Saturday, May 10, 2014

A Pentacon Six lens on a Canon EOS body

So after a long wait thanks to local customs, I finally received my Carl Zeiss Jena Flektogon 4/50 (a medium format lens in Pentacon Six mount) and Pentacon Six to EOS tilt adaptor.  

At about ±US$350 for the combo, they make a much more affordable option if you're interested in venturing into the world of tilt/shift photography with your DSLR, with the cheapest option from Canon being the TS-E 45mm f/2.8 costing you US$1400. Before I ordered them I spent a lot of time researching and one of the unanswered questions I had was: what is the equivalent field of view when I use the medium format lens on my DSLR?

The Pentacon Six use 6x6 medium format film which measures to about 56×56mm, whereas the Canon 5D uses a fullframe 35mm sensor measuring 35.8×23.9mm. Obviously, I intially thought, the lenses from these two different formats would project different sized image circles onto the film plane, which means the 5D would only be capturing a central portion of the image projected by a medium format lens.

However trying to confirm this online just lead to frustration. When people ask "what is the equivalent focal length" it seems to attract crowds that automatically spout the axiom "focal length is focal length and never changes", while completely ignoring the word "equivalent". Then there's another crowd who seemed to answer that the crop ratio is ×1.8 without explaining where they got the number from, leading me to much confusion. I tried working it all out mathematically but I couldn't figure it out, so I just decided to do the practical thing and compare photos.

Flektogon 4/50 Canon EF 28-105mm at 50mm

On the left we have the Carl Zeiss Jena Flektogon 4/50, mounted on a Canon 5D using a Pentacon Six to EOS tilt adaptor. Exposure is f/4, ISO100, 1/800s. On the right we have the Canon EF 28-105mm f/3.5-4.5 USM II, on the same Canon 5D mounted natively. It's zoomed to 50mm, and the exposure is the same at f/4, ISO100 and 1/800s.

As you can see, the focal lengths are almost identical. Playing around a bit I found that the 28-105mm zoomed at 53mm is a closer match to both the Flektogon, and a Carl Zeiss Jena Pancolar 1.8/50 lens (50mm M42 lens).

Canon EF 28-105mm at 53mm Pancolar 1.8/50

So, a medium format lens on a 35mm SLR mount seems to give the same field of view as a native SLR lens on a SLR, which is of course not the case when using a fullframe SLR lens (image circle for 35.8×23.9mm) on a cropped DSLR sensor (22.3×14.9mm). Why? I have no idea.