Depth of Focus ... related to focal length or not?

[Erec Grim]

In View Camera Techniques, Leslie Stroebel writes "Depth of focus is not affecte d by focal length," and gives this formula: depth of focus = 2(f-number)(accepta ble circle of confusion).

In his review of the Walker XL in the July/August 2000 issue of View Camera maga zine, Roger Hicks says that the camera makes it easier to get the front and rear standards parallel. This is critical with wide angle lenses because they have less depth of focus.

This discrepancy is probably results from Hicks and Stroebel using the same term s to describe different things. Could some of you clarify this point for me? I am especially interested in the "problem," if problem there is, of alignment of front & rear standards with short lenses (75 to 120 mm).

Thanks in advance.

[Doug Paramore]

That's odd. Wide angle lenses are supposed to have more depth of field than longer lenses. Maybe I put more emphysis on how it looks than formulas. I don't know how they determine that, unless if it is for the same object size on the neg. Anyway, part of the problem with wide angle lenses is that many users of wide angle on LF do not "focus in" as they should. Wide angle lenses should be focused nearer the camera and let the depth of field take care of the distant subjects due to more curvature of field of the wide angles.

Regards,

[Bob Salomon]

Depth of field is in front of the lens from the nearest point in acceptably sharp focus to the furthest point from the lens accepted as sharp.

Depth of focus lies behind the lens and the film must lie within it in order to be sharp.

Depth of field is less with long lenses and greater with short lenses.

Depth of focus is shorter with wide lenses and deeper with long lenses.

The shorter the focal length the more s=critical the film plane position becomes.

[Bjorn Nilsson]

Hi!

Well, the real answer is that DoF is NOT related to focal length. This is easiest to describe by using pictures, but here goes anyhow. Let's start with a certain scene, say 20 feet wide. If you go back the neccessary distance to fill that with a wideangle, you have to go back e.g. 15 feet. To get the objects that in between 15 and 30 feet from you within the DoF you have to set the aperture to e.g. f/32. Now go back with the camera and mount a long focal length. See to that you fill the same 20 feet. Now you will be at e.g. 60 feet from the scene. In order to achieve the same DoF as before you have to use the same aperture as with the wide angle. The thing that is the most interesting about this is that the DoF using the longer focal length sometimes appear (note: appear) to be larger. The DoF is the same, but the wideangle lets you get closer to the subject to get the shot, that's all. Now, wideangle lenses does give a good DoF on subjects that has a small degree of enlargement. I.e. you get a lot of area covered from a short distance. But the very small distance in between the lens plane and the film plane makes the setting of this distance critical. Why is this? The circle of confusion (CoC) grows very rapidly in the film plane when you move either plane back or forth. Compare this with the relatively slow growth that you get when using a longer focal length. (The cone is as long as the distance between the front and rear standardts.) Given this, a misalignment of e.g. 1 mm (~1/32") may be fully visible with an extreme wideangle lens, while it may not be visible with a long focal length lens. As I said in the beginning, some pictures/drawings would help to describe these issues.

Bjorn

[Mike Mahoney]

Strobels? ?View Camera Technique? offers good information on the subject. The following two links offer interesting information:

http://www.luminous-landscape.com/dof.htm

http://www.luminous-landscape.com/dof2.htm

See also the rather lively discussion thread in the general forum on that same site related to digital DOF, which evolved into a generalized discussion about DOF and circles of confusion.

[Chad Jarvis]

Bob got it right, because it would appear he is the only person who read the question thoroughly enough. Depth of field and depth of focus are two different concepts.

[Bill Glickman]

Just making it perfectly clear since people have commented incorrectly after Bob's correct response. There is two issues here, Depth of Field and Depth of Focus. Depth of Field is what everyone except Bob is talking about above. It is defined as the amount of acceptable "out of focus" which can be tolerated in reference to the point of exact focus, or the plane of sharp focus. This "out of focus tolerance is defined by the Circle Of Confusion. (coc) This is what 99% of people talk about and quite often confuse with Depth of Focus.

However, Depth of Focus, is the opposite of the above, but eqaully defines focus tolerances. Depth of Focus defines the amount the film can be out of alignment vs. the acutal, or true film plane. The film must be within this tolerance as defined below to maintain the desired cc:

f stop * the allowable circle of confusion (used in the Depth of Field calculation)

As you can see from the formula, Depth of Focus has no bearing on the fl of a lens. The answer to this formula defines how much the film can buckle or be out of alignment for any reason, in either direction of the true film plane and still maintain the desire circle of confusion on film. With improper film alignment or film buckle Depth of Field is useless as Depth of Focus becomes the bottleneck to the resolution acheived on the film.

So both Depth of's, are equally important to acheive the resolution you desire on film. Howver, most people tend to only deal with depth of field since the depth of focus is out of most peoples control. Depth of Focus is also the achillies heel of most fast lenses, they are often limited by film flatness, not the quality of the lens.

Also effecting resolution on film is slew of other factors such as moving subject, moving camera from wind, shutter vibration, shutter speeds which spend a majority of their time opening and closing in the light path which adds to diffraction, diffraction limited f stops, lens and film resolving powers defined by the 1/R formula in the Fuji Film handbook, miror slap...etc. It's almost amazing anyone can ever get a sharp image!

to answer your other question.... In his review of the Walker XL in the July/August 2000 issue of View Camera magazine, Roger Hicks says that the camera makes it easier to get the front and rear standards parallel. This is critical with wide angle lenses because they have less depth of focus.

OK, what I think happened here is this... although the depth of focus formula does not account for the fl of the lens, it seems this is a bit of a shortcoming of the formula itself. Or possibly the formula is only really designed for normal lenses and through the years this disclaimer has beend dropped. Because the further the lens is from the film plane, the more shallow the light angles hitting the film. The closer the film plane is to the lens, as in wide angle lenses, the greater the angles of light are approaching the film. So, I do agree with what he is saying, but you are right in that the formula does not account for it. The same is true using the same fl lens and focussing at infinity vs. focussing very close causing the 3x the bellows draw...you have friendly angles to work with at 3x bellows draw, but no allowance in the formula for such. Not all photographic formulas are all encompassing. Even Depth of Field formula does not account for diffraction...it's something you just need to know and apply it accordingly. Hope this helps...

[Hector Pena]

Bob is right.

Also, depth of field is not related to focal lenght. Depth of field is related to magnification. So if you use a 300 mm lens and the subject in the ground glass measure 39mm, and then you change to a 90 mm lens and get closer to your subject in a way it will measure 39 mm in the ground glass, you will have the same depth of field than with the 300 mm lens at the same f stop.

[Matt sampson]

It has always been my understanding that short lenses mean very shallow depth of focus at the film plane. This leads to the rationale behind the Silvestri and Ebony non folding cameras....everything stays absolutely parallel so that theoretically the only variable left is the flatness of the film. Very short lenses such as the Scheneider 47xl produce such shallow depth of focus that it is extremely important that great care is taken when using them. Worn backs and slightly missaligned ground glass will lead to unsharp pictures no matter how good the lens.

On the subject of depth of field, (which as someone eventually pointed out is VERY different to depth of focus),I ,again, have always been lead to believe that regardless of the focal length of the lens, depth of field is always the same only perspective changes and I think that Hector's insightful contribution bears that out. At the end of the day it doesn't matter things are as they are and you have to work with the laws of physics not against them.

[Bob Salomon]

"only perspective changes" Not really. Perspective changes with the angle of the camera to the subject not with the focal length.

What is in or out of focus changes with focal length as does the amout of area captured on the film and the relative sizes of objects. But the perspective remains constant if the camera position remains constant.