stacking Online-- the creators certainly deserve their ad revenue.

For the past few weeks, I have been playing around with focus stacking for ultra-macro photography with the Laowa 25mm 2.5-5x lens. While I initially sought only to find a decent, free alternative to paid programs for use on my laptop, I soon realized that I could quite easily add the constraint of mobile compatibility for the workflow I develop. This would be beneficial for field work/iNatting, since stacks can be created on the go alongside the iNat app, rather than after your opportunity to shoot another bracket has passed.

As of now, my process involves three programs:
PS express mobile app (free version)
Focus stacking online
Lightroom mobile app (free version)

Focus Stacking Online is at the core of my current method. I find that it works impressively well for a free online program, but it has a few weak points. These can be mitigated using PS express both before and after processing in FSO. Lightroom is supplemental to PS express for post-stack processing.

It should be noted that photos on iNaturalist are scientific documentation, and thus should not be altered for the purpose of being more visually appealing. This risks obscuring or misrepresenting aspects of the subject. The usage of editing I will describe in this post is entirely to preserve the genuine appearance of the subject--compensation for either a misaligned bracket, or for the free FSO software's shortcomings. Any processing, including focus stacking itself, should be disclosed in the notes of an observation. The "focus stacking" field should be applied when focus stacking is used. Including some unedited shots, perhaps at a lower magnification ratio, is also advisable, especially when an image is heavily altered.

Shooting:
To begin, I shoot a bracket (typically of 2-8 photos) using the continuous shooting mode on my camera. This can take a few attempts to get right, especially with extremely shallow depth of field (typically around 0.1mm in my case). I usually get the best results by bracing my left hand against a surface, pressing the shutter button, and gently increasing pressure from my right hand while allowing my left to give slightly. I have found that this is the most reliable way to smoothly shift the focus forwards and backwards without a tripod or focusing rail. I make sure there is at least one photo covering every area I intend to be in focus to avoid "focus gaps", which result in blurry bands on the final image.

Using, and "Hacking", Focus Stacking Online:
After achieving a satisfactory bracket, I download the images, import them into FSO (add>photo library), click "stack", and wait for the program to finish processing. This can take a few minutes. Simply importing and stacking often produces satisfactory images, particularly of subjects which have few or no long extremities, such as a a seashell, leaf, caterpillar or ladybug (at least when viewed from above). However, subjects with long legs, spines or antennae can "confuse" the program, especially if these extremities intersect. This is because slight shifts in perspective can produce significant compositional changes between shots, which result in a bracket that cannot be aligned properly. When this happens, parts of the image can become "doubled" or "broken", or the program may fail to stack the images entirely. This is a major issue for handheld stacking, as slight perspective shifts are unavoidable. Sometimes, these issues can be remedied by switching to the "beta" alignment method, selecting a new reference image, or by removing photos which cover the same area of focus as another. However, when all else fails, PS express can be used to eliminate these doubling and breaking effects.

Take, for example, this portrait of a woodlouse spider (The large image is a stack including the bottom three constituent photos):

This is the best I could get by only changing the reference, stacking method, and number of photos within FSO. As you can see, there are significant "ghostly" doubling artifacts produced by misalignment in the chelicerae and pedipalps. To fix this, I used the heal tool in PS express to simply eliminate the problematic areas on the constituent photos in which they were not in focus prior to stacking:

Detail comparison on the problem areas:

This produces a far better result, free from the issues in the first attempt. This method is ideal, because the usable part of the image has been in no way altered-- it simply eliminates the opportunity for "confusing" FSO. However, it may not always work, in which case the heal tool may also be used on an already stacked image to manually erase the artifacts. This is also useful for removing sensor dust, which is often doubled by the stacking process. If you're like me and tend to change lenses in less than sterile environments, you'll notice this issue immediately, and find that healing sensor dust on backgrounds becomes the most commonly needed retouch.

I use PS express mainly because it has the best free heal tool (may also be referred to as "blemish removal" in other programs) of any mobile app I've tried. PS express also has a substantial adjustment section, however having an additional program such as Lightroom is helpful for this stage. Adjusting exposure, contrast etc may be necessary to restore accuracy after processing the stack (stacked photos may come out slightly different from the originals), but this stage should be minimal and can often be skipped altogether.

Below are some examples of photos I have taken using FSO and the techniques described here.

Posted on January 21, 2024 04:22 AM by hsug1747