Is it me ?

[Stephen Lewis]

I was always under the impression that the only factors affecting depth of field (DOF) were the focal length of the lens, the aperture used, and the point of focus distance. DOF is independent of format size, film or digital.

Imagine my suprise then when, this weekend, I opened a landscape photography supplement from one of the big magazines to find an article on (DOF) which showed two tables for hyperfocal distance, one for full size sensors and one for APS-C sized sensors.

For a 50mm lens at F8 one table gives a hyperfocal distance of 55ft, the other 35ft. If I fit a digital back with a 645 sensor to my 5x4 camera, why should that change the DOF characteristics of all my lenses? Am I missing something

[Dan Fromm]

Yes, what matters isn't distance but magnification. The APS-C sensor is smaller, so works at lower magnification than a full size sensor.

Thinking in terms of focused distance is safe only when the format is always the same.

[Stephen Lewis]

Are you saying that the DOF is affected by the format size? Surely changing from a 5x4 sheet to a 6x9 roll film back doesn't affect the characteristics of a lens which has already been fitted, set and focussed? The smaller format simply results in a smaller area of the lens' overall coverage being used?

[Joanna Carter]

Are you saying that the DOF is affected by the format size? Surely changing from a 5x4 sheet to a 6x9 roll film back doesn't affect the characteristics of a lens which has already been fitted, set and focussed? The smaller format simply results in a smaller area of the lens' overall coverage being used?

Not at all. It's all down to something known as the "Circle of Confusion". See this site for an online calculator that will demonstrate the differences CoC can cause.

[Joanna Carter]

Not at all. It's all down to something known as the "Circle of Confusion". See this site for an online calculator that will demonstrate the differences CoC can cause.

Although, I would say that simply putting a different back on the same LF lens wouldn't cause the same problem as a 35mm camera with a 90mm lens compared to a 4x5 camera with a 90mm lens. As Dan says, it's all down to magnification; a 90mm lens on a 35mm camera will not give you the same magnification as the same focal length as on a 4x5. Compare the size of an SLR lens with that of an LF lens.

[Dan Fromm]

Are you saying that the DOF is affected by the format size? Surely changing from a 5x4 sheet to a 6x9 roll film back doesn't affect the characteristics of a lens which has already been fitted, set and focussed? The smaller format simply results in a smaller area of the lens' overall coverage being used?

Yes, and no.

Yes is the image captured is identical, except for size, on the two formats. In this case, magnification is lower on the smaller format. This is not the situation you described.

No if the magnification is the same. This is the situation you described.

You have the advantage on us. You have the article. Would you revisit it and tell us more about what's in it? It might, great stress might, explain which of the situations I've just described the tables cover.

Cheers,

Dan

[Aender Brepsom]

Surely changing from a 5x4 sheet to a 6x9 roll film back doesn't affect the characteristics of a lens which has already been fitted, set and focussed? The smaller format simply results in a smaller area of the lens' overall coverage being used?

Yes, that is exactly how I understand it and that is why I agree with you first post:

I was always under the impression that the only factors affecting depth of field (DOF) were the focal length of the lens, the aperture used, and the point of focus distance. DOF is independent of format size, film or digital.

Of course, the final image will not be the same (might be heavily cropped), but that was not your question if I understood you well. If you try to reproduce the exact same picture on different media (4x5, 6x9, 35mm, full frame DSLR vs crop DSLR, etc.), then things will be different as magnification, different focal lengths and other factors will come in.

If you cut a 4x5 sheet to 6x9 format, it will be identical to the 6x9 shot taken with the same lens and settings.

[Ron Marshall]

Are you saying that the DOF is affected by the format size? Surely changing from a 5x4 sheet to a 6x9 roll film back doesn't affect the characteristics of a lens which has already been fitted, set and focussed? The smaller format simply results in a smaller area of the lens' overall coverage being used?

In this case there is not change in DOF; the magnification is not changed by cropping the ground glass image using the 6x9 back.

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.

[Kirk Gittings]

In this case there is not change in DOF; the magnification is not changed by cropping the ground glass image using the 6x9 back.

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.

Bang on. This was part of the reason I switched to 6x9 roll film from 4x5 for commercial architectural photography a few years back. More DoF at a given aperture meant less strobe fill for interiors etc. The DoF I was getting on 4x5 at F16 with a 90mm, I could get at F11 (more or less) on a 65mm on 6x9. That one stop difference meant allot in the real world of shooting.

[bglick]

Here is a relatively simple cheat sheet for DOF analysis between formats which takes all the variables into consideration....


1) Change format size, change lens fl.....

same DOF exists, when you have the same:


a) recording media (same film type or same density digital sensor)
b) composure (half the format diag, = half the fl)
c) camera to subject distance (same angular view of lenses)
d) resolution on same final print size
(double the format diag. = half cc diam.)


Result ..... 2x format diag, double f stop = 2 stops slower shutter speed for the same DOF. So the penalty for equal DOF in larger format is 2 stops of shutter speed.

or, as Kirk corectly mentioned above, very painful when using strobes, as strobes are dependent on f stop only. Hence why LF is certainly not a flash friendly format.



2) Change lens fl, change camera to subject distance,

same DOF exists, when same:


a) format size (film size or digital sensor size, same density)
b) composure (half fl, half subject to camera distance)
c) focus point (but not focus distance)
d) cc (due to same size format)

Result is..... half fl = 1/2 f stop (2 stops faster).


It's interesting to note, and rarely mentioned, when using larger formats, there is a loss of resolution at the point of exact focus vs. the smaller format. DOF assumes only the max. size cc, not the min. sizes.


However, these are general rules. The reality is, nothing is carved in stone with modern optics. DOF formula was created in the late 1800's IIRC.... there was very few optical variables to contend with back in those days. With modern optics, many things have changed, including the use of aspheres, which can radically change the expected DOF output and sharpness at point of focus. Also, different lens designs react differently to DOF. A math formula that lumps all lenses into a single formula is over generalization today. Aberations vary greatly with each lens type... and at each f stop and even at different focus distances. This is evidenced by viewing MTF data of lenses at different focus distances.

Also, the DOF formula is deceiving.... the reason is, you simply plug in variables, and the formula pumps out answers. The Optical chain is not that simple in the real world. For example, you enter a given cc..... what knowledge do you have that cc can actually be recorded on film / sensor? The formula does not prevent you from using rediculous values that are not achievable, as in real world recording, the MTF values of the lens and the film play a critical roll in what's possible. Example, use DOF formula and shoot with color neg film, then the same with B&W high rez film, compare the results.... the color neg film will probably fall short of your expectations, the B&W film will far exceed your anticipated outcome.

cc is also effected by shutter vibrations, wind, film flatness, etc. etc. too many variables to ever expect an over simplified formula to provide accurate result.

So buyer beware on the DOF formula subject....

I have tested a lot of lenses. I have noticed with newer aspheric designs, the resolution at the point of exact focus on 35mm lenses, will often not vary at f2.8 through f11. This was unheard of 10+ years ago. The aspheres change all the rules as we know them, specially the rules (DOF formula) created in the 1800's.

So IMO, its OK to use these general rules of thumb to get close to what you want to achieve, but often today, it requires testing to be sure you meet your desired objectives...