Hi, David,
Hi all,
This is my first post so if I am in the wrong place please let me know.
I think this is a good as any, and welcome aboard.
My question is this... How does perspective (normal, wide, tele) change with the combinations of focal length and sensor size?
To be more specific, and to give you a reference point, a 50mm lens on a 35mm film camera is considered "normal". I have a Canon 30D, hence a smaller sensor with a 1.6 "crop" factor (apologies to Doug Kerr).
Yes, yes, yes . ..
Based on my research the last few months, a "normal" lens for my camera would be around 30mm. etc.
Well, firstly, the "perspective effect" of a particular shot is a result of where the camera is. Focal length isn't really involved - directly. And there really isn't "wide" or "narrow" perspective. There is the perspective I (or the camera) gets looking down my street from in front of my house, and there is the perspective that I (or the camera) gets looking down the street from in front of the house four houses away.
What are the hallmarks of a particular point of perspective? Well, for one (and we usually don't mention this), from one places we can see certain things and not others, and from a different place that will be different. This is in fact a part of perspective. (And to be really thorough, we need to mention that we get a different perspective looking east from a certain spot and looking west!)
But, thinking of perspective in the more usual context, one manifestation of a certain "perspective" (and I think this was mentioned by one of the others responding to your question) is the relative sizes, on the image, or certain objects (having certain actual sizes and located at certain places).
Another manifestation (actually related to the first) is what points on different objects appear to be aligned on the image.
If we have the camera at a certain site, and shoot the scene with lenses of different focal length, those manifestations will be the same for each image - they are unchanged by change in focal length.
But focal length gets into the deal in two indirect ways (which can help misdirect our attention from the basic principles here). Firstly, if our interest is embracing a certain amount of stuff in our frame, and if the perspective effect we want is obtained at a greater distance (than in some other situation), then we are led to use a lens with a larger focal length (than in that other situation) to effectively fill the frame with the goods - and only for that reason.
Focal length gets into the deal in another indirect way. The perspective effect given by a viewed image will be the same as we would experience in "live" viewing of the scene, from the same location the camera was, if the angular size of objects in the print is the same as they would have had to the eye during the "live" viewing.
To attain this requires some attention to the actual size of the presented image (the dimensions of a print, for example), and the presumed distance from which the image will be viewed. And calculating this involves the focal length of the lens (as well as the image dimensions).
Now to the issue of why a certain focal length is considered "normal" for a certain format size. We hear several stories.
One is that this focal length gives a field of view comparable to that of the human eye. Well, that really doesn't work out. Of course, the field of view of the eye is hard to define, given the complexity of peripheral vision. But if we accept any reasonable definition, that concept of the normal focal length doesn't match.
Another thing we hear (and this links to your inquiry) is that the "normal focal length" gives the same perspective (effect) as is given by the human eye. I think you can see from my discussion above that this is totally meaningless.
So why, for example, is a focal length of somewhere in the area of 40-50 mm considered "normal for a camera with a 36 x 24 mm format size (a "full-frame 35-mm" format)?
My guess is that when 35-mm photography came into widespread use (and considering fixed lens cameras, or the lens to be furnished with an interchangeable lens camera), manufacturers concluded that the widest range of situations would be accommodated for "consumer" users with a focal length in that neighborhood. Period.
It is even possible that their actual finding was biased by the fact that lens design might have been easier for a focal length of 50 mm than, say, 45 mm (which their surveys might have shown was superior). We note that in later years, fixed lens full-frame 35-mm cameras came to be commonly equipped with a 45 mm, rather than 50 mm, lens.
Now lets examine our usual concern with the interaction between focal length and format (sensor) size. Basically, the field of view of the cameras (and that is measured in angular terms) decreases as focal length increases (for any given format size) and increases as format size increases (for any given focal length).
Thus the same field of view is given by a 50 mm lens on a camera with a 22.5 x 15 mm sensor (a "1.6x" camera) and an 80 mm lens on a camera with a 36 x 24 mm sensor (a "full-frame 35-mm" camera).
When we say that a 50 mm lens, used on a "1.6x" camera, has a "full-frame 35-mm equivalent focal length" of 80 mm, we merely mean that the lens of interest, on the camera of interest, gives the same field of view as would be given by an 80 mm lens on a full-frame 35-mm camera.
Now, if we fell comfortable with the notion that a 45-mm lens is a "normal" lens on a full-frame 35-mm camera, then on a "1.6x" camera, a 28-mm lens would be equally appropriate to consider a "normal" lens.
But to circle back to the staring line, all this has nothing (directly) to do with perspective considerations.
