Originally posted by rparmar Hey Marc, I actually know all about focal length, perspective etc. which is why my empirical observations were confusing me.
As with my previous post in response "asdf", I kind of figured as much, and my comment weren't so much directed to you but were also intended to be useful to people reading along who don't have a background in these issues.
Quote: That partially makes sense. Although it might lead to the conclusion that for APS-C we need a 28mm lens with a viewfinder magnification that creates an impression of having a 43mm on the camera.
Otherwise this is finally a good argument for full-frame. Perhaps I am late coming to this realisation. Still learning here, and all.
Me too - that realization about FF hit you several hours before it just hit me while starting to type up this response! It also suggests how the notion of a 50mm focal length as "normal" managed to get so ingrained among 35mm film photographers: everything happened to come together at that focal length / FOV in a way it doesn't for APS-C. Extrapolating, I would assume - never having used one) that larger format cameras have the opposite problem - a lens that provides an equivalent field of view would actually produce an image that looked abnormally large in the viewfinder. On the other hand, larger format cameras are bigger, and maybe that would move the focus screen further away, canceling out the larger size? Of course, view cameras would then provide their own sets of issues.
WARNING: just thinking loud here:
I suspect what might be going on is that what makes 35mm/135/FF "special" is just a happy accident. That is, the image on the focus screen is the size it is - clearly, *MUCH* smaller than the actual scene before us (except for 1:1 macro photography). What makes it appear the "right" size to us is how far away that focus screen is from our eyes (along with whatever additional optics may be enlarging or reducing the image). Given that a camera of a certain size is going to tend to put that focus screen a certain distance away from our eyes, it stands to reason that for every focus screen size, there is one FOV that will happen to 100% magnification, and that the converse is true as well. That is - again assuming a focus screen that is a fixed distance from our eyes - for every FOV, there is only one focus screen size that will render that FOV at 100% magnification. And as far as I know, focus screen size = sensor size (assuming 100% coverage).
So if you take the FOV provided by a 50mm lens on 135 format to be the ideal normal FOV for whatever reason, then indeed, 135 format is the one format that will yield this FOV at 100% magnification. Other formats would need to place the focus screen closer or further away - and/or apply different optics to that image - in order to appear to provide 100% magnification at that FOV.
So it would seem that 135 format just happens to be the one that happens to yield 100% magnification with no additional magnification at the FOV that many would, for whatever subjective reason they care to state - consider "normal". And in that sense, it seems 135 format got something "right"in a way I've never seen discussed before.
I wonder if any of this actually makes sense?