Focus Stacking in Macro Photography
In the world of photography we often work hard to obtain a shallow depth of field. When we take portraits and wish to separate the subject from the background, we use bulky lenses with large apertures just to get that magical 3D effect we're striving for. In the world of macro photography, as demonstrated in a previous article, things are entirely different.
As I’ve mentioned before, depth of field (DOF) depends almost entirely on two factors: aperture value and magnification. The wider the aperture we shoot at, and the closer we get to the subject, the shallower the depth of field becomes. When doing macro work, we often shoot at 1:1 magnification or more, compelling us to be extremely close to the subject. This inevitably means that depth of field is extremely shallow - so shallow that in many cases, most of the subject goes out of focus, even if it’s as tiny as a fly, and even if we close the aperture to f/16 or more.
|This robber fly was shot at f/9, a medium aperture setting, and it’s not
even close to being entirely in focus. Canon EOS 7D, Sigma 150mm
f/2.8 macro, 1.3 second exposure at ISO 200, f/9.
This phenomenon simply results from the rules of optics, and can’t be solved conventionally unless we close the aperture so much that it will critically hurt image quality (and sometime even that doesn’t suffice). Yet it turns out that if we're willing to put in a little more effort and work carefully, we could take macro pictures at any magnification, with close-to-optimal apertures guaranteeing high quality and still get our desired depth of field – all by using a method called focus stacking.
|The same fly, focus stacked from 8 different images, all 1.3 sec
exposures at f/9. ISO 200.
Focus stacking is a process that involves two tasks. The first task is to take a series of pictures at different focal distances, such that the entire depth range we want to have focused is covered by the series. For example, say we’re shooting a fly from the front. We could take one picture where the fly’s head is in focus, one with its thorax (middle body-segment) in focus and one with its abdomen in focus.
This may sound easy, but when shooting a live subject in nature (and moreover, as I usually do it, in natural light and extreme magnifications), there are a number of things that can go wrong. For example, the lighting may change if the clouds move, or the subject might decide it doesn’t feel like staying put, move and destroy this sensitive process. You must remember that it’s critical to get all the images in a stacking series at the exact same conditions and parameters: aperture, ISO, shutter speed and white balance.
This might seem obvious but when the light changes, auto WB might shift and shutter speed could change, altering the images to be stacked, which could result in a strange outcome. I recommend shooting in the shade as during periods of as little wind as possible, to get the consistency needed to produce a good stack.
|A dragonfly, final image stacked from 8 shots, all at 1/50sec, ISO 100, f/5.6 .
Canon EOS 40D, Tamron 180mm f/3.5 macro, Rishon Lake, Israel.
I am a nature photographer, and I only shoot wildlife in the field and not in studios. There are studio-stacking artists out there who produce stacks from hundreds of images, but to do this you have to use some kind of precision rig, as well as studio lights and probably a stone-dead subject, and that’s just not what I personally do.