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11-29-2009, 05:46 AM   #1
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What is the focal length of our eye?

Silly question but I have been asking myself this question for a while now and I hope to get an answer.

Not sure where to post this so I guess the Beginner's Corner might be the most appropriate. Apologies if this is the wrong place.

I searched the net and I think most people agree that 50mm on a FF sensor is the closest to what our eye see. So a 50mm on a FF is about 35mm on a APS-C sensor, right?

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.

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?

11-29-2009, 07:40 AM   #2
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A silly answer, but a 50mm lens mounted on a apc-s is still a 50mm lens.
11-29-2009, 08:33 AM   #3
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Human Eye Focal Length

When considering what you see in the viewfinder, don't forget they don't all cover 100% of what the lens captures, (they don't all magnify at 100% of what they cover).

A Google Search reveals the focal length of the human eye ranges from 17mm to 24mm.

50mm is described as "absurd"

The Physics Textbook, with many citations.
11-29-2009, 08:47 AM   #4
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QuoteOriginally posted by raider Quote
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.

QuoteQuote:
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.

QuoteQuote:
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.

QuoteQuote:
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.

11-29-2009, 08:55 AM   #5
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I understand what you are saying. I tried your test and the "view" through the camera at 55mm is about the same size as what my eye sees.

Your pictures don't exist in the camera viewfinder, they need to be printed or posted for others to see.

Take a shot at 60mm, print it and look at the print at normal viewing distance. Then you will see if 60mm still looks "normal" to you. IMO 35mm looks "normal" to me. Some prefer 28mm or 30mm. Depends on how YOU view the photo.

GP
11-29-2009, 09:17 AM   #6
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Keep in mind also that what we see is not only a function of our eyes, it's a function of our brains. You could say that we don't see with our eyes, we see with our brains.

When you look at a scene, your eyes move all around it - at any one moment, only a small portion is in sharp focus and undistorted, but your brain can fill in a lot of gaps and give you the impression of seeing lots of detail over a wide area. It's similar to the way we're capable of perceiving a far greater dynamic range and depth of field than a camera can record.

To me, this means that there's no one focal length that can accurately record what I see. A camera is incapable of doing that, because what I see is a composite, "recorded" over time and put together by my brain. The best I can do when I go to take a photograph is to think about what it is in a scene that has caught my eye, and decide what focal length is best to capture that part. That usually comes closest to "what I saw".

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11-30-2009, 04:32 AM   #7
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Thank you very much for all your insights. I think I understand better now.

At 60mm, while the object magnification may seem to be almost the same as seen by my eyes, the field of view is much smaller than what my eyes would see. At 28mm or 35mm, the object magnification is much smaller but the field of view seems to be a more accurate reflection of my eye's field of view......I think
11-30-2009, 08:34 AM   #8
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I just did this earlier: I held my K-7 vertically and looked though the VF with my right eye, while my left eye had unobstructed vision, and I just started looking around the room. I then adjusted the zoom ring on my lens until it was as if I wasn't looking though a lens at all, and took a shot. Exif told me the shot was taken at 53mm.
Now I'm not exactly sure what that tells me, but I have a feeling what I found was that for me, a focal length of 53mm is close to the magnification of my eyes.

11-30-2009, 08:50 AM   #9
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You can't know the focal length until you know the crop factor.
12-01-2009, 05:33 AM   #10
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QuoteOriginally posted by Wheatfield Quote
You can't know the focal length until you know the crop factor.
oh yes, just read on the lens, very simple...
12-01-2009, 06:22 AM   #11
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QuoteOriginally posted by debbie Quote
oh yes, just read on the lens, very simple...
But what if you have a cropped format eye?
12-01-2009, 11:12 AM   #12
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Our eyesight has a pretty wide field of view, almost that of a ultra wideangle lens but our attention is usually just focused on a very narrow field. The camera takes that whole field and compresses into a smaller size to where our brain is not registering that scene in quite the same way. I think our eyesight is more like a 50mm with the field of view of a 12mm. Our attention is more like a 300mm.
12-01-2009, 12:15 PM   #13
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When I shoot at 50-55mm I can use both eyes, as what I see in the viewfinder matches what the other eye sees. I'm using the K10D (and -6 glasses...)

Very useful for tracking the kids when they play outside. The camera allmost "disapears", but the AF zone appears superimposed in my brain...sort of
12-01-2009, 04:36 PM   #14
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QuoteOriginally posted by raider Quote
Silly question but I have been asking myself this question for a while now and I hope to get an answer.

Not sure where to post this so I guess the Beginner's Corner might be the most appropriate. Apologies if this is the wrong place.

I searched the net and I think most people agree that 50mm on a FF sensor is the closest to what our eye see. So a 50mm on a FF is about 35mm on a APS-C sensor, right?

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.

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?
Well, The eye has several unique features.
1) the peripheral vision is about 170 degrees, the "focused" vision is around 45 degrees, And only the central part is "really" in focus. but the really cool thing is how the eye can take mixed light and all you see is a perfect WB regardless of the light type (s).
12-02-2009, 10:17 AM   #15
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Just because I hate to leave a question unanswered, the focal length of the human eye is thought to be be 17mm (Reference for FL). The typical range of f/stops for the eye are f/2.1 (for a 7mm diameter pupil) to f/8.3 (3mm pupil on a bright day)-- note this is a bit different than if you were to work it out as you would for a camera, since the eye is filled with fluid rather than air (Reference for eye f-stop).

This information doesn't give us a whole lot, because with the human eye, we do not have fixed 'print' with which to define depth of field or normal perspective. As is stated by the other posters, 'normal' is achieved by simulating relative sizes of objects as they receed into the background of the print-- roughly a 35mm lens on a 35mm camera or a 28mm lens on an APS-C camera when producing on a 4x6" print.

Something pricked at me about your question, too. We always discus normal perspective when wanting to represent what the eye sees, but what about depth of field? If we knew the eye's depth of field, we could then set the 28mm lens to an f-stop that would more closely match what our eyes see.

I fussed around for a little while to see if I could calculate the depth of field for our eyes, and it was HARD! The trouble is that depth of field is intimately related to 'print size', and with the eye, the 'print' is the entire visual field, which is hard to quantify.

I was able to track down a hyperfocal distance of 4 meters (Reference for Eye's hyperfocal distance). Then I turned to the online DOF calculator and plugged in a 28mm lens on a K20D, and then found the f-stop that came closest to 4 meters. The result was f/11.

So, if you wanted to closely represent what they eye sees on a 4x6 print, take a K20D camera with a 28mm lens and shoot at f/11.
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