[CFWhitman]

55mm focal length (physical) will provide you with the perspective of human vision at any distance or field of view.

This may be true to an extent when you are placing the camera to you eye (with current viewfinders, that is), but not when you see the resulting photograph. A "normal" length lens is a lens that captures photos that when viewed from the "normal viewing distance" give the same perspective as human vision. It changes with the size of the film/sensor. It correlates directly only to the angle of view (or field of view when used synonymously), and does not correlate directly to focal length any more than it correlates directly to film/sensor size.

Really, even when placing the camera to your eye, it all depends upon the viewfinder and how much it might magnify or reduce the image to your eye. So you should be able to get the same behind the camera feeling with a normal length lens for any camera by using a viewfinder that magnifies or reduces the image the correct amount to match human vision. Of course such a viewfinder may or may not be available for any given camera.

this is why more and more i have been wanting a full frame camera and enjoy using my film camera once in awhile

this is also why i want a MF camera even more, because with a 50-55mm lens, when you bring the camera up to your eye, you get zero perspective distortion, or, magnification as one could say and a HUGE field of view, to capture the world as i see it.

if you are looking at a bottle 5 feet away, when you bring up your 55mm equiped camera (ANY camera) the bottle will still look as if it is 5 feet away.


when you start increasing the focal length (physical), when you look at that bottle through the viewfinder you will feel as if the bottle is closer to you than 5 feet


when you start decreasing the focal length, when you look through the view finder you will feel as if there is a greater distance between you and the bottle than 5 feet.


again.. ON ANY SIZED SENSOR/FILM whatever

Again this depends entirely on how much your viewfinder reduces or enlarges the image, and does not correspond with the effect on the actual photographic results, which are entirely dependent on the angle of view covered by your focal length/sensor size combination (on whether you are using a normal lens or not).

field of view, is a different beast all together.

a 5-10mm lens on a point and shoot gives you similar FIELDS OF VIEW, but it most definetly does not give you the same sense of perspective that you would have gotten otherwise. When using a point and shoot, you always feel like you are not in the shot, because even though you are standing 5 feet away from your subject, it feels like you are like 50 feet away,

you are shooting someones face on a point and shoot, who's face is more than than the size of the LCD screen, yet they fit on the LCD screen entirely, with their shoulders, and the drunken stranger to the side....

This is entirely the result of using tiny LCD screens on the backs of cameras as viewfinders (I don't like them for that either) rather than anything to do with focal length. Remember, their face is much bigger than a 35mm piece of film as well, yet you manage to fit them on there. A 35mm equivalent sensor camera with a 50mm lens could be equipped with a tiny LCD screen also, and you would still hate the view on the screen just as much as you do now.

What you hate when behind the camera is the apparent size of objects in the viewfinder as compared to when you take the camera away. This is dependent on the viewfinder. Ideally, a normal lens for your camera should always look normal through your viewfinder because that would give you the best feeling of how the photo will turn out. Unfortunately this is not the case, so you have to rely on knowing what is a normal length lens for your camera format to know how the photo will turn out.

The definition of a normal lens is based on the idea that the resulting photo will be viewed from a distance about equal to the diagonal of its print. That is, an 8 inch by 10 inch photo will be viewed from about 13 inches away, while an 8 foot by 10 foot poster will be viewed from about 13 feet away, etc.

Assuming this, to achieve normal vision perspective from the camera you need to focus the image on the film or sensor from a distance of the same one to one ratio to the diagonal of the film or sensor. If you pick a shorter length, then it will put too much in the photo, shrinking the image, and you will get extension distortion that makes everything seem farther away and farther apart. If you pick a longer length, then you will put too little in the photo, magnifying the image, and you will get compression distortion that makes everything seem nearer and closer together.

Of course, in practice, sometimes you want wide angle or telephoto shots, perspective distortion and all, so the options are good.

[Marc Sabatella]

It's not that I gush over "ooo, I get 1.5x the focal length for the same price!"

