Originally posted by raider Silly question but I have been asking myself this question for a while now and I hope to get an answer.
You'll get quite a few, I imagine, most of them contradictory. This topic tends to cause enormous confusion, and often leads to heated arguments. Suggesting right off the bat the answer is, there really is no one "right" answer. But I'll try to answer with something that makes sense.
Quote: I searched the net and I think most people agree that 50mm on a FF sensor is the closest to what our eye see.
There is a sense in which this is correct but also senses in which it is not correct. So first order of business is to be more specific about what you mean by "what our eyes see". There are three *very* different things that one could mean by this:
1. Field of view. Meaning, how much of a scene we can see. 50mm is *way* too long a focal for this on FF; our eyes have a *much* wider field of view than this when you include peripheral vision. You might think, well, maybe 50mm on FF is the right field of view if you *don't* include peripheral vision. But the problem is, there is no simple dividing line between periperhal and non-peripheral vision. So we're left with a *very* subjective notion of how much of a scene we can "focus on" at once. Some people feel 50mm captures this well, some feel they are really focusing one *more* of a scene than that (and hence a shorter lens is a better match), and some feel they are focusing on *less* of a scene than that (and hence a longer lens is a better match. I feel it depends on the scene, but sure, it's not a terrible compromise. If this were all there were to it, I would chose something wider, personally, to capture a sort of "average" sense of what I consider my own natural field of view in "most" situations.
2. Magnification. This seems to be what you are alluding to below: how big things look through the viewfinder. But this is not just a function of the lens - it is also a function of the viewfinder. Some viewfinders magnify more than others. So the same lens on different cameras will have different magnification. It does happen to be the case that a lot of classic 35mm cameras had viewfinders designed to provide something close to 100% magnification with a 50mm lens. probably not an accident; they probably went out of their way to make this the case, because they knew that 50mm would be a commonly preferred focal length for other reasons.
3. "Perspective distortion". The field of view of a 50mm lens may or may not be anything all that special in terms of capturing what our eyes see, but it *does* have a very important quality that is actually the most important of these factors. namely, if you *print* an image taken with a 50mm lens, and then view that print from a "typical" distance for that print, then objects in the print will generally appear to be the same size as they did in real life. And not just sizes of objects - also relative positions, perspective in general. A print made from a 50mm lens will look "normal". This is, again, the most significant of the factors, and it is completely dependent on field of view and field of view only - any lens/sensor combination that produces a field of view that is the same as the field of view of a print viewed from a typical distance will work out exactly the same in this respect.
Quote: If so, I put on my tamron 28-75 and twist the zoom ring to 35mm but the view through the viewfinder is definitely not what I see through my eyes. (I wear glasses by the way). I think 60mm of my tamron is more like what my eyes are seeing.
Then this suggests your viewfinder has a magnification of 80-90% or so. Magnification figures are always quoted with respect to a 50mm lens.
Quote: I asked my wife to test and she said 55mm is closest to what her eyes are seeing. What are we missing here? Is our glasses affecting our eyes focal length?
Glasses could be affecting the magnification, because you are changing the distance from your eye to the viewfinder, and the lens in the glasses might also be increasing or decreasing magnification.
So anyhow, back to your original question: I don't know that it actually makes sense to ask about the focal length of the eye in purely optical terms, although I suppose the eye *does* contain a lens, and presumably has its own focal length, which probably won't mean a whole lot in photographic terms. I just Googled it, and found what appears to be a widely accepted answer: the focal length of the lens in the eye is around 17mm in its relaxed state.
But this isn't really relevant. What matters in term of answering your question are the three factors I listed above. We can answer those questions for APS-C just as we did for FF:
1. The field of view of the eye hasn't gotten any easier to pin down with APS-C than it was with 35mm, but sure, it ranges from a lot shorter than 35mm to a lot longer depending on how much of a scene you are focusing on. There is really no way to put a single answer on this, but to whatever extent "50" was the "right" answer on FF, "35" is on APS-C.
2. If you want to know what lens will produce 100% magnification on your particular camera, look up the viewfinder magnification in the specs, and divide that into 50. That is, as I said before, the answer is going to depend on your viewfinder. But figures of 60-70mm are going to be typical.
3. Since 35mm on APS-C produces the same FOV as 50mm on FF, then 35mm is the focal length that produces a print with no "perpective distortion" on APS-C.