Originally posted by pcarfan
Is there a trick or tool that will tell me, without having to mount the lens, on what a scene will look like at 15, 21, 24, 31, 43, 77, 90, 135 and so on ?
There are general rules, but is there a way to actually visualize it ?
OR is it theoretically impossible due to perspective. It is easy to design a simple tool for FOV, but I don't see how perspective changes can be done without actually seeing through the lens as in a DSLR. I guess that is the point of owning a SLR.
Let me try to sort out the variables here.
First, there's
angle of view, also known as field of view or field/angle of vision. If your position relative to the subject is constant, changes to lens focal length will change your field of view, and that will result in the picture including more or less of the scene.
Second, there's
perspective. If you keep the focal length constant, but actually move—with your feet—you'll change the "look" of the subject in the resulting photo.
Now, there is a third variable here that is often omitted from these discussions, because it's not really a technical variable at all, yet it's one of the most important aspects of the photo you take, indeed, this third variable IS the photo you take. It's the result of the combination of your field of view and your perspective, that is, of your focal length and where you stand. I don't know if it even has a technical name. I'd like to call this "field of view," as distinct from "angle of view," but unfortunately those two terms are generally used as if they were synonyms. What I'm thinking of here is the amount (for lack of a better word) of the subject that you include in the photo. I'll call it "
subject coverage." If I'm shooting a classroom full of children, I can cover the whole room, from the students at their desks to the teacher at the board, using a wide angle lens. Or I can cover a single student with a telephoto lens. The reason subject coverage has to be mentioned here is that it is what we are usually most concerned about.
If you're shooting with a prime lens and you can't or don't want to change your focal length by changing the lens, then subject coverage is more or less the same thing as field of view, or more precisely, they're the two ends of the same thing. Field of view is the camera's end, subject coverage is the subject's end. But if you're shooting with a zoom lens, well, it's not so simple.
What's easy
Now, field of view is fairly easy to visualize, roughly, with practice. When I started in photography a very long time ago, I shot for years with cameras that had just one lens and just one focal length. I remember that it was pretty easy for me to figure out where I needed to position the camera in order to get a particular field of view. Shooting with zoom lenses really confused my ability to see field of view without looking through the camera's viewfinder, but now that I've gone back to shooting mostly with prime lenses, this ability is coming back to me. Without looking through the camera's viewfinder, if my position is a given, I can pretty accurately predict what I'll see with the three focal lengths I use most often: 28mm, 40mm, 70mm. Or if the focal length is a given (because I have mounted a lens on the camera and I can't or don't want to change the lens), I can pretty accurately predict where I need to stand in order to get the field of view I want. I still fine-tune the framing through the finder, but I can position myself pretty accurately without raising the camera to my face. It's an essential skill, I think, especially if you shoot with primes. But it isn't anything special. Anybody can do it; it just takes a little bit of practice.
Perspective is even easier to visualize than field of view. LOOK. What you see is (roughly) what you will get. The main problem with perspective is that we notice it more in photos than we do with our naked eyes. Stand in front of a tall building and look up. Unless you're trying to think like a camera, you probably do NOT notice that the sides of the building appear to get closer higher up. We don't notice this because our brains adjust for perspective. That's precisely why perspective was a great technical DISCOVERY in the Renaissance. It took a long time for human beings to become conscious of what they'd been seeing since forever.
What's hard
How these variables interact depends on which ones you modify. As I said, if you've already committed yourself, say, to a 28mm focal length by mounting a 28mm prime lens on your camera, your field or angle of view is fixed, and the only way to change the subject coverage is to MOVE. But that changes your perspective.
On the other hand, with a zoom lens, you can change the subject coverage by zooming in or out, without changing the position of the camera. Since you're not moving the camera, perspective doesn't change. Since you're changing the focal length, field or angle of view does change.
Here are three photos that illustrate these ideas. Here's a shot taken at a focal length of 17mm, looking across my (messy) dining room and living room.
In the photo above, notice the far wall, from the print hanging on the left, to the print hanging on the right above the bench. Here's a view with that horizontally-defined subject coverage. Focal length = 45mm. As you can tell, I've moved forward a little to take this shot:
Now here's more or less the same horizontal subject coverage, taken at 17mm. Obviously I had to move much closer to the focal plane (the wall):
Between them, these three pictures illustrate the interplay of the three variables I mentioned at the top: field/angle of view, perspective and subject coverage. The first and third photos were both taken at 17mm, so have the same angle of view, but obviously have dramatically different subject coverage—illustrating that angle of view and subject coverage aren't the same thing at all. The second and third photos have the same horizontal subject coverage, but fairly dramatically different angles of view AND different perspectives, causing one of them (the third photo) to show pretty clear "perspectival distortion."
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How do you internalize all of these variables? I find it hard to do when I'm shooting with a zoom lens and all three variables are in play. It's relatively easy to do if you go completely old-school and allow yourself just one lens, with a single focal length. Then angle of view is no longer really a variable at all. Shooting with a couple of prime lenses puts the level of difficulty somewhere in between and this is where experience really comes in. If I'm shooting with two cameras hanging around my neck, one with (say) a 28mm lens and the other with a 70mm lens, I know from experience which camera to grab to take the shot I want to take, and I know whether I have to move or not. And because focal length is, for me, a much simpler variable (since I don't have an infinite number of prime lenses and usually only have one or two at hand), I can concentrate much more on perspective and subject coverage.
Will