> You really need to know the sensor size to adjust accordingly.

Absolutely. Focus length without sensor size (or film size) is a meaningless indicator for FOV.

But when it comes to a FOV being equivalent to the human eye, surely we should take display size into account as well. A photo made with a 500mm lens displayed at 4"x6" has exactly the same perspective as a photo taken from the same place with a 50m lens, displayed at 40"x60". In fact that small picture is basically the same as the large one cropped to the size of the small one.

It's a factor I never see mentioned. The usual explanation is that a "normal" lens gives more or less the same image when we look through the viewfinder as when we look directly, without the camera. But we don't look at photographs through a camera's viewfinder; we see them on our screens, on billboards, on prints.

Then you can go all the way and factor in viewing distance as well, then with the physical dimensions of the medium and the viewing distance you get an actual FOV, which better to be in the same order of magnitude as the FOV of the camera when the picture was taken to look realistic.

That's right. Here's a handy chart with common sensor sizes: https://en.wikipedia.org/wiki/Full-frame_DSLR#/media/File:Se...

E.g. on a Canon 450D (crop factor 1.6) the equivalent of a 50mm full frame lens is a 30mm lens

I've always heard it was 35mm. Weird.

Then again, it's also weird to try and guess what an average perspective would even be, as it's dependent on screen size and distance from face. I wonder how consistent the "arc-length" is between people and devices

My understanding is that it's actually ~40mm (or even 42mm). But of course that's not a standard focal length, so either 50mm or 35mm is closest.

EDIT: Ah yes, I remembered correctly https://theonlinephotographer.typepad.com/the_online_photogr...

~40mm isn't that unusual. I have a 40mm pancake lens for my full-frame DSLR and I've had rangefinders with about that fixed focal length and it was pretty common as the low-end of a lot of digital point and shoots.

It's true. But it's also not nearly as common as 50mm (or even 35mm) on DSLRs

Certainly. Before zooms became common, 50mm was absolutely the standard "kit" lens for an SLR. Personally, I always preferred something a bit wider for day-to-day. Of course, most people generally use zooms these days.

35 is 50 on a cropped sensor. But I also feel like my vision is closer to 35mm full frame (23 aps-c).

With no crop factor, 50mm is pretty close to what's "in frame" for me, with my glasses, and 35mm is pretty close to what's "in frame" for my total field of vision. You'd have to defocus pretty hard to make a 35mm lens match what my uncorrected eyes actually see, though...

This conversion doesn't actually make any technical sense. Yes, you can calculate the field of view, but the depth of field, the angles of sight lines etc. are that of a 35mm lens.

For an intuitive way to visualize this: look at a few dolly zooms and think about what you are seeing in terms of geometry.