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 -

If I understand correctly:

You’re saying if I take my D850 and any given lens, and stick it on a tripod and take a photo, then stick my D500 on the same lens changing nothing else, absolutely every factor the same, that the Depth Of Field changes?

I hope that’s not what you’re saying because that is completely incorrect. Wildly incorrect. A crop sensor is a sensor that is smaller, that’s it. No action is taken on the focus of the light by the detector. The only change resulting from replacing a large detector with a smaller one in the same plane is that you capture a crop of the image. Like, if you were to crop in post. Go, try it for yourself. I did.

Also, a teleconverter does not change DOF. I made sure to run a physical test before replying.

Are you... maybe confusing Depth of Field with Field of View? Or, are you comparing a crop image directly to a full frame image and taking measurements 1:1, without adjusting for the fact that the crop image is just a crop of the full frame image, and needs to be scaled accordingly? 🤨

Cropping:

I focus a lens, there is a range in front and behind the actual focal point that is in acceptable focus. I take a photo. I print that photo. I cut the edges off to reduce it by, say, 1.5x
... the range of acceptable focus in front of and behind the focal point doesn’t change.

I’m really not trying to be rude here, super really honestly, but since you keep arguing things that are easily testable and so very verifiably false I’m gonna have to just... move on.

Pixel Pooper wrote:

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.

Darren James wrote:

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.

For your statement to be true you would have to have the same focal length and the same sensor size so that the angle of view would be fixed.

Any time we have the same angle of view, the same focus distance, and the same pupil diameter we will have the same DOF, but changing any one of those factors will change the DOF.

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!

If you change nothing but sensor size the DOF does change. This is because we compare DOF at the same display size, so the image from the smaller sensor is enlarged more, which enlarges the blur circles and reduces the DOF. This is why the COC value scales with sensor size.

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.

A teleconverter makes the focal length longer and if you keep the same entrance pupil diameter the f-number becomes smaller. It also reduces the angle of view which changes the DOF.

Focal length doesn’t affect DOF if the entrance pupil remains the same diameter, all other things being equal.

If you change only the focal length you cannot keep all other things equal because the angle of view will change and the DOF will change. If you maintain the angle of view by using a smaller sensor or cropping, then you will get the same DOF (all else 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.

Everything I mentioned was with the same subject distance and pupil size.

I’m always happy to be proven wrong, so by all means point me to some sources and I’ll take a look

You can confirm what I said with a DOF calculator. See the screenshots below from dofmaster.com

These all use the same entrance pupil diameter (12.5mm) and the same distance to subject (10ft) yet the DOF changes when the angle of view changes.

Shot #1 is a FF sensor with a 50mm lens at f/4 which gives us a DOF of 2.94ft.

Shot #2 shows that if we change the sensor size to APS-C and change nothing else, the DOF changes to 1.94ft.

Shot #3 uses a 35mm focal length at f/2.8 which is the same as using a .71x speed booster. This gives us a wider angle of view and the DOF changes to 4.36ft

Shot #4 uses a 70mm lens at f/5.6 which is the same as using a 1.4x teleconverter. This gives us a narrower angle of view and the DOF changes to 2.08ft.

Notice that shots #2 and #4 have very similar DOF. This is because they have the same pupil diameter, the same focus distance, and very similar angles of view. If shot #4 was at 75mm and f/6 the angle of view and pupil diameter would be the same, and so would the DOF.

You will not find any combination of parameters that gives you the same DOF with the same entrance pupil diameter and subject distance unless the angle of view is also the same.

Shot #1: FF sensor, 50mm lens, 12.5mm pupil, 10ft subject distance.

Shot #2: APS-C sensor, 50mm lens, 12.5mm pupil, 10ft subject distance.

Shot# 3: FF sensor, 35mm lens, 12.5mm pupil, 10ft subject distance.

Shot #4: FF sensor, 70mm lens, 12.5mm pupil, 10ft subject distance.

 Darren James's gear list:Darren James's gear list
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