Measuring Focal Length, Aperture and F-Stop
By the way, adjusting ISO rather than shutter speed will work just as well. Doubling the ISO is equivalent to a one stop
aperture change. Therefore in the example above, increasing the ISO in the reference shot from 400 to 1600 should have
produced the same results as decreasing the shutter speed from 1/60 to 1/15 second. If you choose to do this measurement
using a flash (which I do not recommend), then adjusting ISO is your only option because adjusting shutter speed will not work.
This method gives you an approximate effective focal ratio which takes into account all the optical properties of the
lens including light loss due to glass and coating quality.
One final note: If you've built a working SLR/DSLR lens with a focal length that measures less than 15 mm or a focal ratio
that measures under f/ 0.75 , please let me know as soon as possible. ;-)
OK, back to the lab.