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The Pentacon Six System
by TRA Equivalent focal lengths with 35mm cameras Many people “move up”, as they say, from a 35mm camera to Medium Format, to obtain better quality than they have experienced with their 35mm SLR. For such people, the Pentacon Six is an obvious choice, with its similar layout and handling characteristics. They are often familiar with the effect of various focal lengths on full-frame 35mm SLRS:
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 not only by the focal length of the lens, but also by the size of the recording medium behind it. 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 lens on a 6×6 camera records as much of the scene before it as a wide-angle lens would on a 35mm camera. 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:
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.
Notes 1 Horizontal value taken from Hasselblad data 2 Interpolated values in 35mm columns calculated on basis of other data available. 3 “CZO” = Carl Zeiss Oberkochen: lenses in Hasselblad 500C mount 4 Normal (unshifted) 5 Horizontal value derived from Schneider Xenotar MF data 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.
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