Depth of Field: Long focals versus tele lenses

[Paul Schilliger]

Would the theory confirm my observation that a long lens has more DOF than a tel e lens of same power at same working aperture? My observation was with lenses of different focals so I'm not quite sure (450 long versus 600T). In other words, does the DOF depend of the lens design or of the angle of the lens? (In my compa rison shots, the crop in the 450 shot corresponding to the 600 frame has MUCH mo re DOF than in the image from the 600T at same f-stop. Has someone tested lenses of same power for DOF?

[Pete Andrews]

Hmm, interesting question.My instinctive reaction is "of course there's no difference", but then all the formulae for DOF make the assumption that an approximation to a simple lens is 'near enough', and that the back focus and the EFL are one and the same.

I don't remember seeing any references to DOF changing with the telephoto ratio, so I'll have to draw myself a few diagrams and do a few calculations before jumping to any conclusion about this one.DoF is the result of changes in subject distance causing the focal plane to shift, played off against depth of focus. If the projection angle of a lens is different from the theoretical angle relating to its EFL and aperture, then one might expect the depth of focus, and hence the depth of field, to differ as well.The implications for a zoom lens with a fixed back focus are interesting as well.I'll get back to you.

[Geoffrey_1456]

Paul,

I believe your intuition is correct. I did a similar comparison between an Apo-germinar 600 and a Nikkor 600T. Same subject, same aperture (F32), same film (Provia F). The results showed that the Apo-Germinar produced more DOF, from center to corners, than the Nikkor T. So I also reasoned whether a plasmat type lens would give more DOF than a tele type lens. In theory, if all assumptions hold true, as Pete said "of course there's no difference". However, in practice, the assumption(s) for a plasmat type lens might be slightly different from ones for a tele type, and maybe that's all the diference there is. The difference is munite, but noticeable. A dear friend told me that I should look into one of those SPIE series optics books to find math evidence. Unfortunately, I don't have the books, and probably wouldn't understand the math any more....

Pete, we are looking up to you for a confirmation.

PS How does lens quality contribute to DOF, if there is any? And in theory, how is this evaluated or assumed?

[Duane]

I have a cunning theory about this; perhaps Pete can back it up.

A telephoto lens in canonical form is a lens group with the desired focal length followed by another lens group to shorten the back focal distance and thus shorten the overall lens length. (Here's a link to a page with a diagram of a 500 mm tele lens: http://sr5.xoom.com/nathandayton/page25.html )

I'll assume the 1st lens group is on the left and the 2nd is on the right. You'll also have to menatlly add the light rays from a typical object coming to a focus on the right.

Removing the 2nd group for a moment, the light rays from the object pass thru the 1st element and converge in a cone with the apex at the focal plane. The angle of this cone determines the depth of field. A narrower light cone with a smaller angle corresponds to the lens being stopped down. This makes sense because as you move the object the focal plane moves and with a smaller aperture the light cone is narrower and therefore can tolerate more movement in the object for acceptably sharp focus. (Words fail me; does everybody follow this?)

Conversely, with a greater aperture, the cone has a greater angle and the object has a much smaller range of movement before it is out of acceptable focus.

OK, now add the 2nd lens group. To shorten the focal length the 2nd group causes the light rays to focus to a plane closer to the 1st lens. To do this, the angle of the light cone must increase. This then causes the depth of field to decrease because a smaller change in the position of the object is needed to cause the image to be out of focus.

I'd imagine that this effect would be much more pronounced at large apertures as the light cone angles would be greater.

Hmmm, given that a retrofocus lens as used on an SLR camera has a back focus longer than the focal length of the lens will it also have a greater depth of field than a standard lens?

Cheers,

Duane

[David A. Goldfarb]

I suspect that things work as they really should (DOF for a given aperture and a given focal length is the same for a given circle of confusion), but that true long focals are likely to be better corrected than teles, so they appear to have more depth of field.

