Standard Lens?

Focal length maths!


I was thinking yesterday about my new to me Texas Leica, (Fuji GSW 690 III) which takes 6×9 images and considering if I should have waited and brought the not so wide version with the 90mm lens rather than the one with the 65mm. Give or take, in 35mm terms the 50mm instead of the 28mm. I was also contemplating whether I still don’t prefer the square format. This lead me to the thought, well what do I get if I just crop the odd photograph to 6×6?

I took my EOS for a walk the other day with a roll of Ilford 400 Delta and the 40mm Pancake. This lens seems to suit me well, in fact these days I tend to naturally fall toward 35mm focal length or close. I often find 50mm a bit too tele sometimes but 28mm a bit wide – hence my concern. So back to my question what do I get if I make an image on 6×9 at 65mm (28mm equiv) and crop to 6×6? I started to surf to find an answer or rather, gather some facts to make up my own mind!

I remember reading years ago that the natural “normal” lens for 35mm would be closer to 40mm rather than the 50mm it is, and that this was mostly adopted as the case, because Leica had the design for a 50mm on the drawing board when Oscar Barnak thought about using movie film in a smaller camera for stills. What I hadn’t realized is what seems to be a link to these focal lengths and format diagonals. A quick bit of research reveals that, staring with the small, 4/3rds or indeed Micro 4/3rds has a diameter of 21.6mm, the often referred to crop factor is 2X to get the 35mm equiv, and thus the Panasonic 20mm pancake becomes a 40mm, same as I use on my EOS 30! My Fuji X-Pro 2 has a diagonal of 30.1 mm apparently, (APS-C) size, this has a reported crop factor of 1.5X and sure enough to get the 35mm equiv, if we do the math, we end up with 45mm. So can see that 35mm film with a diagonal of 43mm, should have this as it’s “standard/normal” lens – perhaps that is why I fall towards that?

Continuing on we see that 6×6 has a diagonal of 79.2mm and for a long time 80mm has been a “normal”, so my Fuji with a diagonal of 101mm, normal would be 100mm, the 90mm a tiny bit wide, and thus my 65mm gives me the equiv of 28mm.

So with some simple thinking, I figure that 50mm on 6×6=28mm in 35mm, 80mm on 6×6 = 50mm I’m halfway between the two so should end up with something like 39mm in 35mm equiv – spookily close to my favourite – all is well with the world!

Please don’t comment about vertical field of view compared to horizontal and changing the ratio of the image – I’m done with maths of off to take some pictures!

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