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

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 lens 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.
Lenses designed for use on 35mm SLRs may be usable on bellows or extension tubes on a medium format camera, as at the greater distance from the film the projected image may be large enough to cover the frame fully.  (Think of projecting a slide onto a screen.  If you move the screen further away from the projector, a larger image will be projected onto the screen.)  In fact, sometimes suitable “35mm format” lenses – especially enlarger lenses – make great Medium Format macro lenses.

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.

Medium Format      35mm 
Name Maximum
Aperture
Focal
Length
Angle
 of View Notes Name Focal
Length
Angle of View
. . . Diagonal Horizontal . (Data source) . (Diagonal)
Zodiak f/3.5 30mm 180° 112° 1 Minolta 16mm 180°
Curtagon f/4 40mm 89° 69.5° . . 22mm 89°
CZO Distagon f/4 40mm 88° 69° 2, 3 . . .
Mir-26B f/3.5 45mm 83° . . Minolta 24mm 84°
Flektogon f/4 50mm 78° 60° 1 . 26.5mm 78°
. . . . . . Minolta 28mm 75°
PCS Super-Angulon f/4.5 55mm 71° 53.5° 4 Pentacon 30mm 71°
Curtagon f/3.5 60mm 66.5° 50° . . 33mm 67°
Mir 38B f/3.5 65mm 66° . . . 33.5mm 66°
Flektogon f/2.8 65mm 66° . . . 33.5mm 66°
. . . . . . Minolta 35mm 63°
Variogon f/4.5 75mm 56.3° 41° . . . .
Biometar f/2.8 80mm 54° 39.5° 5 . . .
Xenotar f/2.8 80mm 52° 38° . . . .
Biometar f/2.8 120mm 41° . . . . .
Variogon f/5.6 140mm 31.3° 22.5° . . . .
Variogon f/4.5 150mm 30° 21° . Minolta 85mm 29°
CZJ Sonnar f/2.8 180mm 24.5° . . Minolta 100mm 24°
. . . . . . Sonnar 120mm 21.5°
CZO Sonnar f/5.6 250mm 18° 13° . Minolta 135mm 18°
Variogon f/5.6 280mm 16° 11.3° . . . .
CZJ Sonnar f/4 300mm 15° . . . . .
. . . . . . CZJ Sonnar 180mm 14°
Pentacon f/5.6 500mm 10° . Minolta 250mm 10°
. . . . . . Sonnar 300mm 8.1°
3M-3B f/8 600mm 7.5° . . . . .
Spiegelobjektiv f/5.6 1000mm . . Pentacon 500mm
. . . . . . Spiegelobjektiv 1000mm 2.5°
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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© TRA November 2005, January 2012