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# Thread: DOF - OK, I'm confused!

1. Originally Posted by RalphLambrecht
Correct, but who works in actual aperture dimensions?
Perhaps we should for some caculations. For prints that are to be viewed in correct perspective, the negative's DOF and hyperfocal distance are a function of the entrance pupil. Multiplying the entrance pupil by a constant gives the hyperfocal distance. A constant of 2000 is sometimes cited as being appropriate for critical work.

The print viewing distance for correct perspective varies in proportion to the camera's focal length. The hyperfocal distance can be corrected for this by another simple calculation. Basic math that we may have rarely used since elementary school lets make these approximations in the field without charts or calculators.

The photo's subject can be an important factor in DOF, but is often neglected. A Weston seashell or nude is more about light and form than about texture, while a bare tree silhouetted against the sky demands sharp detail. Individuals' perceptions of sharpness vary widely. Charts and calculations are handy tools, but a thorough understanding of DOF lets the serious photographer develop a more effective personal approach.

2. Jim

Maybe I'm missing your point, but it's too complicated for practical use in my opinion. Talking about 'actual apertures' quickly drifts toward 'effective aperture', and who knows what the 'real' diameter of the entrance pupil really is after that. Also, who really cares about the viewing distance for correct perspective, and how do you want to enforce it?

3. Originally Posted by RalphLambrecht
Nope. Given the same f/stop, the 'normal' lens of a smaller format will always produce more DoF than the 'normal' lens of a larger format.

Did it. See post #10.
Originally Posted by RalphLambrecht
Nope. Given the same f/stop, the 'normal' lens of a smaller format will always produce more DoF than the 'normal' lens of a larger format.

Did it. See post #10.
Right..Pardon my Greek But it would be an easy test staying with one format and switching lenses but not distance from subject with constant DOF. Then like your pictures he would see the difference..

PS Nice German house you got.

4. This illustration is clear, that when you enlarge the shorter length lens to the same perspective there seems to be an DOF advantage. I have some problem with this conceptually because the circles of confusion should be enlarging in proportion and there shouldn't be an advantage. My question then is are the apertures really the same between different length lenses. Is 5.6 the same diameter on a 28mm as a 85mm lens? I always assumed they were the same.

Originally Posted by RalphLambrecht
Simply speaking: you are correct in all points.

A bit more complicated: The equation for DoF has four variables (subject distance u, focal length f, circle of confusion c, and aperture setting N). Only c is format dependent, and all but f are linear (see attached equation).

The picture comparison illustrates the increase in DoF with smaller formats.
Picture 1 shows that a 24mm lens has the same viewing angle with a small format as an 85mm lens with a larger format. Picture 2 shows that, if enlarged to the same image size, the smaller format has a DoF advantage at the same aperture setting, due to the shorter focal length.

Hope this helps.

5. Originally Posted by Saganich
... My question then is are the apertures really the same between different length lenses. Is 5.6 the same diameter on a 28mm as a 85mm lens? I always assumed they were the same.
No. The aperture setting 'N' (f/stop) is calculated as:

N = f/d

where 'f' is the focal length and 'd' is the circular opening.

Hence, in your example the opening at f/5.6 is 5 mm with a 28mm lens and 15 mm with a 85mm lens. (note the way I write f/stops, I hope you can see why)

6. Sorry for even more confusion, but that's not completely right. It's not the physical size of the aperture that goes into that calculation but the entrance pupil - the aperture as you see it when you look through the lens from the front.
These are different, because the elements in front of the iris can make the aperture look smaller (concave elements in most wide-angle lenses) or bigger (convex elements in most normal to long lenses) than the physical size of the aperture opening. It's not easy to comprehend and I only understood that fact after taking apart and rebuilding a few lenses.

Here is a (in my opinion) very good explanation of DoF with different lenses: http://silverbased.org/blur-entrance-pupil/

7. Simply speaking, an 80mm (For instance) lens has the same depth of field at any given aperture regardless of format. The longer the lens, the less depth of field. Your choice of camera format, I.E.-- medium-format or 35mm, dictates the apparent angle of view for your image frame. A smaller format like 35mm uses less of the image circle, which gives you a telephoto effect, while a larger image format, like 6X9 Centimeter for instance, utilizes more of the image circle...creating a wide-angle effect.

If you shot both film formats with the same lens at the same aperture, you would have identical depth of field in both frames.

8. It's not the physical size of the aperture that goes into that calculation but the entrance pupil - the aperture as you see it when you look through the lens from the front.
Yes, that is correct. I should have been more specific.

If you shot both film formats with the same lens at the same aperture, you would have identical depth of field in both frames.
No, since the CoC are not the same. In this case the AOV is not the same either.

9. Referring to Tom's comment:

Is that actually correct? Wouldn't different size of CoC (Circle of Confusion) on each format make some difference?? The equation suggests it does.

10. I think the real solution is to take out the gear you have and shoot pictures and look at your results. Experience will tell you what aperture to pick.
I am always mildly amused by these lengthy technical discussions.

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