In this case there is not change in DOF; the magnification is not changed by cropping the ground glass image using the 6x9 back.Originally Posted by Stephen Lewis
But, in order to have the same framing (and perspective) on 6x9 that you have with 4x5 you must use a shorter focal length on the 6x9; shorter focal length, less magnification, greater DOF.
Ah, you've nailed it perfectly, and obtained a useless result, since the two pictures won't be the same content, now will they? How about not being extraordinarily pedantic for a moment, and consider the desired result (a picture of some specific composition from a specific shooting position).
Sheesh, will this dead horse every stop twitching?
Language is as bad as mathematics in its ability to confuse simplicity. (Photographic proverb).
Nate Potter, Austin TX.
I prefer the old Monty Python line...
"A wink is as good as a nod to a blind man."
Vaughn
Proving once again that nothing is as contentious or as confusing among photographers as Depth of Field.
It is Ken Lee that nailed it. Depth of field calculations require assumptions, if not knowledge, about how much magnification will take place from the negative to the print, as well as the viewing distance for the print. Focal length, aperture, object distance and focus distance will uniquely determine the size of the Circle of Confusion, but the negative that gets more enlarging, requires a smaller CoC to look as sharp in the print.
I really, really find it very unhelpful to discuss the differences in DoF between formats, if you fail to scale the focal length to the negative height or width so that essentially identical compositions can be compared.
The Cliff's Notes on DOF is:
- If you have a lens that represents a fixed horizontal angle of view, say 45°, DOF will increase inversely to sensor size. The larger the sensor, the less DOF for the same HFOV.
- If you keep the lens and aperture the same, DOF does not change for different sensor sizes. While a 90mm on 4x5 will give you a radically wider photo than the same lens on 35mm, a 35mm-sized crop of the 4x5 will look like what you expect a 90mm shot on 35 to look like. We say the 90mm on 4x5 has "more" DOF than on 35 because you're not likely to print it to the same magnification as the 35mm crop. Printing smaller increases apparent DOF.
So it's me then!
Well, sort of
YES! Depth of field is absolutely dependent on the format size!
DOF calculations are based on the amount of blur that can be present in the image but will still be perceived as "in focus" by the viewer. This is obviously a subjective thing, and it requires us to begin with certain assumptions.
The first assumption is a standard print. For reasons lost in antiquity, the standard print was chosen to be an 8x10, viewed from something like 10 inches away. Or maybe it was 12 inches. Let's go with 12; it's not critical to this discussion, just work with me. The rest of the numbers I'm using are correct.
On that 8x10 print, viewed from 12 inches away, is a blur circle. It turns out that people with good vision (or whatever kind of vision they chose to define DOF) will perceive a blur 0.2mm across as a single point. That's because persons with this variety of good vision can just separate two points that are 0.2mm apart. Put the two points closer than that and our "standard person" will see only one point. So our second assumption is that we are allowed a circle of confusion (blur circle) on the print of 0.2mm.
Now we have a standard for "in focus," which can be used to calculate depth of field. It's based on a standard print (8x10) which is viewed at a standard distance (12 inches) by a person with standard vision (can resolve 0.2mm at 12 inches.)
[Disclaimer: some people have different vision, make different sized prints and view them at different distances. That's nice. Nobody cares. DOF is a very subjective thing, but we're talking about standards right now.]
On the film, you can have a circle of confusion which when enlarged to the 8x10 print will be at or below the limit of 0.2mm. For 4x5 film, which only needs 2X enlargement for an 8x10, you are allowed a 0.1mm circle of confusion on the film without being perceived as "out of focus." Change to 120 film and a 6cmX7cm format, and you now have to enlarge 3.6X to get an 8x10 print, so you can only have a 0.05mm circle of confusion on the film.
Let's take an example, using a 150mm lens at f/8, focused at 10 meters. With 4x5 film, allowing a 0.1mm CoC, our DOF will extend from 7.4m to 15.4m, for a total DOF of just about 8 meters. Leaving the lens alone and changing to 6x7 format rollfilm, the DOF will go from 8.5m to 12.1m, a total DOF of 3.6 meters. Thus the depth of field characteristics have changed, even though we changed nothing except the size of the film.
I'll stop now.
[edit: I used f/calc to get the numbers for CoC examples. Apparently f/calc assumes 4X enlargement for 6x7 film to get an 8x10 print.]
10 inches is taken as a standard close focus capability of person with good vision. At that distance an 8x10 print pretty much fills the field of vision that a person can view without losing the corners to degraded peripheral vision. So, while one can clearly make a larger print and view it from closer, or even view it with a magnifier (If that is your standard, then you need to apply a smaller circle of confusion.) but for most people to do so they are no longer looking at the photograph as a whole.
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