Field Photography for Post Process Focus-Stacking
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Loc: Orangevale, California
Hiking, while shooting wildlife, is always a constant challenge. I am sometimes stalking, other times sticking myself into the solitude of a photo blind while the rest of the world works around me. I live for this kind of adventure, whether it be out in the desert capturing deer mice in an abandoned mine shaft, calling coyotes across the winter landscape of sage, or just working with some wildflowers on a hillside, basking in the late afternoon sun. It is all the same to me.
When it comes to outdoor shooting, there is a particular venue of shooting that incorporates thinking and frustration. But if executed properly, it will combine exactness and accomplishment, blossoming into a stunningly sharp image. Exploration into the world up close is that venue. Close-up and macro hunting are in whole different realms of their own. Close examination of a specific location can yield many opportunities to the patient photographer. One of the ways to capture stunning imagery of this world is to focus-stack images.
This article is targeted for those who have some knowledge of the camera/lens workings, but I always attempt to back-peddle a bit in hope that others will understand the problems and possible solutions. I always encourage you to use any or all of the information I provide and to execute your own tests as well. You may find something that works better for you in the field.
When we set up a shot, and this is any shot, Depth of Field (DoF) is one of the ingredients we should always consider when time permits. I have had opportunities when events are happening too rapidly to muck around with various apertures (f/stops) and shutter durations. I have a couple of ideal apertures that I use for these situations, and they seem to work in a pinch. But ultimately, the choice of a specific f/stop will provide you with a very specific DoF. Each focal length, aperture, and chosen distance from subject, will be individualized for that subject.
Let me provide an example. Let us say the sample flower in the illustration in Figure #1 is shot at f/2.8. The DoF is very shallow, meaning the plane on which I focused is very sharp, but narrow, resulting in the space in front, and space in back of the focus plane, begin to get blurry, away from the focus plane. You can see by the yellow area representing the focus and resolvable area. Not too difficult to visualize. The top diagram illustrates the focus spot with the lens set at f/2.8, which is delineated by the blue line. The yellow in front and in back of the subject would be the DoF. The farther the distance from the focus spot, the lighter the yellow and hence the lack of focus represented in the final image.
If I stop down to f/11 (Figure #2), the plane on which I focused is still the sharpest location, but the space in front, and in back, is more RESOLVED (deeper focus). Point to emphasize here is that the increased area is more resolvable, not in focus. The specific plane at which the lens is focused will always be the sharpest part. This is important to remember. Fig 2 illustrates what that DoF might be at f/11.
If I were photographing an insect, I could stop down to include all of the insect in the DoF, but I would still focus on the eyes. Why? I do this because that is usually the most important part of the image. If I focused in the center of the insect, the head and eyes would be in DoF and reasonably resolvable, but they wouldn't be as sharp. See example below.
In the first image, you can see I have reasonable focus on most of the stick insect, but the truly sharpest portion is located in the head portion of the image. That point at which I focused.
Image stacking, on the other hand, provides a whole new perception to your imagery. The increase in Depth of Field (DoF) is often amazing in itself. You also open yourself to seeing the "whole picture". Observing the foreground and background in the same resolution is almost like viewing the images three dimensionally.
Second insect image is an example of the same basic capture using focus-stacking.
Some subjects are desirable to capture with more DoF than the single image can provide. This can be done with image stacking, where a series of images are taken at varying distances throughout the subject. Ideally the individual image DoFs overlap, so that the final image appears to be perfectly sharp throughout the entire range of the subject. In Figure #3, you can visualize how each captured layer is recording sharpness throughout the image. This is a sample illustration of a very minimal focus-stack. Notice segments where the shot is not sharp - less yellow.
The third drawing attempts to illustrate how stacking at different distances will help to provide more clarity throughout the image by stacking. As the resolution drops in front and back of the subject, you maintain a level of sharpness.
Figure #4, the most significant diagram, illustrates how, when I increase the number of images, shooting them closer together, the overall image will be much sharper.
So that is the "nitty gritty" why I use stacking in some situations. And what are the considerations, difficulties, and solutions involved with these captures? First of all, the captures involve several, to sometimes a dozen or more shots combine into one final image. This process takes time. Subjects can move about of their own accord, or are passively moved by even the slightest breeze as in wildflowers. When this happens within the series of captures, you will have to begin the whole series over again. Now, when we factor in being in the field where the air is never still even in the best of conditions, we need to utilize techniques that help to make captures when the best timing is available.
