Focus stacking put simply is taking a number of pictures of the subject, while moving the plane of focus for each shot but changing the framing of the shot as little as possible. Then software is used to combine the in focus sections of the images you shot to obtain a composite image with more of the subject in focus than you could with a single shot.
Why not just stop down the aperture?
You soon realise this is not an option when you start to shoot at extreme magnification - at 10:1 and more, even stopping down to F22 will not generally get you enough DOF for more than a very flat subject - three dimensional subjects like small insect faces are too deep for one shot to get everything in focus.
The main reason though is that at proper macro magnification (1:1 and beyond) stopping down a lens will soon start to cause diffraction. The more magnification, the sooner this starts to degrade your photographs.
Here's an example, even resized down for web, the diffraction blur is obvious in this shot of a tortoise beetle, shot at F11 and around 3-4:1 magnification. I'm not talking about the lower part of the image where the rear of the subject is going out of focus, that is simply demonstating that F11 does not give enough DOF to get the whole subject in focus, I'm talking about the general resolution of the in focus section of the image.
If the loss of resolution isn't obvious in that shot alone, compare the detail to the following stack of the same bug (admittedly the other side of it), a composite of 47 images shot at f4.0;
The reason diffraction starts to become a problem is that the aperture you set on the lens actually appears to become smaller from the camera's point of view as you add extension between the camera and the lens. This is known as the effective aperture.
The formula is:
Effective Aperture = Marked Aperture x (Focal Length + Total Extension) / Focal Length
As an example, with a 20mm bellows lens and 200mm of bellows extension for 10:1 magnification, the lens is set to f8.0 then:
Effective Aperture = 8 x (20 + 200) / 20 = f88
An effective aperture of f88 is very small, and is enough to cause diffraction on any modern camera (well any camera with less than medium-large format sized film/sensor.)
If the lens can open up to say f2.0 then the numbers are quite different:-
Effective Aperture = 2 x (20 + 200) / 20 = f22.
An aperture of f22 is still just into diffraction territory on smaller sensor DSLRs like my Olympus, and APS sized sensors, but only to a small degree. This lens would be quite usable for stacking on a DSLR at f2.0 but at f8.0 will produce a seriously degraded image. (At this extension/magnification).
Okay that sounds interesting, so how do I do it??
Well, there are two ways of doing this - Method 1 in the field with live bugs; you can try to get a small number of shots closely framed, the first with the eyes of the bug in focus, then move forward by 1-2mm and grab another shot with the wings/torso in focus. In my experience, 2 or 3 shot stacks of wild bugs can be done but longer stacks rarely work well. Here is a short tutorial from the master of hand held focus stacking.
Method 2 is the way I make my bug portraits. It involves killing the bugs and shooting them in an indoor studio, with completely controlled lighting, and a mechanism to shift the camera or the subject on an extremely small scale to adjust focus. This latter method is what I will focus on in these articles.
Here is an example 2 frame animated gif showing a focus stacked shot of a tiny insect (a very small gnat, around 2mm long) compared to a single slice from the stack. Shot with a Nikon 10x microscope objective with a fixed aperture, roughly equivalent to a 15mm/f1.6 camera lens*. The full stack is 103 images deep.
Note I picked the slice from the stack which had the front of the eye in focus to make the slice look as good as possible!
* Microscope objectives have apertures markes as NA (Numeric Aperture). The way to work out the equivalent f-stop aperture for an objective marked in this way is F-stop = 1/(2xNA). So for an objective with an NA of 0.3, F = 1/(2x0.3) = 1/0.6 = 1.667, so f1.6/1.7
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