This post references my last article on stacking.
When you take a photograph, whether you make the decision or the camera, there are essentially only three settings involved: aperture, shutter speed, sensor sensitivity.
how wide an aperture you use is denoted by its F stop number. Perhaps counterintuitively a small number means a big aperture (hole for the light to come through and hit the sensor). A bigger number means a smaller aperture. The important aspect is that big apertures create shallow depth of field (DoF) = how much of the image is in focus in front of and behind the chosen focus point. Small apertures give increasing DoF as the aperture narrows.
How long the aperture remains open to allow the light to hit the sensor.
is like the old ISO (or ASA) of film. Film had grain and a low ISO number meant fine grain. Sensors have digital noise and this is a very unscientific equivalent of grain.
In focus stacking the critical aspect is DoF. When I shot the flowers I wanted only the flowers in focus, not all the distractions around them. To get them in focus from front to back needed a small aperture. Probably something like F16 or F22. The problem then was that all the distractions are also sharply in focus. So how did I achieve the end result?
I selected a large aperture (F5) that gave me a shallow DoF. So when I took my picture only part of the flower could be in focus. However providing the flowers did not move I could and indeed did take a series of photographs with each frame focussed on a different part of the flower. Usually I try to start with the point furthest away and work forwards but it does not matter. In this case I took 15 frames. Each one looks odd in isolation because only a thin segment of the subject is sharp.
The next step is to import or transfer the 15 files into a software programme that will select the sharpest part of each frame. The 15 sharp portions are blended so that they form a single in-focus subject whilst leaving everything else out of focus. I used a programme called Helicon Focus. Photoshop will do it. In some cases PS is better but Helicon Focus is very, very fast compared to PS. I then saved the blended frame into Adobe Lightroom.
Shutter speed has little relevance here. I want as little grain or noise as possible so I set the ISO to as low a number as possible. I used ISO 200. The shutter speed in effect is the balancing item to give me the correct exposure. If the breeze had been blowing the flowers I might have tried a higher shutter speed and allowed the ISO to be the balancing item. More likely I would have given up.
There is no formula for how many frames you need. In a studio you might shoot 100. With something that itself has a shallow plane you might use 3 or 4. For a landscape you could use a high F number – maybe 11 or even 16 and just shoot 3 to make sure the frame is sharp from front to back. It is test and learn.
Finally I used PS to remove two tiny blemishes on one of the petals and added a vignette to help focus the viewer’s eye on the flowers. I did some additional burning (darkening) of any remaining lighter areas. The whole processing time was less than five minutes.
There are people out there who are far more expert than I and if they wish to add or correct my explanation please leave a comment.
Here is an example of a single frame – I’m not sure whether the crop is identical – it is for illustration purposes only. You can easily see that only one leaf – and everything that falls within the same plane of focus – is sharp. The software will identify that and the sharpest areas of the other 14 frames to generate the final image.