Help me calculate/understand the physics of aperture, DOF and BG blur -

Started 7 months ago | Discussions thread
Darren James New Member • Posts: 12
Re: Help me calculate/understand the physics of aperture, DOF and BG blur -

Uhh... I guess, I still don't understand if you actually have a question you're asking?

I mean, math is math. There aren't actually "different" ways to derive a variable, such as DOF. You can rearrange equations, include different variables, (for example, you can replace N by putting in f/D instead, and replace f with something else that equals f). You can even have what appears to be an entirely separate equation, but in the end it isn't - it's the exact same equation written a different way.

Such an equation incorporates all the different necessary variables at some level, even if they don't appear to be part of the equation.

I couldn't access your dropbox paper, and I don't know who Alan Robinson is, and I'm not sure what relevance either have. The fact is that the math behind all of this is already a known factor and can be found in textbooks on the subject. It's not a matter of opinion, and no one says whether something can be "important" or not.

If there is a valid proof that contains a certain variable then the variable is part of the equation. If DOF cannot be derived using a certain other variable then that variable doesn't contribute.

In this case the DOF equation using N, which is equal to f/D (where D is the entrance pupil) seems to be fairly simple, and the most simple equation is probably the most convenient to use. If you instead want to replace D with Exit Pupil Diameter/Pupil Magnification then you can, but I don't know why you would?

J A C S wrote:

Darren James wrote:


Can you elaborate? I am not sure what you are specifically asking.

See the paper linked earlier:

DOF+WorkingFnumber-2020-06-14.pdf (

There are several equivalent formulas there for DOF, some of them include the pupil magnification, some do not. My point is that you cannot say that the pupil magnification is an important factor or not; it is a matter of taste if you want it in your formula. You can choose various sets of independent variables for the same quantity.

I should say that I have not checked everything in that article but Alan Robinson (the author) is somebody I respect a lot.

J A C S wrote:

Darren James wrote:

Hi Bill,

Indeed, the entrance and exit pupils have different virtual diameters, which is easy to experience by looking at both ends of a lens.

Every source I have read references the entrance pupil and distance to the exact external plane in focus (I simplified this as referencing distance to subject). This seems to be a possible knowledge gap for me, as I am taking these sources at their face value which may be a mistake.

In that same spirit: Can you provide me an independent source that is not your own website for your correction about exit pupil being the important factor for depth of field as opposed to entrance pupil? I don’t mean to cause offence, but as I don’t know you or your website in any great detail I don’t have any cause to prioritize your information over any other source. I hope you understand.

How do you measure the importance of a certain factor? You can have a formula that involves it and a formula that does not, both correct. In fact, there was a link earlier with more than two formulas.

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