Measuring Focal Length, Aperture and F-Stop
You've probably figured it out by now, but let me spell it out, step-by-step. Put your camera on a tripod and point it
at an object with distinct edges. I like to use a bookcase. Now take two photos: one using your mystery lens, and one
using a reference lens with a known focal length. If your reference lens is a zoom, try to adjust it so that the object
appears to be the same size as it is with the mystery lens. If you are able to get a match, just look at where the zoom is
set. That is the focal length of your mystery lens.
However, if your mystery lens seems to be outside the range of your zoom, or if you are comparing it to a fixed (prime) lens,
then there are a few more steps. Choose the biggest object that is clearly defined in both images. I like to use the distance
between two shelves in my bookcase. Get your ruler out again because you will need to measure the size of this object as it
appears in each of your two photos. I usually do a crude measurement using the LCD on the back of my camera. A more accurate
measurement can be done by uploading them to your computer. Either way, just make sure you are comparing full size images shown
at the same size. The focal length of your mystery lens is calculated by multiplying the focal length of your reference lens by
the ratio of the two measurements.
Example: suppose your reference lens has a focal length of 50 mm. And suppose that when you measure the distance between
two shelves in the image taken with the mystery lens you get 35 mm, while in the image taken with the reference lens, the
distance between two shelves measures 25 mm. This means that your mystery lens has a focal length of 50 x (35 / 25) = 70 mm.
By the way, there's absolutely no reason why the above method can't be used with film. It just won't be as fast.