Measuring Focal Length, Aperture and F-Stop
Now what about f-stop? Once again, I prefer to measure the effective
focal ratio (F-stop) of a homemade or mystery lens by comparing it to a reference lens.
Use a reference lens with a focal length that is as close as possible to your mystery lens. Mount your camera on a tripod
and point it at a scene in which the light level is not changing. I generally do this inside, under artificial light.
Starting with the mystery lens and the camera set to manual, adjust the shutter speed (and ISO if necessary) to get a
correctly exposed photo. Now, leaving the shutter speed and ISO unchanged, try to get a matching photo using your reference
lens by adjusting only the aperture. The histogram is very helpful. Keep in mind that if you are testing a homemade lens,
it probably has much lower contrast than your reference lens, which will cause your histogram to be "scrunched
together." You're looking for an approximate match of the centers of the curves. Just as when we were testing
focal length, there are two possible outcomes. If you find an approximate match, you're done. Just read off the f-stop
from the reference lens.
However, if your mystery lens is outside the aperture range of your reference lens, (most likely faster), then
you're not quite done. I frequently have this case when I am using that nasty kit lens as my reference lens. The trick
is to adjust the shutter speed or ISO until you have the closest match and then use this as an offset to the aperture reading
on the reference lens.
Let's take an example: Suppose that the image from the mystery lens is brighter than the reference lens even when the
reference lens is at its widest aperture (lowest f-stop). Suppose that the shutter speed and ISO (for both the mystery
lens shot and the reference lens shot) is 1/60 second at ISO 400. Let's assume that your reference lens aperture is f/ 5.6.
You notice that when you change the shutter speed of the reference shot to 1/15 second, you get an approximate match in image
brightness between the mystery lens image and the reference image. Since each time you change the shutter speed by a factor
of two is equivalent to a 1 stop change in aperture, changing from 1/60 to 1/15 is a two stop difference. So your mystery lens
is two stops faster than f/ 5.6, which is f/ 2.8. (A one f-stop change is a factor of 1.4. Standard f-stops are 1, 1.4, 2,
2.8, 4, 5.6, 8, 11, 16, 22, 32.)