Is it possible to estimate (or calculate), in advance, the range of focal lengths you might need for shooting in a given scenario?

For instance, suppose you will be going to the zoo (with your APS-C DSLR) and you want to pack light, and assuming that you will be between 30ft(10m) and 60ft(20m) from your subjects, is it possible to calculate which zoom lens(es) to take to cover this range so that you can get wide angle to reasonable close-ups?

Is there a forumla (or rule-of-thumb estimatation method) that you can use?

  • \$\begingroup\$ Looks like you're new here. Welcome to photo.SE! \$\endgroup\$
    – Reid
    Sep 26, 2010 at 23:47
  • \$\begingroup\$ Yes, first post, though I was an early committer to the site and have just been lurking for several weeks! Thanks! \$\endgroup\$
    – seanmc
    Sep 27, 2010 at 2:15

4 Answers 4


If you know or can estimate the distances, use this equation:

Focal Length = Sensor Dimension * Distance / Scene Dimension

Where you match the dimension of the sensor and the scene. e.g:

Focal Length = Sensor Width * Distance / Scene Width

Note that the the advertised size of your sensor is typically not the width, height, or even the diagonal, which are the appropriate dimensions you can use. See this wiki article for the size.

For non-Canon APS-C, the width is 23.6mm. Say your scene / subject is 20m away and you want a width of 5m at that distance. You can use feet as it is just a ratio. This equation says to use a focal length of 93.6mm.

If you are out in the field already and want to know what lens to put on, it might be good to give yourself a good hand calibration:

Hold your fist in front of your face with your elbow bent at a right angle; use your knuckles to select the lens you want for the scene behind your hand

From newarts on PentaxForums

4knuckles:50mm, 2:100, 1:200.

The trick is figuring out where to hold your fist so the above rule holds. Try it a few times.

Take a photo of a scene with a 50mm lens (or look through the camera's viewfinder with a 50mm lens in place.) Put the camera down & hold your fist in front of your face such that the 4 knuckles just fill that scene height. Remember where to hold your fist next time you want to select a lens.

  • \$\begingroup\$ Super cool trick. I haven never heard of the knuckle trick! \$\endgroup\$
    – dpollitt
    Jan 18, 2012 at 14:57

For normal and longer lenses you can approximate the field of view

angle (radians) = imager size / focal length

Typical imager width is 36mm (full frame), 25mm (aps) 18mm (OM 4:3)

The reason for using radians is that something one meter (or foot) away and one meter (or foot) wide is one radian.

So if you want a picture of an object 1m wide and 10m away you want a field of 1/10 radian and so from the above you want a lens 10x the film (or sensor) size.

(it's not quite accurate enough for wide angle lenses)

  • \$\begingroup\$ So, on an APS sensor, your example would require a 250mm focal length? Interesting, if that holds, this is a very simple way to make a good guess. Thanks. \$\endgroup\$
    – seanmc
    Sep 27, 2010 at 2:22
  • \$\begingroup\$ yes, it's the same as the more complete answer by Eruditass. \$\endgroup\$ Sep 27, 2010 at 11:19

This doesn't exactly answer your questions, but Nikon has a cool focal length visualization tool that may help you.

The short answer on your question is yes, you can compute that with some trigonometry. I don't know the formulas offhand but it's nothing fancy if you remember your high-school trig. The other factor you need is the size of your subjects. What you would do is compute the necessary angle of view and then translate that into focal length (which you can find in your lens specifications or I'm sure there are many calculators on the internet).

  • 1
    \$\begingroup\$ I would give you +1 for the link but I don't have enough reputation yet to do that! Cool tool, though. Thanks! \$\endgroup\$
    – seanmc
    Sep 27, 2010 at 2:17
  • \$\begingroup\$ @seanmc you do now :) \$\endgroup\$ Sep 27, 2010 at 11:53

mgb is correct, but I wanted to make an additional comment.

A Focal Length Calculator is available, along with a more detailed explanation of how lenses work, at this website: http://www.cambridgeincolour.com/tutorials/camera-lenses.htm

It performs much like mgb's approximate rule of thumb, but gives more precise values and allows you to select different camera types. So if you are at a computer and feeling lazy or need to perform this type of calculation with greater (but not perfect) accuracy, this will help you out.


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