The original comparison isn't really fair, because it is the focal length and aperture that determines the DOF, not the angle of view on film. That is why photos made with Minox cameras and consumer digital cameras have so much DOF--the normal lens is around 15mm.

[Cesar Barreto]

Paul, There are some points to be positive about and one more question to add. Of course, a 450mm lens will show better DOF. And lenses design can make some indirect influence over perceived DOF, as the comparision within sharp and unsharpness can be confusing if the sharpness limits also varies. A bad lens may probably show great apparent DOF, as nowhere you'll find a sharp image to compare. Now, if telephoto design can change anything, I'd suggest (avoiding all the math involved) it may only deals with depth of focus. I guess Mr. Merklinger would be rather welcome here!

Cesar B.

[Geoffrey_1456]

Armin,

You are absolutely right. DOF, for a given lens, depends on its focal length, aperture used, and the distance between the lens and the object. This is simply known as object-based depth of field (ODOF). I was under the impression that Paul was comparing the two shots made by two different lenses on film, like what I did, so we are actually looking at a different type of DOF, or it's commonly referred to as image-based depth of field (IDOF). IDOF is a bit more complicated than ODOF. Let's agree on that DOF, in general, is a zone of acceptable definition. On film, the limit of such a definition is the distinguishable distance between two tiny dots. We all know that, besides a lens' ODOF and Scheimpflug principle, its sharpness/resolution, contrast, and the film's resolution all contribute to a given IDOF. Since I used the same film, almost same focal lengths, same aperture, processing by the same lab and in the same developer, the variables become lens resolution and contrast for the least. I have not conducted any tests regarding their resolution and contrast, so I can not conclude that the comparison was valid at large. Since you know the Apo Germinar lens is sharper, and I know the Apo Germinar lens is contrastier (my particular example), then the apparent better DOF (IDOF) of Apo Germinar, I observed, over Nikkor T is still true. Furthermore, it's still reasonable to ask the question whether a plasmat type lens will have a better IDOF over a tele type lens, and whether there is any math model(s) to support such a claim. Hope Chris and Kerry would have some test data to show that a plasmat lens in the 600 mm range is generally sharper and contrastier than a tele type lens in the similar focal length range. Cheers,

Duane,

I'm trying to figure out what you are trying to tell me. Frankly, I have a difficulty following it. But don't worry, it's probably my fault. I believe it's a sign of aging :-) I need to study it more carefully.

[Geoffrey_1456]

After I presses the "submit" key, I found two more posts. Thanks Cesar. That's the name I tried so hard to recall. Yes, Dr. Harold M Merklinger. Did he write a book about DOF for view camera?

[Pete Andrews]

Well, I've had a good ponder and a sleep on this, and, from a purely geometric and theoretical point of view, I think my first instinct was correct. There should be no difference.The projection cone of both a long focus and a telephoto lens with the same EFL, at the same aperture, should be exactly the same. In fact the angle of the cone is dictated purely by the numerical aperture of any lens.It's well known, however, that spherical and chromatic aberrations, and astigmatism, can increase the apparent DOF, by 'stretching' the plane of focus, and this must be what you're seeing. If the effect is due to spherical aberration alone, then the difference between the two lens should diminish as the apertures are stopped down.

This effect shouldn't be confused with what happens in SLR camera lenses.In many zoom and all internal focusing lenses, the focus is changed by shortening the EFL. This naturally increases the DOF at closer focusing distances over what would be expected from the marked focal length of the lens.I'm not saying that this is the full story, all I'm saying is that DOF calculations, which make the assumption of a perfect thin lens, can't be used to explain any difference between a telephoto and a long focus lens.

[Ken Burns]

Geoffrey:

Yes, Merklinger wrote two books, "The Ins and Outs of Focus" and "Focusing the View Camera". He also has an interesting website at http://fox.nstn.ca/~hmmerk/.