In fact, it's quite the opposite. I could spend about $400 US on the DA Limited 21mm...only to find out that its not even as wide as my Rikenon 28mm.

Of course. It's a two edged sword. Telephotos lenses become more telephoto, wide angle becomes less so. For people with no specific preference in their shooting, these cancel each other out. For people who take more telephoto than wide shots, it's great. For people who take more wide than telephoto, it sucks. No news there.

Not to mention the noise benefits from larger sensels, etc.

That and DOF control possibilities, yes. This affects everyone no matter their wide/telephoto preferences.

Still, until/unless technology reaches the point where a FF camera can be made as cheap and as small as current APS-C cameras, I'd still say APS-C was a wonderful compromise. That is, had 35mm film never existed and someone were trying to pick an sensor size to balance camera size, complexity & cost against noise, DOF, lens size & size at a range of different focal lengths, I don't know that they could have done much better.

how hard would it be to put a 35mm sensor in the K20D's body?

If the offerings from the other manufacturers are any indication, it wouldn't be "hard", it would just be expensive, and there would be very little market for it, making the ROI look pretty slim.

[pingflood]

ramblings

As people have pointed out, perspective is all about distance to subject. Not sure where you got the whole 55mm idea from.

[rparmar]

The fact that the diagonal is 43mm is not up for debate, but where is this technical definition of "normal"? I hear it bandied about and it's even shown up in photography books, but is there any real technical/engineering backing behind the claim?

I don't know. I have accepted it as is since it makes some mathematical sense and furthermore has an empirical basis. I have never done technical research into optics, but perhaps a trawl through Google Scholar will help?

[Marc Sabatella]

I don't know. I have accepted it as is since it makes some mathematical sense and furthermore has an empirical basis.

I'd put it this way: there is no doubt that, as the measurement of the diagonal of a frame of 35mm film, 43mm makes sense as a definition of *something*. And maybe it even makes sense to call that thing "normal". However, whether or not it this definition of "normal" has any of the other properties sometimes claimed of it - "closest to the FOV of our eye", "most natural perspective as viewed through the viewfinder", "yields most natural perspective when printed at typical sizes and viewed at typical distances", etc - that's an entirely different matter.

[pingflood]

I don't know. I have accepted it as is since it makes some mathematical sense and furthermore has an empirical basis. I have never done technical research into optics, but perhaps a trawl through Google Scholar will help?

I've poked around a bit and honestly have not found anything. I wonder if it's one of those things that have just been perpetuated and nobody really knows the true origin.

I mean, why the diagonal of the frame? Why not the diagonal of the square made from the longer side? Then the 'normal' would be 50.9mm for film...

[mk07138]

Ok, to throw some more empirical evidence on the whole a 50mm is a 50mm is a 50mm thing. I just put my M-50mm f2 junk lens on my k20d and looked through the viewfinder with my right eye and kept my left eye open. With the lens properly focused I could just about see whatever I was looking at as if I didn't have the camera to my right eye. The perspective of the 50mm through my k20d viewfinder is almost exactly the perspective of my naked eye. Not exactly but darn close.

With any other focal length, for example my A-28mm f2.8 I can't do that, the perspectives between my two eyes are different at that point.

[Marc Sabatella]

With the lens properly focused I could just about see whatever I was looking at as if I didn't have the camera to my right eye. The perspective of the 50mm through my k20d viewfinder is almost exactly the perspective of my naked eye.

Emphasis on "through my k20d viewfinder". Use a bigger or smaller viewfinder and you get a different result. In other words, what you're talking about isn't perspective - it's viewfinder magnification.

[EricT]

Ok, to throw some more empirical evidence on the whole a 50mm is a 50mm is a 50mm thing. I just put my M-50mm f2 junk lens on my k20d and looked through the viewfinder with my right eye and kept my left eye open. With the lens properly focused I could just about see whatever I was looking at as if I didn't have the camera to my right eye. The perspective of the 50mm through my k20d viewfinder is almost exactly the perspective of my naked eye. Not exactly but darn close.

