boBQuincy
<font color=green>I am not carrying three pods<br>
- Joined
- Nov 26, 2002
- Messages
- 5,083
Since challenging the common wisdom on crop factor was so much fun, I thought I would take on another of our photographic "truths", the normal lens.
A "normal" lens is often considered to be one whose focal length is approximately the diagonal of the sensor, 50mm for full frame, 27mm for 1.6 crop. The idea of this being a normal lens is that it most closely approximates how our eyes see a scene. But how do our eyes see a scene?
One of the factors is the field of view, which is difficult to measure for our eyes. Our total field of view is quite wide, almost 180 degrees. Clearly, no 50mm lens is going to come anywhere close to that. What we see and what we perceive is quite different, our field of view that we normally acknowledge is much more narrow, maybe only about 50 degrees. This was perhaps one of the targets for a normal lens. With a field of view similar to what we recognize the image would look natural to us.
I am going to take another approach and suggest a "normal" lens is one that closely matches the magnification of our eyes in order to produce an image that looks natural to us. One way to check this is to set up a camera with a mid-range zoom lens, look through the camera with one eye while looking at the target with the other eye, and adjust the focal length until the image matches what we see. We must take into account any magnification of the viewfinder system, 0.9x in the case of a Canon 30D, so the lens zoom would be about 1.1X the length of the proposed normal lens. On my Canon 30D the lens setting that matches the magnification of my eyes is about 55mm. Multiplied by the 0.9x of the viewfinder this is right about 50mm!!! Hmm...
So much for magnfication, what about perspective? Lenses do not change perspective, relative distances do. If we replace the 50mm lens with a 24mm lens the perspective stays the same but the image size is half. In order to get the same magnification the 24mm lens would have to be half the distance from the subject as a 50mm lens would.
If a 24mm lens is placed at half the distance from the main subject as a 50mm, the main subject will be the same size, but subjects at other distances would not. Thus the image looks "wrong" to us, the proportions appear incorrect.
If the main subject is 5' distant and a secondary subject is 10' distant a "normal" lens would make both of these look to be the same magnification as with our eyes. In this case the subject at 10' would appear half the size of the subject at 5'. If we use a 24mm lens but get twice as close the main subject (now at 2.5') looks the same as it did with the 50mm at 5' but the distant subject is at 7.5', now 3x the distance of the main subject, instead of the 2x as it was before. Of course, the distant subject will now appear 1/3 the size of the main subject, giving the scene that "wide angle" look.
Similarly,if we use a 200mm lens and get 40' from the main subject, the secondary subject at 45' will appear almost the same size, giving that telephoto "flattened" look. From this we can see that only one focal length will make near and far subjects appear to be the proper magnification, and that would be our normal lens, the one that most closely corresponds to how our eyes see the scene. The 50mm.
A shorter (or longer) lens could be moved to make one or the other subjects the same magnification but not both. Only the "normal" lens would match our eyes magnification for subjects both near and far. And a 50mm does just that, even on a 1.6x crop camera. But why? Isn't the focal length of the normal lens supposed to be approximately the diagonal measurement of the sensor? And does this mean a 50mm would be a normal lens on *any* camera?
No, not really. What is *not* changing here is the lens to sensor distance, even though the sensor size itself is different. If we kept everything else the same and downsized the sensor even more, the 50mm would still give the same image, just less of it due to the cropping factor.
So the 50mm is still the normal lens, for full frame, for 1.3x, and even for 1.6x crop cameras, as long as they use the same geometry as their film camera counterparts. This holds true for Canon, Nikon, and any other dSLR that uses lenses from the film camera days.
These are my rantings and ravings for today, and I welcome any comments and/or corrections.
boB
A "normal" lens is often considered to be one whose focal length is approximately the diagonal of the sensor, 50mm for full frame, 27mm for 1.6 crop. The idea of this being a normal lens is that it most closely approximates how our eyes see a scene. But how do our eyes see a scene?
One of the factors is the field of view, which is difficult to measure for our eyes. Our total field of view is quite wide, almost 180 degrees. Clearly, no 50mm lens is going to come anywhere close to that. What we see and what we perceive is quite different, our field of view that we normally acknowledge is much more narrow, maybe only about 50 degrees. This was perhaps one of the targets for a normal lens. With a field of view similar to what we recognize the image would look natural to us.
I am going to take another approach and suggest a "normal" lens is one that closely matches the magnification of our eyes in order to produce an image that looks natural to us. One way to check this is to set up a camera with a mid-range zoom lens, look through the camera with one eye while looking at the target with the other eye, and adjust the focal length until the image matches what we see. We must take into account any magnification of the viewfinder system, 0.9x in the case of a Canon 30D, so the lens zoom would be about 1.1X the length of the proposed normal lens. On my Canon 30D the lens setting that matches the magnification of my eyes is about 55mm. Multiplied by the 0.9x of the viewfinder this is right about 50mm!!! Hmm...
So much for magnfication, what about perspective? Lenses do not change perspective, relative distances do. If we replace the 50mm lens with a 24mm lens the perspective stays the same but the image size is half. In order to get the same magnification the 24mm lens would have to be half the distance from the subject as a 50mm lens would.
If a 24mm lens is placed at half the distance from the main subject as a 50mm, the main subject will be the same size, but subjects at other distances would not. Thus the image looks "wrong" to us, the proportions appear incorrect.
If the main subject is 5' distant and a secondary subject is 10' distant a "normal" lens would make both of these look to be the same magnification as with our eyes. In this case the subject at 10' would appear half the size of the subject at 5'. If we use a 24mm lens but get twice as close the main subject (now at 2.5') looks the same as it did with the 50mm at 5' but the distant subject is at 7.5', now 3x the distance of the main subject, instead of the 2x as it was before. Of course, the distant subject will now appear 1/3 the size of the main subject, giving the scene that "wide angle" look.
Similarly,if we use a 200mm lens and get 40' from the main subject, the secondary subject at 45' will appear almost the same size, giving that telephoto "flattened" look. From this we can see that only one focal length will make near and far subjects appear to be the proper magnification, and that would be our normal lens, the one that most closely corresponds to how our eyes see the scene. The 50mm.
A shorter (or longer) lens could be moved to make one or the other subjects the same magnification but not both. Only the "normal" lens would match our eyes magnification for subjects both near and far. And a 50mm does just that, even on a 1.6x crop camera. But why? Isn't the focal length of the normal lens supposed to be approximately the diagonal measurement of the sensor? And does this mean a 50mm would be a normal lens on *any* camera?
No, not really. What is *not* changing here is the lens to sensor distance, even though the sensor size itself is different. If we kept everything else the same and downsized the sensor even more, the 50mm would still give the same image, just less of it due to the cropping factor.
So the 50mm is still the normal lens, for full frame, for 1.3x, and even for 1.6x crop cameras, as long as they use the same geometry as their film camera counterparts. This holds true for Canon, Nikon, and any other dSLR that uses lenses from the film camera days.
These are my rantings and ravings for today, and I welcome any comments and/or corrections.
boB
Every real photographer knows the "normal lens" is the lens that's normally on the camera. 
