Have you ever asked or been asked; “What focal length should I use (for this picture from where I’m standing)?” It is a common question asked by many people as they take their first steps into photography. As an answer; “It depends.“, doesn’t really cut the mustard but if you wear an analog wrist watch then the real answer is staring you in the face!
Caveat: This article assumes you are using lens designed to cover a 24x36mm sensor size (what used to be 135mm and is now sometimes called FX size). If not, then you need to check your camera’s manual for the “crop factor” and adjust the results accordingly.
But first a little background. Every lens has a well defined Angle or Field of View (AoV/FoV or coverage). Dig out your lens specification sheet or check on the www for the coverage of your lens. Caution; some lens manufactuers quote the AoV as the diagonal of a 24x36mm sensor instead of its horizontal (azimuth) coverage. Tricky, but a little bit of basic geometry using the len’s Focal Length (FL) and the FX (24x36mm) sensor size will do the trick. Or you could use this calculator to create a table for you:
http://www.howardedin.com/articles/fov.html Scroll down to the Field of View Calculator.
Drummed into us from an early age is that a circle can be divided into 360 degrees. With that “fact” we can do interesting things, such as divide a pie in various ways by using degrees as our measure. Your analog (sorry digital users) watch has a face that shows a “circle of time”.It is normally divided into 60 minutes. A quick bit of division shows us that one minute of clock time must equal 6 degrees of circle angle. And most of us have learnt one way or another that 15 minutes is 90 degrees, half an hour is 180 degrees and 45 minutes is 270 degrees. All useful stuff but we need to refine things a bit. It is easy to see that each 5 minute period of time equates to 30 degrees of angle and so on.
How does this help me calculate the focal length to use?
Easy! First, point your watch’s 12 O’clock at the subject of your shot. Next, decide what is to be the left side of the frame and imagine the minute hand pointing at the spot. Remember the minute number the hand is over, e.g. 5 minutes to the hour. Now decide on the right side of the picture and again, imagine the minute hand pointing there, e.g. 7 past the hour.
Some of you have no doubt raced ahead and figured this out but for the rest of us the final step is to add the two “minute” values (5 + 7 == 12) and multiply by the magic number 6, giving 72 .
And there you have it, the horizontal (azimuth) angle of view is 72 degrees. Now you pick the lens that has this angle/field of view or the nearest coverage and blaze away. Of course, you may have to walk back or forward a little if you don’t have a lens with the exact coverage, but that is good, no? And zoom lens users have it easy too 🙂
With the almost universal use of smart phones you can create a table of your lens’ coverages and keep it as a Note or construct a little diagram like this.
If your specific FL is not shown you can interpolate as it is an arithmetic progression. For example, if you have a 100mm lens then it is half of the 50mm len’s 43° Angle of View, giving 21.5° but let’s not get too fussy here. 20° s is close enough and that is close enough to 3.5 minutes on your watch face “compass”!
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