DOFocus and Lens Focal Length

[stevewillard]

I just read that DOFous and DOField improve as you stop down

which I knew.

 

However, unlike aDOField, DOFocus improves as the focal

length of the lens increases. That is, wide angle lenses have a

shallower DOFocus then Telephoto lenses. This I did not know.

Is this really true?

[dhananjay_n]

Its a little more than that. DOField exists on the subject side of the lens i.e., how much away from the plane of focus can subject matter move before the circle of confusion becomes large enough to show up as blurred? DOFocus exists on the film side of the lens i.e., asuuming you are focussed at a particular subject plane, how much can you move your rear standard before that subject plane goes out of focus.

 

When you are comparing short focal lengths (wide angles) and long focal lengths, assuming both are at the same aperture, DOFocus does get more critical with wide angles. Think of it this way. At the same aperture, the apparent size of the opening is the same. If you drew triangles from the lens to the film plane, the traingle for the wide angle will be contained in the other (i.e., only theposition of the lens changes).

 

However, in actual use, it might be worth saying this. Because long focal lengths have shallowere DOField, you will have to stop them down more to get the same DOField. Once you do this, the apparent size of the lens opening at the film plane changes. The traingles I referred to earlier will no longer share two of the edges but one of them (th elong focal length one) will be narrower. This means that the light rays from the wide angle are reaching the film plane at a steeper angle (compared to light rays from the long focal length lens) and a small movement of the film plane will result in a more substantial increase in the CoC for the wide angle. Therefore, the DOFocus will be shallower for the wide angle.

 

Hope this is clear - some things are easier to show with diagrams. Cheers, DJ

[john_cook1]

Depth of focus is inversely proportional to the SIZE of the image on the film. That is the ONLY factor which matters.

 

As you move closer to the subject, its size increases and the depth of focus decreases. (Thus the shallow focus with macro subjects.)

 

Keeping the camera and subject stationary and using a longer lens also increases the size of the image and therefore reduces the depth of focus.

 

Wide-angle lenses definitely do NOT have a shallower depth of focus, all other things being equal.

[donald_miller1]

Are two different things being addressed here? I understand DJ's explanation because when I speak of depth of focus as opposed to depth of field I mean the discrepency that is tolerated at the camera negative before the circle of confusion becomes apparent. I wonder if everyone is on the "same page" here. Probably another instance where the use of language becomes inadequate. At any rate please clarify for me.

[leonard_evens]

The short answer is that depth of focus is independent of focal length.

 

Here is the long answer.

 

Unfortunately, the term 'depth of focus' can mean different things to different people, so you can't have a sensible discussion of the matter without a careful definition.

 

The usual definition of 'depth of focus' is the following. Imagine that you are focusing on ONE SUBJECT PLANE. You are for the moment only interested in having everything in that plane in good focus. There will of course be one position of the standard on the rail for which that plane will be in exact focus. But you can ask how much variation is possible without having the focus degrade too much. That distance is called the depth of focus.

 

The exact formulas for depth of focus are as follows. Let c be the diameter of the largest acceptable circle of confusion that applies when you are focusing, i.e., the largest disc you can't distinguish from a point. That would generally depend on many factors, e.g., the fineness of the ground glass, the level of illumination, how good your eyes are, and most important the amount of magnification you use when focusing. It would be half the size for an 8 X loupe than for a 4 X loupe. Let N be the f-number, and let M be the scale of reproduction which is the ratio of image size to subject size for the plane of exact focus.

 

Then Nc(1+M)/(1 - Nc/f) and Nc(1+M)/(1 + Nc/f) give the distances along the rail on either side of the position of exact focus. However, Nc/f in all but extreme cases is so small it can be ignored. (The exception would be ultrawide lenses at very small apertures with a relatively small degree of magnification.) If you ignore that, then both of these are very close to Nc(1+M), and the total depth of field is twice that or 2Nc(1+M).

 

Note that after dropping the Nc/f term, the result doesn't depend explicitly on the focal length. Since the scale of reproduction M does depend on the focal length, it does enter implicitly. That is at the same distance from the lens, a shorter focal length lens will have a smaller value of M, and hence will have less depth of focus at the same f-stop. Be that as it may, except for closeups, the factor M is so small that it is usually ignored. That means for distant subjects, which means most situations, the depth of focus is very close to 2Nc, so it is essentially independent of focal length.

 

Finally, note that difficulty in focusing may be affected by factors other than depth of focus. For example, at the same distance from the lens, objects are smaller with a shorter focal length lens. What depth of focus relates to is issues such as how far the gg position can differ from the film position or how much the film can 'bow'. It also relates to how consistently you can focus on the same object if you do so several times in a row or what your typical focusing error might be in practice. Those are all related to depth of focus and in most situations not affected by focal length. For example, I found that for me, at about 2 X, my average focusing error can be as large as 1.5 mm, at 4 X, it is more like 0.7 mm, and at 7 X, it is less than 0.3 mm. It doesn't matter much which lens I use as long as the aperture is the same, and the numbers are consistent with the theory.

But depth of focus won't tell you what your subjective experience in focusing a short focal length lens will be when compared to a longer lens.

[leonard_evens]

Dhananjay,

 

Your analysis is qualitatively correct, but you really do have to look at it quantitatively. The formulas I gave are derived by analyzing the triangles you describe. But you can't stop there. A neglible difference may be a difference, but it is also not relevant. In practice the only time depth of focus is usually affected by focal length is in closeups, and only if the distance to the subject is fixed. However, for closeups one usually tries to keep the scale of reproduction constant. If one does that, then, except for the quibble I mentioned, depth of focus is also independent of focal length.

[dhananjay_n]

Leonard, that is a very detailed and correct answer. And yes, depth of focus is independent of focal length (as long as other variables, including magnification are kept constant). I was only trying to point out that in practical situations, a long focal length is typically stopped down more for enough depth of field and that results in the greater depth of focus. This, of course, does not mean that depth of focus is dependent on focal length - merely that we don't hold everything constant. I was trying to find an intuitive way to explain this, which I didn't do too well, I guess. Thank you for taking the trouble to spell out the math - it does make everything clearer and less confusing. Cheers, DJ