Think about the definition of f stop. it's a ratio of the diamenter of the aperture to the focal length of the lens. f32 on a 400 mm lens is a bigger hole than f 32 on a 35 mm lens. The size of the hole (aperture) determines DOF. So...are they related or not? Any lights going on?
I totally agree with Mr. Grim: there is a clear discrepancy between the writings of Hicks and Stroebel. Although it may seem counterintuitive, I agree with the fact that depth of focus depends only of the f-stop (and of course of the chosen value for the circle of confusion). This is indeed easy to demonstrate using a little drawing showing the light cone (for instance emitted by a luminous point) coming from the lens onto the focus plane. The depth of focus depends only of the angle of this cone which depends itself of the ratio "focal length/physical aperture of the lens" which is the definition of the f-stop. Therefore, I cannot understand why the alignment of the front and rear standards should be more critical with short focal length lenses. The only explanation that I can find for this putative small depth of focus of the short lenses is that, due to their large depth of field, short lenses are often used more open (smaller f-stop) than long lenses.
"I cannot understand why the alignment of the front and rear standards should be more critical with short focal length lenses"
Because when no movements are applied the entire area at infinity should be equally sharp. This means that the film plane must be totally within the area of depth of focus. If not some areas will be unsharp. This requires that a camera be in proper alignment.
We had a case with the NY Times where they were trying to use a 47mm on a 23 Technika that was 25 years old. While the camera appeared to have no visible wear it was impossible to focus the lens at infinity and have an overall sharp image.
To cure the problem the camera had to go back to the factory and be re-built to the original factory specs.
Depth of focus is very critical with wide angle lenses.
Perhaps an easy way to remember the difference is that Depth of field is on the subject side of the lens. Depth of focus is on the image side of the lens.
Bob, how do you reconcile your statement about depth of focus varying by focal length with Stroebel's formula (which doesn't contain focal length)? This would seem to be critical to the optimum sharpness f stop selection technique that I have seen on the LF homepage and in a magazine (maybe View Camera, I don't remember).
"Bob, how do you reconcile your statement about depth of focus varying by focal length" He obviously never made a camera that uses extreme wide angle lenses that needs to be sharp all over at infinity.
I am more concerned what the makers of cameras and lenses say then theorists. They don't have to make the product.
Bob, Noshir asked a very good question above. You did not answer his question, but have been around long enough to maybe know the answer. Can you elaborate on this?
Almost everyone can see from diagrams and drawings that wide angle lenses require much more precision in film flatness and the paralellness of the standards. But the question still remains, why doen't any of the Depth of Focus formulas utilize the fl into the formula? I have never seen any Depth of Focus formulas utilize fl as a variable. Does anyone have a good answer for this?
I am sticking by my theory. The formula was originaly designed for normal fl lenses. Longer lenses are are easier to work with than normal lenses in this regard, so no reason to re write the formula. But wide angle lenses require even greater precision than normal and long lenses....and no one every bothered to figure out what the exact relationship between fl and Depth of Foucus is, hence the formula really only applies to normal and longer lenses...
' Almost everyone can see from diagrams and drawings that wide angle lenses require much more precision in film flatness and the paralellness of the standards. '
In that case 'almost everyone' needs to do their diagrams and drawings more accurately. The depth of focus is determined by the angle subtended by the aperture at the film plane. This is the same, for any specified f-number, regardless of focal length because the f-number is equal to the focal length divided by the aperture diameter. Stroebel's formula is correct, and could be verified by any reasonably bright child with a decent grasp of elementary geometry.
What might be confusing the issue is that with a short lens an error in focussing of, say, 1mm will make a much larger difference in the position of the object plane of focus than it would with a longer lens. However, the effect on the circle of confusion of a point source at the intended object plane will be the same - which is just another way of saying that for a given aperture and a given focus distance the shorter lens has more depth of field.
Depth of Focus is related to the focal length and Stroebel stated the right formula for it. This is best known in Astrophotography, because there is all about Depth of Focus and they don't care about Depth Of Field. Take a look at http://www.astropix.com/HTML/I_ASTROP/FOCUS/FOCUS025.HTM#Depth for further explanations.
Another important issue with short lenses is "Field Curvature". The focal plane is not a plane, but merely a part of a sphere. The shorter the depth of focus the more critical is field curvature and this is the reason why short lenses do require higher precision.
Regards,
Sorry, for the addional confusion, I forgot the essential word "not". Depth of Focus is *not* related to the focal length. But the focal sphere has a smaller radius with a shorter lens and is thus more aparent.
Regards,
I think we're now getting into the realms of equivocation - if we weren't there already.
You could indeed say that depth of focus is related to focal length if you are speaking in the context of fixed aperture size. If, OTOH, you speak in terms of a fixed f-number then it is independent of focal length.
I must say, though, that I find the entire discussion somewhat bizarre, since the whole point of LF cameras in general is the ability to change the relationship of the front and rear standards in order to exercise control over the image, including the tilting and/or swinging of the plane of focus - all of which is done by eye and not by theoretical considerations. To do this reliably means that you need the alignment of the ground glass to be as accurate as possible in relation to the positioning of the film holder. However this is vital regardless of whether short or long lenses are in use.
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