• 50-58mm is a “standard” focal length
• 80-90mm is an excellent portrait lens
• 35mm is a general-purpose wide-angle

When they come to Medium Format, a whole new set of focal lengths needs to be learnt.  Why?

What determines how much of the scene before you ends on on the film is determined by the angle of view of the lens, not its focal length. 

If a 50mm focal length lens is projecting the scene onto an area 24mm high × 36mm wide (the size of a full-frame 35mm exposure), a certain amount of the scene will be recorded.  If a 50mm focal length lens is projecting the same scene onto an area 56mm high × 56mm wide (the size of a 6×6 or 2¼" square frame), obviously a much larger amount of the scene will be recorded (more than three and a half times as much). 

So while a 50mm lens on a 35mm camera produces what we might call a “standard” field of view, a 50mm camera on a 6×6 camera produces a wide-angle field of view.

In practice, even if a mount could be made for it,  a 50mm lens designed for use on a 35mm camera could not be used directly on a medium format camera, for two reasons:

• Lenses are designed to produce a sharp image with no vignetting with a coverage adequate for the format intended, so the image brightness and definition would fall off dramatically beyond the 24×36mm area, probably completely disappearing well before the edges of the 56×56mm MF frame;
• The greater physical depth of the MF camera would mean that it would be impossible to obtain infinity focus using a lens designed for a 35mm SLR – you just would not be able to get the lens close enough to the film.

Various mathematically-inclined people have designed complex formulae to “convert” 35mm format focal lengths to 2¼ square format focal lengths.  Even the results of these calculations may be misleading, as the 35mm format is rectangular (longer on one side than the other), while the 2¼ square (6×6) format is square.  Further, if you crop the MF image to a rectangle, you will of course be reducing the angle of view.

A detailed comparison

A number of years ago I made a comparison of 6×6 and 35mm angles of view, based on published specifications available to me at the time.  Be aware that most manufacturers quote the diagonal angle of view (it is wider than the horizontal angle!), though some manufacturers of lenses for medium format cameras (for instance Carl Zeiss Oberkochen for Hasselblad) routinely quote both the diagonal and horizontal angles of view.

Here are the results of that comparison.

Note that certain focal lengths become popular, and many manufacturers consider that they must offer lenses in them.  For instance, for 35mm format cameras, it was for years popular to offer a lens with a focal length of 135mm.  However, many lenses labelled “135mm” are not exactly that focal length.  A given manufacturer’s “135mm” lens may in fact have a focal length of as little as 130mm, or as much as 139mm (or perhaps even greater), which is why different lenses that purport to have the same focal length may have slightly different angles of view – see the “80mm” Biometar and Xenotar lenses in the above table, for instance.  The 120mm lenses from Carl Zeiss Jena and Arsenal are also slightly different from each other.  The CZJ Biometar has a slightly wider angle of view than the Arsenal Vega (so either it is slightly shorter than 120mm, or the Arsenal Vega lens is slightly longer than 120mm – I do not have the technical equipment that would enable me to check the exact focal length of lenses).

A simpler approach

Having done all the hard work, I prefer a simpler approach!

To get a good approximation of the relationship between 6×6 and 35mm, take the 6×6 focal length, halve it, and add a bit (about 10% of the resulting number).

Take, for instance, a 50mm MF lens.  Divide by 2 = 25mm.  Add 10%: 2.5mm = 27.5mm  So a 50mm 6×6 lens is roughly equivalent to a 28mm lens on a 35mm SLR

Another example:  40mm MF lens ÷ 2 = 20mm + 10%: 2mm = 22mm

The longer the focal length, the less significant the 10%.  So a 1000mm lens on an MF camera gives approximately the same angle of view as a 500mm lens on a 35mm camera.