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02-22-2010, 04:58 PM   #1
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How to determine which focal legth will work for a scene?

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.

But, I want to hear from others as related to this topic.

Thank you

02-22-2010, 06:09 PM   #2
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Perspective is controlled by camera position and is independent of the lens used.
You could cut yourself a cropping tool out of a sheet of cardboard and figure out where to hold it for various focal lengths.
02-22-2010, 10:07 PM   #3
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Depends on camera

You mention using a SLR and looking through the lens. Don't know about all Pentax SLRs but my K20D does have a depth of field preview. It's switchable with Live View.

If you're not using a SLR depth of field becomes tricky. The smaller the sensor, the wider the depth of field at any given aperture. My Panasonic DMC-FZ35K super zoom has a much greater depth of field at minimum aperture of f8 than my SLR does at f22 That makes it great for insect closeups because I can get much more of the bugs in focus than I can with my macro lens. On the other hand, it makes a poor portrait camera because it's near impossible to get a very soft background.

Another issue is focal length. The shorter the focal length the greater the depth of field. At 18mm, just about everything from 30 ft out to infinity is in apparent focus. At 200mm, depth of field is just a few feet. That goes back to the point and shoot issue. Focal lengths are given in 35mm equivalents. I don't remember the actual focal length numbers but I think that the 27mm equivalent is actually 4 or 5mm. So we're looking at an multiplier of depth of field of at least 5.

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02-22-2010, 10:12 PM   #4
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That depends on what you mean by perspective.

QuoteOriginally posted by Wheatfield Quote
Perspective is controlled by camera position and is independent of the lens used.
You could cut yourself a cropping tool out of a sheet of cardboard and figure out where to hold it for various focal lengths.
The shorter the focal length the wider the field of view. A 50mm lens is more or less equivalent to the way people see things. Shorter than that perspective changes. With a 12mm lens, very close objects appear disproportionately large and the distances between things can seem much larger than normal. With a long telephoto the apparent distance between objects is flattened.

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02-23-2010, 12:35 AM   #5
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No magic tricks -- learn by doing. You can experiment with FOV and perspective with a zoom lens, but it's hard to judge DOF that way. DOF control IMHO is best done with an old manual prime, using its inscribed DOF computer -- decide what you want to be in focus, then set the aperture and hyperfocus accordingly. And as mentioned above, perspective depends ENTIRELY on distance from lens to subject.

Some of the greatest work of the film era was done with fixed 'normal' primes, with the photographer's only controls being position and aperture. Change perspective by moving in or out. Change angles by moving sideways, or up or down. Change emphasis by exploiting light and aperture and shutter.

No focal length is 'perfect' for any class of image. Look through some galleries here and you'll see, for example, splendid portraits shot with everything from ultrawide to long tele. Certain FLs and distances may be stereotyped for some specific purposes, but playing by those rules too often results in boring images. Sometimes boring is good, like ID photos. Yawn.

My suggestions:

1) Go to a public library, look at the Time-Life books THE CAMERA and LIGHT AND FILM and THE ART OF PHOTOGRAPHY, and all the rest of course. These go deeply into the craft of seeing. Eye and lens see differently. Learn how and why.

2) Get some old primes, doesn't matter if they're super-crisp or not, just that they're not too slow nor too crusty. For an APS-C camera like most modern dSLRs, try 21-24mm (wide), 28-35mm ('normal'), 50-58mm (portrait), 85-105mm (tele), etc. Shoot a lot. Practice using the distance+aperture marks to strictly control DOF. And move around a lot. A LOT!! Don't change lenses -- use one lens all day, every day, for a week. The next week, another lens. Learn how they see. And learn how to judge distance, angle, perspective. Practice practice practice.

3) Outside, don't shoot when the sun is bright overhead. Restrict yourself to early and late, with light at oblique angles, or foggy-cloudy-drizzly days. Learn how light and shadow can round or flatten your subjects. Learn to exploit reflections and glare and gloom. And learn to look for shape and form and texture, not color.

Like I said, there's no magic tricks, just experience. Have fun.
02-23-2010, 07:12 AM   #6
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QuoteOriginally posted by mysticcowboy Quote
The shorter the focal length the wider the field of view. A 50mm lens is more or less equivalent to the way people see things. Shorter than that perspective changes. With a 12mm lens, very close objects appear disproportionately large and the distances between things can seem much larger than normal. With a long telephoto the apparent distance between objects is flattened.
Take a picture with a telephoto lens, then take a picture with a wide angle lens from precisely the same camera position.
Crop the matching section out of the wide angle picture and get back to me about how the perspective has changed.
02-23-2010, 07:44 AM   #7
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Thank you for all the replies.

