Focal Length of 6×6 compared to 35mm Lenses

by Ken Williams on 27 June 2011

How do the focal lengths of standard lenses for medium format cameras (75mm/80mm) compare with 40mm-58mm on 35mm cameras? When I was younger, I just accepted that the 75mm lens in my Rolleicord was equal to the 50mm lens in my Praktica. This is not so, nor is it clear cut. It depends on how you calculate it. The focal length of a ‘standard’ lens is calculated by measuring the diagonal of the film format. As every schoolboy knows that the square on the hypotenuse of a right-angled triangle is equal to the sum of the squares on the other two sides. From the diagonal of a film format can be easily calculated. For those who have forgotten: to calculate the diagonal of a 35mm negative use the following formula. (24 x 24) + (36 x 36) and find the square root of the answer.

You can compare the focal lengths of 6 x 6 and 35mm by:

  1. Leaving the formats as they are.
  2. Making the 6 x 6 format the same proportions as 35mm format,
  3. Making the 35mm format the same proportions as the 645 format.

Let’s look at leaving the formats as they are. 6 x 6 is actually 56 x 56 from which the diagonal is 79.20mm. This means that 80mm is about right or maybe 75mm.  But when it comes to 35mm the diagonal measures 43.2666153055mm – So a focal length of 45mm would be closer to the medium format standard  lens of 80mm. There are some cameras with a standard lens of 45mm – notably the Contax G series. The popular Rollei 35 series used a 40mm as did the Leica CL and Minolta CLE. But most 35mm cameras used the 50mm.But even this doesn’t work as the 6 x 6 format is really meant to be 645 which is 41.5mm x 56mm, unless of course you take pictures for calanders. I am assuming that Rolleiflexes/cords use a square format, as the right-way-around pentaprism had not been invented when the cameras were introduced . The TLR can be turned on its side to get a vertical format but it is scarcely convenient. From that, you think in 645 terms when taking the picture but can have either vertical or horizontal at the time of printing. So with that in mind, the diagonal of 645 is 69.70mm so a 70mm would be closer to the correct standard lens. Even this is not quite right as the 35mm format has a ratio of 1:1.5 whilst 645 is about 1:1.35.

So now you could alter the 645 format to 35mm proportions which works out as 56mm x 37.33mm. The diagonal of which is 67.30mm making the standard lens as 65mm possibly 70mm. Now, for those who like the 35mm format in those proportions then this is a correct comparison. From that you can calculate other focal lengths.

Medium Format TO 35mm equivalent   (divide by 67.3 and multiply  by 43.26)

40mm 25.71mm
50mm 32.13mm
60mm 38.56mm
75mm 48.20mm
80mm 51.42mm
90mm 57.85mm
120mm 77.13mm
150mm 96.41mm
180mm 115.70mm
250mm 160.69mm

35mm format TO Medium format equivalent (Divide by 43.26 and multiply by 67.3)

21mm 32.67mm
24mm 37.33mm
35mm 54.44mm
40mm 62.22mm
50mm 77.78mm
90mm 140.01mm
135mm 210.02mm
200mm 311.14mm

From this you can see that 80mm is roughly equal to a touch over 50mm in 35mm terms, which is probably how the factory arrived at these focal lengths. IF YOU DO IT THAT WAY !

Now, I don’t like the proportions of 35mm. It is OK for landscapes but portraits look odd, as the picture is too tall. On top of that very few sizes of standard B & W printing papers are of the same proportions. None conform to 35mm format with the exception of the enprint size of 6 x 4 recently available for that very purpose. 8 x 10 needs only 30 x 24 as does 16 x 20. I have found that 24mm x 32mm would be better. It is interesting to note that the original Nikon cameras, made just after the war, were this very size. The USA forbade their import as it didn’t suit Kodachrome processing mounts of 24mm x 36mm. It is also interesting to note that 24mm x 32mm conforms to this 4/3rds format frequently mentioned in the photo magazines in relation to digital photography. It is also interesting to note that the 645 format is all but the same ! If you want to be awkward then 42mm x 56mm IS the same proportions rather than 41.5mm x 56mm. So the calculation now alters as the diagonal of a 32mm x 24mm is exactly 40mm. Perhaps that is why the famous Rollei 35 series had a 40mm lens !

Medium Format TO 35mm equivalent (Divide by 69.7 and multiply by 40)

40mm 22.95mm
50mm 28.69mm
60mm 34.43mm
75mm 43.04mm
80mm 45.91mm
90mm 51.64mm
120mm 68.86mm
150mm 86.08mm
180mm 103.29mm
250mm 143.47mm

35mm format TO Medium format equivalent (divide by 40 and multiply by 69.7)

21mm 36.59mm
24mm 41.82mm
28mm 48.79mm
35mm 60.98mm
40mm 68.70mm
50mm 87.12mm
90mm 156.82mm
135mm 235.23mm
200mm 348.50mm

So there is not a lot of difference between the two. Making the 645 format the same proportions as 35mm is roughly in agreement with the manufacturers. 50mm being slightly less than 80mm and slightly more than 75mm in medium format terms using the first method. Using the second method which fits the paper better anyway changes the situation making 90mm the equivalent focal length of a 50mm lens on a 35mm camera. It is also interesting to note that a 50mm lens is based on a 2” lens which is a hint longer in focal length. The theoretical focal length of a Zeiss (Kyocera/Contax) 50mm f1.7 is actually 51.9mm making 51.64mm even closer to 90mm in medium format terms. It is interesting to note that a 42mm x 56mm format has a diagonal of exactly 70mm so calculating becomes easy to remember and easy to do. So it is up to you whichever format suits you best as to which are the correct equivalent focal lengths.

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