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

[Ken Lee]

Adding Circle of Confusion to the calculation (see the web site to which Joanna has provided a link), takes into consideration the fact that different sized film or sensors, require differing amounts of enlargement to make a print of the same size.

What might be an insignificant degree of blur in a contact print, or a mere 2X degree of enlargement, begins to look genuinely out of focus, at greater magnification.

For example, since 8x10 film requires 1/2 the enlargement of 4x5, the calculator allows a correspondingly larger circle of confusion, twice as large. Since 6x7 film requires roughly twice the enlargement of 4x5 film, the allowable circle of confusion used in the formula is roughly 1/2 that given for 4x5... etc.

[Martin Miksch]

At least we reached the circles of confusion, I hope all will found out again^^

[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.

[Ken Lee]

"At least we reached the circles of confusion, I hope all will found out again^^"

Dante would be proud !