aperature number vs f number

Jethro B-UK wrote:

Len Philpot wrote:

f-stops are merely fractions. For example, f/8 means the aperture (opening) in the lens is 1/8 of its focal length. f/11 is 1/11 of the focal length, and so on. It's just a ratio.

That's why as the number gets larger, the opening -- and amount of passed light -- gets smaller. If you'll do the arithmetic you'll see that an aperture (circle) that's twice as large as another one will admit 4x the light. Conversely, an aperture half as large will admit 1/4 the light.

I didn't know that.

Thanks for the tip.

In my other hobby of astronomy, we deal with f/ratios all the time. A 10 inch f/5 telescope has an aperture of 10 inches (254mm) and a focal length of 50 inches (1270mm). A 10 inch f/10 scope would have twice the focal length and therefore twice the magnification with the same eyepiece, but it collects the same amount of light. Focal ratios are static since almost never is the aperture variable on a telescope. It's the same relationship as with a camera lens, just a slightly different context.

"Faster", i.e., lower f number, telescopes are currently en vogue, but they're much more difficult (and expensive!) to make and are far more demanding on eyepiece / corrector design and quality.

In astronomy there's far less interest in DOF since everything is focused at infinity. But there's interest in things like true field of view, exit pupil, image brightness, etc.