With any other focal length, for example my A-28mm f2.8 I can't do that, the perspectives between my two eyes are different at that point.

This is what I've found with my FA 50 on my K10D as well, and it's part of the reason why I really enjoy using this lens.

[mk07138]

Well, the same trick works on my KX viewfinder, and my K1000 viewfinder and my fujica st801 viewfinder with my 50mm f2 fuji lens. All of these viewfinders are "close" to 1x viewfinders, slightly less but still close. My point being that all things near 1x 50mm matches the eyes perspective. Sure a 2X viewfinder is going to change that but who really uses a 2x viewfinder?

This is the reason that people have been called a 50mm a normal lens on 35mm for so long. It just matches the eye on a normal viewfinder.

[Wheatfield]

Emphasis on "through my k20d viewfinder". Use a bigger or smaller viewfinder and you get a different result. In other words, what you're talking about isn't perspective - it's viewfinder magnification.

Most viewfinders are approximately the same magnification. I did the same test when the istD came out, comparing it to my LX finders.
The image size was close to the same, the istD finder was cropped compared to the LX finder.

When I was learning this stuff 40 years ago, the 50mm lens was acknowledged to be a slightly longer than normal lens.
The diagonal of the format has been considered to be "normal" for as long as I've been doing this stuff, it was explained to me that this length will very closely match the angle of view of the human eye, though as people tend to ignore their peripheral vision, the slight telephoto of the 50mm gives people what they perceive as a natural angle of view.
It is also enough of a jump from the obviously wide angle 35mm focal length that people would buy the 35mm length.
FWIW, the 85mm length was standardized for portraiture because it gave the most flattering rendering in a head and shoulders portrait, with just the right amount of flattening to be nice, without so much as to turn the subject into a butterface.

[CFWhitman]

What makes a normal lens a normal lens is not that difficult of a concept, really. It's just based on a certain assumption.

That assumption is that photographs have a "normal viewing distance," which is about equal to the diagonal of the print. In theory, that is the distance at which people will be viewing the photograph. In practice, whether from training or some natural feeling about a photo, that is the perspective that we seem to expect even when we are viewing small photos from a greater distance, or standing too close to large photos. (You could make an argument here, I suppose, that viewing 4 inch by 6 inch photos from a distance greater than their diagonal is the reason why slightly longer lenses are often perceived as "normal").

If you accept this assumption (and it's not really far off in general), then the formula for a normal lens is easy enough to understand. For a lens to have the same perspective as actual vision when a print from it is viewed at the 1 to 1 distance/diagonal ratio, then the focal distance/capture diagonal has to have the same 1 to 1 ratio.

Incidentally, the view through the viewfinder can be normal, wide-angle, or telephoto perspective, and normal perspective through the viewfinder is the one at which things look the same as they do without the camera in front of you (this doesn't even depend on an assumption). However, the idea that the perspective through the viewfinder corresponds to the perspective of the resulting photo is not necessarily true at all. There's a good chance it won't be true. This would be nice, but it often isn't the case. As far as 50mm being the "normal" perspective through a viewfinder without magnification, the distance the focus screen is from your eye changes this, so it's not carved in stone either.

[Marc Sabatella]

Well, the same trick works on my KX viewfinder, and my K1000 viewfinder and my fujica st801 viewfinder with my 50mm f2 fuji lens. All of these viewfinders are "close" to 1x viewfinders, slightly less but still close. My point being that all things near 1x 50mm matches the eyes perspective. Sure a 2X viewfinder is going to change that but who really uses a 2x viewfinder?

It is true that most SLR's - digital or otherwise - aim for close to 100% *magnification* with a 50mm lens, So most will indeed give you the effect you describe. I still object to the word "perspective" in this context, though. That term has a fairly precise definition in terms of vanishing points and so forth, and that is *not* affected by focal length *or* by viewfinder size. But I will grant that most SLR *viewfinders* provide something close to 100% *magnification* when used with a 50mm lens.