First of all, air movement is probably the most difficult thing to deal with. How can we reduce the effect of air movement? A simple starter is "the closer to the ground you can get, the better". Many wildflowers present themselves at the end of a long thin stalk. These move with the slightest breeze. So bring along a closed cell foam pad to sit or lay on while shooting. Heck, it is even great to use in catching up on your sleep with a mid-day nap. The pads that I have interlock together. It is sometimes helpful to use a couple of the pads to block the breeze while performing a capture series. (see below).
The main task with stacking involves keeping things still. I use a small tripod because I am usually attempting to be as close to the ground as possible. I also have a small softbox that folds up into my pack. This not only serves to block the wind, but also softens light during midday which is usually not the best quality anyway. So it is helpful for lighting and blocking the wind. I have cut out the bottom of the box so that I can set it over the subject somewhat like shooting inside a tent. The frame of the structure is spring steel, so it folds right up into a small disk for my pack. (see above)
Here is my shooting technique: First, I want to keep my camera as steady as possible. Second I want to time the movement of my subject. This will enable me to capture as many images for the eventual focus-stack as possible, during the least amount of movement. "Sit and Watch Your Subject". If it is an insect, learn some of the behavior patterns. Some species of invertebrates freeze if you blow a slight puff of air at them. This is your time to shoot. Others will panic and flee immediately. Take time to study these reactions.
Placing the organism on a new substrate,(rock, stick, leaf), may cause them to stop and analyse their situation before actually proceeding. Some will react just the opposite. Know your subject.
If you are lucky, you have found a subject that is maybe basking in the morning light. Which, by the way is an excellent time to search for live specimens, not quite ready to move around for the day yet. Or you have noticed that by late afternoon, the wind is at a minimum for the location you have chosen to shoot your wildflowers. But regardless of the why-for, you have a subject ready to shoot.
Set the camera in just the proper angle you desire for the shot. Look at the background and make any necessary changes so that there isn't any background distractions. Focus on the tip of the closest object you desire in the final image, and then turn the focus ring just a bit closer. You will be beginning the capture series just in front of the closest part. The reason for this is if your subject should move just a tiny bit, it will be included within the capture series.
Now here is where you can save the time that you normally don't have with regular stacking. I try to use 1/350-sec at f/11. I will adjust the ISO until the proper exposure is achieved. With camera on tripod, and 1/350-sec, you should be pretty safe with any slight movement from the subject. The image of the beetle and nightshade was actually taken hand-held.
I set my camera on continuous at one frame-per-second (1fps), or 2fps. This means I do not have to be pushing the button consecutively throughout the series - I just hold it down. All I have to do is move the focus ring after each exposure. You will be able to work out what fps is within your comfort zone. When you approach the end of the capture series, take one or two extra for the same reason you did at the beginning of the capture. Shoot just beyond the desired capture range. It is like taking an extra shot at the ends of a panorama, you can always delete what you don't need in the final processing.
So here is a recap on the process. You will hold the shutter release button down, click,click, click, click, until you capture it all. Moving the focus ring, only between shutter releases. Do not change focus during the release times. That is why you will have to find the number of fps that suites your ability to change focus. Once you have practiced this technique, you will cut the time for stacking down by a large factor. Time is essential for stacking. Many things can, and often do, happen during a series. Watch carefully as you are focusing to catch any stray movements of the subjects during the exposure series. If some parts move while they are out of focus, the software will not use them in the final, so don't worry about it. If you have passed by the insect or other subject, and then it moves a bit when it is very blurry, the software will only use the sharp images. If all else fails for that stack, you can always shoot another series.
Now, when I am shooting the series, I will also make a conscious effort to use smaller incremental movements in visually important spots - like through the head and eyes, legs, etc. For the reason illustrated in Fig 4. It also saves time you might lose shooting through areas with no subject.
You are now ready to capture images for stacking. There are several software products on the market for processing the final image, and they can vary a lot in pricing. I use Zerene Stacker. They have a trial version you can download to see if it will work for you. This particular blog does not address using the software, but most of the companies have video tutorials for such occasions.
I hope you have the time to try field stacking. It is a slow task, but can be very rewarding as well. Watch your timing for stillness, capture the images in a timely manner, and have fun with the processing later, and beyond all else, happy shooting.
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