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 -

My example is for the same lens, ie: the only absolute actions you can take with a given lens to affect DOF is move relative to the subject or to change your aperture. That said....

To my knowledge:
I believe some or all of your comments about angle of view are incorrect. Sensor size doesn’t affect DOF, at all. It might cause you to move relative to your subject to achieve the same framing as a full frame camera, but the action being taken here is moving relative to subject. You can easily test this with a full frame camera: put camera on tripod, focus, take photo. Now change nothing and put camera into DX or Super 35 mode take photo. DOF is identical, framing changes as the edges are cropped. Put a crop body on the same lens, change nothing else, DOF is the same. Fun experiment to try at home!

A speed booster/teleconverter doesn’t affect the DOF of the lens it is put on, to my knowledge, unless it includes an aperture stop that sets up a new (smaller) entrance and exit pupil. The only way it can alter the depth of field would seem to be by altering the circle of least confusion through degradation of focus. If the entrance pupil (virtual image of aperture stop on object side of lens) stays the same and the distance to subject doesn’t change then DOF is the same. Don’t take my word for it, try this yourself! I just did.

Focal length doesn’t affect DOF if the entrance pupil remains the same diameter, all other things being equal. This doesn’t mean the f-number. For example, with a 14mm F2.8 and a 24mm F2.8 lens: A 14mm F2.8 has an entrance pupil of 5 at F2.8. To maintain this entrance pupil at 24mm requires an f-number of 4.8. Comparing a DOF at 14mm F2.8 and 24mm F2.8 is comparing an entrance pupil of 5 to 8.57. The DOF changes not because of the focal length, but because of the entrance pupil. Any focal length lens at the same absolute distance from a subject, with the same entrance pupil size, will have the same DOF (ignoring individual lens imperfections). The active variable being changed is the pupil diameter, the focal length changes the field of view but this doesn’t meaningfully affect DOF, unless you move relative to the subject.

Everything you mentioned seems to be changing the DOF by causing a change in distance to the subject or a change to the size of the pupil. So it’s not actually the focal length or extender/booster doing the work.
I’m always happy to be proven wrong, so by all means point me to some sources and I’ll take a look

Pixel Pooper wrote:

Darren James wrote:

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.

Not quite. Depth of field depends on entrance pupil diameter, focus distance, and angle of view.

If you keep the same entrance pupil diameter and focus distance, anything that widens your angle of view will make your DOF deeper, and anything that narrows your angle of view will make your DOF shallower.

If you use a shorter focal length, larger sensor (or crop area), or a speed booster you will get a wider angle of view and your DOF will be deeper.

If you use a longer focal length, smaller sensor (or crop area), or a teleconverter you will get a narrower field of view and your DOF will be shallower.

This of course assumes that the viewing conditions are equal, so the images are displayed at the same size regardless of crop or sensor size.

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