Anyhow, my main point here is you are talking about a phenomenon that has *everything* to do with what you are seeing during the few seconds the camera is up to your eye, and *nothing* to do with the image that gets recorded and is preserved forever in a print. And I would think that basing a focal length choice on a quality that is *not* going to captured in the photography but seems kind of silly.

The image actually recorded by a 50mm lens will look totally different depending on the sensor size - APS-C versus 35mm versus "medium format", etc - and if you want to claim that one of them "matches the eye" or anything like that, you're going to have to grant that the others *don't*.

[Marc Sabatella]

When I was learning this stuff 40 years ago, the 50mm lens was acknowledged to be a slightly longer than normal lens.
The diagonal of the format has been considered to be "normal" for as long as I've been doing this stuff, it was explained to me that this length will very closely match the angle of view of the human eye, though as people tend to ignore their peripheral vision, the slight telephoto of the 50mm gives people what they perceive as a natural angle of view.

Point being, it's all about angle of view, not focal length. In whatever sense 50mm was "normal" or "natural angle of view" for 35mm film, it is *not* that for APS-C - 33mm would be the equivalent focal length.

FWIW, I do *kind of* agree with the assessment of this field of view as being somewhat "natural", although in kind of an artificial way. That is, if I *don't* ignore my peripheral vision, I can very clearly see *far* more than this. Like, my 18-55 on APS-C doesn't go wide enough capture everything I can see. at the other extreme, if I look at a scene and think about what part of it am I most focused on, something longer than the sort of focal length we are talking about makes most sense to me. I think the 40mm nails it pretty well most of the time on APS-C. But if I think about how much of the scene I am really *aware* of - the part I am interested in plus the surrounding area I can't help but notice - then somewhere in the 28-35mm range on APS-C seems to capture this best. And yes, I have actually sat down and thought about this in a few different contexts, looking at a scene, forming mental boundaries to represent what I can see, what I am most aware of, and what I am most focused on, then putting various lenses on to see which match those best.

FWIW, the 85mm length was standardized for portraiture because it gave the most flattering rendering in a head and shoulders portrait, with just the right amount of flattening to be nice, without so much as to turn the subject into a butterface.

And again, it's not that the focal length does this - all lenses have the same perspective. It's that the *field of view* of an 85mm lens on 35mm film is such that it encourages you to stand at a *position* relative to your subject that makes the above true. Standing from the same place, you could shoot with any rectilinear lens, crop accordingly, and get precisely the same degree of "flattening".

[CFWhitman]

And again, it's not that the focal length does this - all lenses have the same perspective. It's that the *field of view* of an 85mm lens on 35mm film is such that it encourages you to stand at a *position* relative to your subject that makes the above true. Standing from the same place, you could shoot with any rectilinear lens, crop accordingly, and get precisely the same degree of "flattening".

When you crop the photo, it's the same thing as using a smaller film/sensor size, which changes the perspective just like changing focal lengths does. Angle of view, and therefore perspective, is based entirely on the ratios of focal length to sensor size and viewing distance to print size. When these two ratios match (when the angle of view of the capture matches the angle of view of the presentation), then the perspective looks normal.

When you make the ratio of focal length to film/sensor size higher (assuming the other ratio is constant), either by using a longer focal length or a smaller sensor (or cropping the photo and blowing it up, which is using a fraction of the sensor), then you make everything in the photo look the same percentage larger, which skews the geometry and changes the perspective, creating compression distortion. This is because when you actually walk toward objects that are far away, their images don't really grow in your field of vision at the same rate. The images of nearer objects get larger faster than the images of farther objects.

Likewise, when you make the ratio of focal length to film/sensor size lower, either by using a shorter focal length or a larger sensor, then you make everything in the photo look the same percentage smaller, which also skews the geometry and changes perspective, creating extension distortion. Again this is because actually moving back from a scene would cause the images of nearer objects to get smaller in your field of vision faster than the images of farther objects.

The distortion that occurs has nothing to do with camera to subject distance (other than the fact that two different distances have to exist for you to be able to see any perspective). That can only make the distortion more or less apparent; it cannot change it.