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:




Ultrawide 8-14mm 14-22mm
Wide 15-22mm 24-35mm
Normal 24-32mm 40-55mm
Moderate 50-60mm 80-100mm
Long 80mm+ 130mm+

(* 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.

The views and opinions expressed in this article are those of the author and do not necessarily reflect the views and opinions held by dpreview.com or any affiliated companies.


Total comments: 11
By becstanley (Apr 27, 2013)

WOW this is very helpful! Thanks for your advice!!

Port Royal Dad
By Port Royal Dad (Jun 28, 2012)

Nice Article WilbaW. Well written, great visuals to support the ideas. You've certainly given me something to think about and try using.

Regards, Mike

By WilbaW (Jul 1, 2012)

Thanks. Let me know how you go.

Dima R
By Dima R (Jun 21, 2012)

:) It is interesting, that something similar to this is used by snipers, or more generally in artillery. They use objects available (hands, pens, cloth buttons, etc...) to determine the approximate distance to the target.

By WilbaW (Jul 1, 2012)

Yeah, that makes sense and works well with a standard target.

By carnaga (May 30, 2012)

So far I have used an old slide-film frame to find proper composition with ease, but I have never heard that you can use your arms to measure focal lengths: you are always carrying this awesome tool with you and it's really easy to use ;)

Thank you so much WilbaW!

Comment edited 2 minutes after posting
By WilbaW (Jun 3, 2012)

You're very welcome. :-)

By janzu77 (May 26, 2012)

Nice one, never though about using arms like that to evaluate the view, thanks Wilba!

By WilbaW (May 28, 2012)

No worries, have fun with it. :-)

By AvyMan (May 24, 2012)

Thanks for this useful alternative, Wilba. I haven't tried it yet, but wondering off the top of my head if it works equally well for any subject distance. It must. I'll give it a shot...so to speak.

By WilbaW (May 24, 2012)

Yeah, the distance to the subject doesn't matter, since we're only working with angles of view. I'll update the article with something about that. You also made me realise I should say something about closing one eye. Thanks a lot for your help Nick.

Total comments: 11