No, the DoF stays the same for a given focal length no matter what size the sensor is. But the smaller the sensor, the shorter the focal length (with less DoF) you need to use in order to achieve the same FoV.
Sigh. Some people just don't seem to be able to read. My comment clearly had NOTHING WHATSOEVER to do with DoF scales or CoCs. I replied to Eric T (as indicated by the quote box), in regards to the effect (or lack thereof) of sensor size on the DoF for a given focal length.
Sorry to say, but you were still completely wrong. DOF indeed does depend on the sensor size, leaving all other image parameters unchanged. Try it out yourself, for example using the Silverlight DOF calculator (
http://www.silverlight.co.uk/resources/dof_calc.html) - dial in a focal length, an aperture and a distance and watch DOF change. Just try it out yourself, but be rigorous about changing only one parameter (sensor size) and leaving the other parameters unchanged (focal length, viewing distance, aperture) as per your first posting above. Preferably don't use DOFMaster for trying this out, because DOFMaster changes the underlying CoC assumptions when you change format and hence is not rigorous enough. There is a mathematical reason for why this is how it is, but this is not the place to go into formulas; refer to the article quoted below if you're interested.
You could also take a look at the DOF scales of a medium format and a 35mm lens of the same focal length and wonder why they're so different, but you didn't want to talk about DOF scales.
🙂
DOF is not some kind of inherent property of an image - it comes about because our brain percieves things as sharp as soon as they're small enough. Hence it does not make sense to talk about DOF without talking about things like circles of confusion, viewing distances and print sizes. It seems to be surprisingly difficult to accept that DOF indeed does not correspond ty physical reality (in the sense that a picture indeed does not have one single unique DOF, no matter if you look at it in 400x600 pixels on-screen or at a 40x60" print of it), and that in-camera and on-lens DOF scales are based on solely conventions and assumptions about print sizes and enlargement rations, making DOF a highly volatile thing dependent on a lot of non-obvious parameters like the acceptable circle of confusion. This seems very counter-intuitive to many people who think that an image either is in focus or it isn't. People don't like mathematics, and they also don't like to change their mind on what seems obvious to them, and hence DOF discussions tend to go bad rather quickly.
Nevertheless, that's how it is, and any advanced photography textbook (and even the
Wikipedia article on DOF) will contain a highly mathematical section on how DOF comes about and why it is so relative.
This is my last post in this discussion, because I've seen many DOF discussions; they all tend to go pointless and angry at some point, and I'm on RFF for fun after all.