Wheatfield, you are right of course with the crop thingy, as long as the camera is in the same spot the cropped scene from a wide angle will look the same as the image from the telephoto.

But, my visualization had to do with the missing part. The perspective I was talking about is the relationship of the missing part to the cropped scene. This relationship changes the entire look of the picture, and I wonder whether there was a way to imagine this.

Lets say I take a scene with the my 77mm and another with the 43. If I take it from the same spot and crop the 43 to an equivalent 77 FOV then the images will be identical. However, including the foreground in the 43 will push the cropped part of the image to the background and make it less significant for the overall scene. So, the whole look changes.

I was wondering whether there is a way to easily visualize this other than just knowing the general rules.

My feelings were it has to come with experience, but in case if there was another way, I didn't want to miss it.
02-23-2010, 08:02 AM   #8
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QuoteOriginally posted by pcarfan Quote
Thank you for all the replies.

Wheatfield, you are right of course with the crop thingy, as long as the camera is in the same spot the cropped scene from a wide angle will look the same as the image from the telephoto.

But, my visualization had to do with the missing part. The perspective I was talking about is the relationship of the missing part to the cropped scene. This relationship changes the entire look of the picture, and I wonder whether there was a way to imagine this.

Lets say I take a scene with the my 77mm and another with the 43. If I take it from the same spot and crop the 43 to an equivalent 77 FOV then the images will be identical. However, including the foreground in the 43 will push the cropped part of the image to the background and make it less significant for the overall scene. So, the whole look changes.

I was wondering whether there is a way to easily visualize this other than just knowing the general rules.

My feelings were it has to come with experience, but in case if there was another way, I didn't want to miss it.
A crop window really is the best way to learn this stuff.
Hold the card closer to your eye to mimic a wide angle, farther away to mimic a telephoto.
And use one eye only to mimic the non stereo view of the camera.

Regarding the perspective thing, you are talking about near/far relationships, which is somewhat not the same thing. If you are going to compare a wide angle to a telephoto and introduce a variable that one lens cannot reproduce, which is what mysticcowboy is advocating, you are not making a valid comparison and so have not come up with a valid result.

02-23-2010, 10:57 AM   #9
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QuoteOriginally posted by pcarfan Quote
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."

*

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
02-23-2010, 02:34 PM   #10
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My answer: experience. Live with a lens for a while and you can come to have a pretty good sense of how its FOV will look in any given scene before mounting the lens. But untily ou gain that experience, sure, you can use a physical device as an aid. Recently someone posted some rules of thumb (quite literally) for using your hand held out at arms length to visualize different telephoto focal lengths. All I remember is the one I had figured out on my own: 100mm is about the size of my first.
02-24-2010, 04:58 AM   #11
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Will,

Thank you for that extensive explanation and answer. What you describe as 'field of view' is what I was thinking when I posed the question. Anyhow, using just one prime is a lot to ask, but a couple should be fine for 90% of the shots. Thank you for the time.
02-24-2010, 05:01 AM   #12
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QuoteOriginally posted by Marc Sabatella Quote
My answer: experience. Live with a lens for a while and you can come to have a pretty good sense of how its FOV will look in any given scene before mounting the lens. But untily ou gain that experience, sure, you can use a physical device as an aid. Recently someone posted some rules of thumb (quite literally) for using your hand held out at arms length to visualize different telephoto focal lengths. All I remember is the one I had figured out on my own: 100mm is about the size of my first.
I would love to see that thread on how focal lengths relate to the size of fingers/hand at certain distance. If anyone remembers where this is, I would appreciate it.

Thank you.
02-24-2010, 05:08 AM   #13
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Well, I found this tool which shows what an image looks like at various focal lengths keeping the same distance, sort of useful, so thought I'd put it here.

Focal length comparison tool, Tamron USA

Last edited by pcarfan; 02-24-2010 at 07:28 AM.
02-24-2010, 07:26 AM   #14
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QuoteOriginally posted by WMBP Quote
If I'm shooting a classroom full of children
Here's one you don't want to get used out of context...
02-24-2010, 08:53 AM   #15
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QuoteOriginally posted by pcarfan Quote
Well, I found this tool which shows what an image looks like at various focal lengths keeping the same distance, sort of useful, so thought I'd put it here.

Focal length comparison tool, Tamron USA

Right. I've seen that page on Tamron's site before. The limitation of the page is that it doesn't—and of course probably cannot—take into consideration the camera's distance from the subject, in other words, where you're standing.

Where you stand—well, more precisely, where the camera is positioned relative to the subject—is extremely important, not just in a positive way, that is, as a reflection of how moving around changes the photo, but also in a negative way, that is, as a result of the fact that sometimes you simply cannot move around as you would like.

Will
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