A question for the Gurus

[Banjo]

Conventional wisdom has it that the longer the focal length, the shallower the depth of field: thus on a 35mm body, the DOF for a 28mm lens (at any given aperture) is greater than for a 200mm lens (at the same aperture).

OK, how does this translate to medium format lenses?

That is, for example, does a 200mm focal length (35mm) lens have a shallower DOF than a 200mm (MF 6x7) lens, or do both have the same DOF?

[Himself]

http://www.dofmaster.com/dofjs.html

[jjphoto]

[caspert79]

In other words: a fullframe lens on an APS-sized dSLR gives a crop of the actual image. Parts of the actual image fall outside your sensor.
Because of the crop, the image of the APS-sized dSLR looks like it's taken with a longer focal lenght while the DOF remains the same.
To take exactly the same picture (without these crops) with an APS-sized dSLR as with the fullframe camera, you'll have to use a lens with a shorter focal length. This will result in a deeper DOF.
This explains why digital pocket camera have such a huge DOF. Because of their super small sensors, they have to use super small focal lengths, resulting in huge DOF.

[sichko]

[Sevo]

It is actually quite a complex question as the circle of confusion depends on the final magnification factor (of he print, projection, screen view or whatever).

A 100mm 6x9 medium format lens has the same depth of field at f/2.8 as a 35mm lens provided that the MF image is printed to 60x90cm and the small format image to 24x36cm, and both are viewed from the same distance. If both are enlarged to the same print size, the small negative has much less depth of field (being in essence a factor 3 crop).

On the other hand, the relationship between focal length and negative size scales proportionally for equal angles and identical final result. So if we compare normal (42mm@24x36mm) to normal (100mm@6x9cm) for the same print size and the same CoC on the final print the former has twice the DOF.

To confuse matters (and some explanations), this also means that the DOF is exactly the same for an aperture of same absolute size (i.e. in mm radius rather than relative to focal length).

[sichko]