if you take a 35mm SLR with a 50mm lens and rotate it vertically (portrait) and hold the viewfinder up to one of your eyes, and leave the other eye open, your binocular vision will merge the two images with no problem/distortion, as if you were not holding a set of lenses up to one eye.
since what you see through the viewvinder is what the taken picture will look like, it is neutral like/wrt your eyes, at the zero middle between wide angle and telephoto. (it's worth considering "who says eyes are neutral?" it's the system we are used to and our brain develops to understand)
it's non obvious to a casual observer that the mm units chosen for the image size (the image gets focused on a 35mm rectangle (you need to know the aspect ratio)) and mm for the focal length are measuring different things, but that's why you just need to "know" that 35mm and 50mm "equal neutral". there are more things measured in mm as well, like the actual width of the primary lens which indicates how much light is gathered to be focused onto the same square.
i'm not a photographer. i don't quite know the mm lingo for what happens when the image sensor/film is wider then 35mm, the large/full formats. the focal lengths "work" the same, but a larger image would need to be focused and that seems like it would require some larger distances within the lens system.
The large format ones get a higher FOV in degrees. IIRC if you keep absolute aperture the same and change focal length to keep FOV the same, the DOF won't perceptually change.
Now, when you realize that there are geometric limitations to how wide an aperture can be relative to the focal length without having to stray from vaguely traditional _shapes_ of the objectives ("camera lens"), you can see that at the expense of fancier abberation corrections and of course larger/heavier glass lenses making up the larger objective, one could use a proportionally wider aperture with large format cameras.
For example, the infamous Barry Lyndon objectives were actually "just" 0.7x teleconverted spinoffs from an originally 70mm f/1 design. https://web.archive.org/web/20090309005033/http://ogiroux.bl...
The phenomenon you describe is a function of viewfinder magnification. It so happens that many SLRs had their magnification such that it worked well at 50mm to shoot with both eyes open. There are SLRs that have different magnification so this trick doesn’t always work.
You can get a rangefinder style camera with a viewfinder that lets you shoot with both eyes open but has a 35mm POV.
People have a variety of theories as to why 50mm is considered the standard lens and why people say it mimics human vision. I have heard so many explanations that I am inclined to say that there’s not really much but opinion behind it. It might just be that it was the most common first lens and because it is cheap and relatively simple to make a good, fast 50mm lens.
if that trick doesn't work, then either 1. your viewfinder is not showing what you will shoot which is what everybody expects because otherwise how can you frame your shot, 2. you are not using a 50mm lens or 3. you are not using a 35mm SLR
the point of a "single lens reflex" system is that you can see what the picture will look like by looking through the same (single) optics
No. As I stated, if the trick doesn’t work at 50mm it is because you are using a viewfinder with a different magnification.
A Pentax MX for example shows .97x magnification at 50mm. It will work great for your trick. Meanwhile a Canon AE-1 has .83x magnification at 50mm meaning one eye will be seeing an image where everything is 17% different in size. It will be like one eye is looking at a 55 inch TV and the other eye is looking at a 45 inch TV. Or more accurately, one eye is looking at the same TV but from 17% farther away.
If you throw a 58mm lens on that Canon, the trick will work again because you are zooming in to compensate for the zooming out that is happening in the viewfinder.
Of course, none of this has anything to do with 50mm lenses being “standard”.
Don’t believe me? Go slap a 50mm lens on an SLR with very low magnification. Or read one of the dozens of articles and threads out there explaining your misconception. Here’s a great one: https://www.lomography.com/magazine/319909-cameras-in-depth-...
technically speaking, if your viewfinder has a different magnification, that is (to coin a word) Multiple Lens Reflex; you have added a lens. SLRs were invented to show you "what the camera sees" so you can tweak it perfectly on different dimensions.
you are describing a different system that does not show you what the camera sees. I'm not saying what you are talking about doesn't exist, I'm saying that your over-inclusivity takes away the value of describing what I described and is telling people "there's really nothing you can say, a million different things could be going on"