Aperture equivalent to full frame cameras

[tensor]

Comparing the full frame lenses with the medium lenses there is always an equivalent concerning the focal length, but I didn't saw any hint about the maximum aperture. Does anyone know if there is a table, a formula or a rule of thump were this relationship is described? Or even does it make sense to compare such a lens parameter.

[Buddy]

In terms of DOF you can use the tables for each format and compare. Generally a f/2.8 on medium format would be something like a f/2.0 on 35mm. But off course it depends what size the MF sensor is (33x44, 40x53, etc.).
For example: a MF 100mm lens on a H6D-100c (Sensor: 40x53.4mm) at 2.5 m focus and at f2.8 delivers roughly a DOF of 15.8 cm; the same/similar DOF is reached with a 70mm lens of a FF 35mm body at about f2.0.
There are tables and even apps for such calculations.

[Tangomaniac]

The connection is easy: The DOF is dependant only on the absolute diameter of the aperture.

For example: The Planar f/2 110mm has an aperture diameter of 55mm with fully open aperture. 
I.e. if you are comparing this lens (used for 6*6) to a FF-lens of the same image angle, you have to divide the focal length by about 1.5.
If you are using this lens with an X1D you would have to divide by 1.2

Used with a V-body (factor 1.5), this lens gives you the same image angle and DOF like a 1.35/70mm lens in FF.
Used with an X1D (factor 1.2), this lens gives you the same impression like a 1,7/85 mm in FF.

(You can either divide the 2.0 by the same factor 1.5 or the 70mm equivalent focal length by the diameter of 55mm to get the same result.)

BTW: This shows, that the f/2.2 100mm in the H-Series which has "only" about 45mm aperture diagonal can't give the same impression like the 2/110 when fully open, it corresponds to a 2,4/110 regarding the DOF.
It shows likewise, that the f/2.8 150 mm with 54mm aperture diagonal gives about the same DOF like the 2/110 and the 4/210.

[jwillson]

Unfortunately, since the aspect ratio is different with medium format vs. full frame, you can't really give a simple, universal answer.  To a pretty good first approximation, though, just take the full frame focal ratio you want to emulate and divide by the crop factor to get the "equivalent" aperture.  So, for a 44x33 sensor you would divide by 0.79.  For the larger medium format chips you will find in cameras like the H6D, divide by the crop factor of 0.65.

So, an f/2.8 lens in full frame, for example, should give roughly the same DOF for a given subject distance as a 44x33 camera at f/3.5.  On a Hasselblad H6D you would get the same depth of field at 4.18. 

This matches the earlier advice you got on the diameter of the entrance pupil, too.  A 35mm lens at f/2.8 has diameter of 12.5mm.  A 45mm lens at f/3.5 has about the same aperture diameter. So these two are "equivalent" in more ways than one.