Where do you set the focus? This is a question we should always consider. How accurately do you want to focus? That is another critical question is on what basis are you adjusting the focus? Does it suit your needs and style?
The X-Pro range of cameras I use has two options for ‘Depth of Field Scale‘: Pixel basis and Film format basis. This will enable you to adjust the camera setting to suit your need and style.
Technically, the only region that is in focus is one particular plane parallel to the optical axis. All other areas will be out of focus, even when moved by 1mm. All other planes are in ‘bokeh‘; theoretically that is. The reality is that the amount of bokeh is so tiny that it appears to be sharp. You can basically ignore it. ‘Depth of Field‘ is about the plane in focus and areas in front and back of the plane that appear to be in focus (although it is defocused in theory).
The bokeh in the defocused area is referred to as ‘circle of confusion’. The ‘permissible circle of confusion‘ is the bokeh that is almost indistinguishable. Bigger the maximum permissible circle of confusion, the deeper the depth of field it gets. They are proportionally related.
The problem is this ‘permissible circle of confusion‘ changes depending on the image sensor resolution and the viewing condition.
The resolution of the image sensor is much higher than that of the silver-halide films, and the circle of confusion is therefore smaller. In addition, pixel-peeping has gotten popular, so the ‘permissible‘ circle of confusion is much more restricted—the ‘shallower‘ depth of field demands for much more accurate focus position and area. The depth of field scale on a pixel basis is optimised for such needs.
However, for some people, the depth of field becomes useful only when it is deep. Snap shooting, for example, takes advantages of the deepness of the depth of field and does not demand a more accurate and strict scale.
Go out in the street. Set the aperture to f/8. Search for the light. Find the composition. Predict the subject movement. Set the focus position based on the prediction. Do not get overly concerned about the accuracy; the ‘depth of field‘ will cover the error…. You can see that in such a style of photography, one benefits from the greater permissible area.
This is something that is inherited from the silver-halide film days. And to match this sense of feeling, we have the ‘Film format basis’. (*The value is based on the 4P print viewed at a standard distance.)
There is no correct answer. You should make the selection based on your style and needs. If the viewing size is already determined, then you can make your choice based on it. You do not have to stick to one basis either; you can always go back and forth.
For your information, the XF14mm, XF16mm, and XF23mm has a depth of field scale on the lens barrel based on the film format. If your style is to grab a shot by eye measurement or manual focus, these three lenses can help you.