What Focal Length Do I Need for a Particular Shot?
When you see a scene or subject you want to shoot with your interchangeable-lens camera, how do you know which of your lenses to use? Experience? Instinct? Look through the viewfinder and change to a longer or shorter lenses if the one on the camera doesn't suit?
I guess some people judge the "range" in which the scene fits, and think their way through it with a table something like this:
(* Those ranges, their names, and the focal length values are shown just to illustrate the idea of a lookup table in personal memory. No claim is made that these values are in any way definitive, universal, or even typical. No correspondence will be entered into. :-))
Here's another way that's simple, takes a few seconds to execute, and doesn't require any equipment or technology other than arms and hands. I'll illustrate with focal lengths for an APS-C camera. This image was taken at 15mm and shows the approximate framing you would get for three particular focal lengths.
|17, 50, and 300mm framing on APS-C (click on the image to see it full size).|
I've chosen to show the 17mm frame because it corresponds with a standard wide angle view, such as you get with an 18-55mm kit zoom, "upgrade" third-party zoom such as a Sigma or Tamron 17-50mm, or premium zoom such as the Canon EF-S 17-55mm f/2.8.
50mm is very handy for Canon shooters with the "poverty trio" of EF-S 18-55mm, EF 50/1.8, and EF-S 55-250mm.
My longest zoom is 300mm, so that's a handy one for me to judge how much I could fill the frame with a distant subject.
The trick is to use your arms and hands to cover the width of each frame as you look at the scene with the naked eye (not looking through the viewfinder).
If I hold my arms straight out and parallel like a sleepwalker, turn my hands out as far as I can, then look with a single naked eye, fingertip to fingertip spans a wide-angle frame.
|Arms parallel, hands turned out = 17mm framing on APS-C, or 28mm on full-frame (click on the image to see it full size).|
I had to stand next to the camera to take that shot. When you do it, of course your eye will be looking from between your arms rather than off to the side.
If I extend my arm and spread my fingers, thumbtip to little-fingertip spans a 50mm frame.
|Thumbtip to little-fingertip = 50mm framing on APS-C, or 80mm on full-frame (click on the image to see it full size).|
And if I extend my arm, across the peaks of three knuckles spans a 300mm frame.
|Three knuckles = 300mm framing on APS-C, or 500mm on full-frame (click on the image to see it full size).|
You will have to find what works for you and your body dimensions. There are many combinations you can use, for instance, two fingers together, peace sign, four knuckles, devil horns hand, four fingers spread, middle finger sideways, four knuckles plus thumb extended, two fists together, thumbs of parallel arms, or two spread hands together.
You can use this technique with any combination of focal lengths and formats you like. For instance, if you have a 24-70mm and a 70-200mm, you'd find poses for 24, 70, and 200mm, and if you use those lenses on APS-C and full-frame bodies, you need a set of poses for each format.
It doesn't matter how far away the subject is since you are working with angles, just comparing the angle subtended by the subject with angles subtended by your hands in various poses.
I use it all the time. Don't be surprised if you make some new friends while you're out practicing.