After having spent some time reading about DOF and the derivation of its formula, I tried calculating the theoretical DOF of different lenses. Now to my knowledge, a camera using a lens with a focal length of 100mm, stopped down to an f-stop of 16, and the same camera on a 25mm with an f-stop of 4, should produce the same DOF. I am not trying to achieve the same framing and the object distance to the camera would remain the same, which means that both DOF and perspective should be the same. However, when plugging my values into the calculator, they are not?
- Circle of confusion (c) = 0.029mm
- Object distance (o) = 5000mm
|DOF 1||DOF 2|
|f = 25||f = 100|
|N = 4||N = 16|
|DOF = 109m||DOF = 2.49m|
I also am aware that the variable of the circle of confusion can be very subjective, with all the aspects of visual acuity, viewing distances, and image enlargement. Nevertheless, as I am in theory using the same camera and assuming the output of the photo would be the same, I would think it is fair to assume that c is constant. In this case, I am using a c value of 0.029 (common full frame value from Wikipedia).
There obviously must be some confusion on my part, but after seeing and reading through Manuel Luebbers's tests (https://manuelluebbers.com/large-format-look-alexa-65-vs-alexa-mini/), and Steve Yedlins's article on matching lens blur on different format sizes, I can't see why the calculations shouldn't match up.
I also have checked that an object distance of 5m is not the Hyperfocal distance when using the 25mm lens, as for reasons I don't quite understand yet, that tends to mess the calculations up.
An explanation from an expert or anyone knowledgeable on this topic would be greatly appreciated.