Focus stacking is an important technique to master when photographing subjects that are close and you need deep depth of field. Stopping down the lens only brings limited improvements, except if you’re working with a very wide-angle lens, which isn’t often the case in closeup photography. It also brings on the risk of diffraction, which degrades the image quality. Therefore, it is better to set your lens at its “sweet spot”, the aperture value at which it performs best, and stack focus.
Now, to stack focus, you need to move the focusing distance by very small and very equal increments, so that the software you will subsequently use to combine all those photos together, retaining only the sharpest parts of each, will have an easier job and not force you to make many retouchings by hand. That can of course be achieved most easily if your camera has a built-in function that allows you to program what you want, click a button and let the camera do the work while you enjoy a drink and a good book —a bit like interval photography. However, when you do that, the risk is that, as the camera automatically turns that focusing ring to focus on points farther and farther away, a phenomenon known as “focus breathing” appears. I will not expand on this as it is a complex subject and those of you who are interested can read all about it online; I will simply say that it is another issue that will make the post-production stitching less reliable and precise.
In order to avoid this, the best way I know of is to manually adjust the position of the entire camera + lens group to make it all move closer and closer (or the other way, of course) to the subject without touching the focusing ring. In order to do that, especially when you’re clumsy like me, you need a very good focusing rail that will allow very smooth action through high-quality gearing. The rail also needs to have precise markings so that you will see exactly how much you’re moving it towards the subject, or away from it, every time you make an adjustment. The smaller the adjustment, the better for the quality of the subsequent stitching, but you also don’t want to spend
entirely too much time on this, nor end up with a stack of 200 photos that will be a nightmare to process... It is a domain in which everyone has to make judgment calls based on personal experience and the subject at hand.
The photo below is my first try at stacking focus manually. I used a general photography lens, an 85mm, to which I added an extension tube to allow it to reach 1:1, or possibly even a little beyond that. I took 20 focus-stacked exposures, using the Novoflex focusing rail that appears on the behind-the-scenes snaps below. Those explain everything so I will not dwell on the technical aspects any more. Let me just say that I found manual focus stacking not so difficult, even enjoyable. It is of course, by nature, a repetitive and thus boring process, but doing 20 photos with one hand on the knob of the focusing rail and the other on the radio trigger to actuate the shutter really didn’t take up too much time. It is the pre-start thinking that was more difficult: the “How many photos do I need?” part is easily answered by looking at the actual results on the back screen of the camera (or, even better, a much larger computer screen if you’re shooting tethered), but the “How much should each focusing increment be?” is a lot more difficult. I followed what had worked for me previously when using the auto function on the camera, and the result was apparently OK for Helicon Focus, which did a good job, as usual.