Originally posted by Winder The 135mm format was created by taking 70mm film and cutting it in half. That has nothing to do with the lens mount.
I know the story behind the 135 format and actually you could just as well call it "double frame". The 36mmx24mm image size is not the result of taking 70mm and cutting it in half. Cutting 70mm film in half gives you 35mm film. The latter has an image size of 24mmx18mm. Only if you combine two 35mm frames you get the 36mmx24mm format we are talking about. Hence, you could call it "double frame", if you like.
However, I think referencing an original ancestor film medium size (no matter which way you cut it; "half" or "double") is not helpful at all. Such historic references miss the point that 36mmx24mm is a standard size for still images and if you use a camera whose mount diameter and registration distance are designed to support that standard, however only a portion of that standard size is covered by the imager then you are dealing with a "crop" imager.
To me, the most reasonable interpretation of "full-frame" is that the imager is not cropping, it captures the "full-frame".
Note that the term is "full-
frame", not "full-
format". It does not make reference to the size of a medium, it refers to the fact that a camera is able to capture all of the frame.
Originally posted by Winder Then you have to apply the same logic to the Fuji GFX since it has a sensor that is designed for specifically for the GFX mount.
Since the 36x24 format was the most common and popular format to receive a cropping treatment, the term "full-frame" is often used synonymously with the 36x24 format. Other uses of "full-frame" are much rarer, but logically it makes sense to refer to any sensor that is not cropping as a "full-frame" sensor, independently of the actual format size.
I hope Fuji was not so short-sighted (or shall I say "narrow-sighted"?) to design the G-mount so that it only supports the current 43.8mm x 32.9mm image size. That's quite a bit smaller than the original 56 mm × 41.5 mm 645 film size. The current GFX image size is only 1.67 x bigger than that of a K-1, which only translates to a 0.74 stop advantage. If you want to use a 3:2 aspect ratio than the GFX format is only 1.48 x bigger than that of the K-1, yielding an ever smaller 0.57 stop advantage.
In other words, the difference between a GFX image in 3:2 format and a K-1 shot is marginal and if you use PixelShift on the K-1, the K-1 will be ahead. In that light, referring to the GFX as a medium format camera, is a bit of stretch, AFAIC.
Originally posted by Winder So relative to the mount, the Fuji GFX has a full frame sensor. The 645z has a cropped sensor relative to what the mount is designed for, so that means the same sensor would be a crop in one mount, but a full format in the other mount.
Yes, exactly.
Sensors (or film sizes) are not "cropping" or "non-cropping" per-se. It depends in which camera you use them in. Either they waste light or they don't.