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

Started 7 months ago | Discussions thread
Re: Help me calculate/understand the physics of aperture, DOF and BG blur -

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.

Indeed I miss-used depth of focus. Even as a film shooter I have trouble keeping those terms straight.

bclaff wrote:

Darren James wrote:

...

Here is what I know (note, I didn’t bother to read the whole thread, apologies if this is redundant). I don’t know all the exact equations behind this ...

Indeed, depth of focus is due almost entirely to entrance pupil diameter and distance to subject. Not 100%, but most of the time it can be generalized to these two metrics with only a small rounding error.

[Bold above is mine for emphasis.]

Perhaps you meant Depth Of Field as opposed to Depth Of Focus; they are different things.

And ... it's the exit pupil not the entrance pupil; although most assume pupil magnification of unity where entrance pupil is the same size as the exit pupil and this helps perpetuate the idea it's the entrance pupil.

Ans, FWIW, it's focal length and distance to front focal point or magnification; take your pick.

...

Optics are pretty neat!

Agreed. You may find the PhotonsToPhotos Optics Primer interesting in particular the article on Depth of Field although reading the primer sequentially is ideal.

Darren James's gear list:Darren James's gear list
Nikon Coolpix S33 Canon EOS 5D Mark III Canon EOS 6D Nikon 1 V2 Canon EOS 70D +12 more
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