# Study of lenses

1. May 9, 2012

### moatasim23

I am a bit of confused with the following terms involved in the study of lenses:
Diameter of lens
Aperture of Lens
Are these terms synonomous?
And how are they related to focal length of a lens?

2. May 9, 2012

### Andy Resnick

Re: Lenses

'diameter' and 'aperture' are often interchangeable, and they are unrelated to the focal length. The ratio of focal length and diameter is given by the f-number or the numerical aperture.

3. May 9, 2012

### HallsofIvy

Re: Lenses

The "diameter of a lens" is measured from one edge of the lens to the opposite, through the center. It is, in other words, just the "diameter" of the circular edge of the lens. The "aperture" is the opening that allows light through and, as a single number, refers to the diameter of that opemimg. That can be as large as the diameter of the lens but most devices using lenses, telescopes, microscopes, cameras, have a "shutter" that allows you to close off part of the lens, giving an aperture that is smaller that the physical diameter of the lense.

The "focal ratio", or "f-stop" on a camera, is the focal length of the lens divided by the diameter of the aperture. Reducing the size of the aperture increases the "f-stop".