**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][1] 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.  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][2]:  

> 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][3]

> 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.


  [1]: http://en.wikipedia.org/wiki/Image_sensor_format
  [2]: http://www.pentaxforums.com/forums/1106422-post6.html
  [3]: https://i.sstatic.net/Ys4JI.jpg