Originally posted by jogiba Here is what you posted :
"you do understand that an F -stop is a measure of light transmission and doesn't directly relate to DoF don't you?
You have three camera setting choices :
shutter speed
iso
aperture
Take a guess what setting is directly related to DOF.
F stop DOF example Take a guess.
YOu need to buck up on your basic terminology.
Start with what is exposure.
Exposure is the combination of ISO (sensitivity) Shutter Speed and Aperture.
Any definition of Aperture that ignores the absolute irrefutable fact that it is an element of exposure is misleading.
Any definition of Aperture that would lead to confusion in exposure is a misuse of the term.
F 2.8 on APS-c is F 2.8 on any format. Get used to it.
Once you've got that under control... the rest will be easy.
As discussed else where you can argue rough equivalency in terms of field of view for lenses, but it doesn't really exist. There are characteristics of a 35 mm lens, that are not changed by what size sensor is behind it, and those characteristics are formed by many factors. The number of elements, the shape of the elements etc. If you think I might be in error, try and take a picture with a simple lens. Or better yet, get a lens from one of those old cheap throw away cameras. The parabolic lenses in those will totally destroy your concept of the link between F-stop and DoF forever. So while for the purposes of understanding the relationship between DoF and Aperture may be a great teaching tool, there are so many elements in lens design it doesn't address, it's not practical as more than a teaching concept. When I see a DoF chart that includes the number, shape, and composition of the elements for a given lens, I might pay a bit of attention to it. It doesn't matter how your graph is linked. When you use such an imprecise measure as describing one of your units with a simple lens, it's pretty meaningless, except in a conceptual way.
If you want to talk about DoF fine, measure it. It's measured in inches or mm. It's a distance.
If you want to talk about Aperture, think of it a a level of illumiation that's consistent across all systems.
If you want to think about different lenses, think about distortion, CA, vignetting etc.
Trying to bundle those three things, each of which come with it's own variables, and is an area of design unto itself, reducing them to their simplest form, and combining them into one concept is a recipe for mis-understanding.