Quote:
Originally Posted by barx1963
I could be wrong, but my understanding is that they are exactly the same, focal length divided by aperture. The main difference is that in most cases, the f ratio is fixed in a telescope, while it is variable in a camera lense.
There is a discussion here https://en.wikipedia.org/wiki/F-number
Malcolm |
f-number is exactly the same for a telescope or a camera lens: Focal Length divided by Aperture. As you say, camera lenses typically have a variable f-number, by using an adjustable diaphragm to reduce the aperture (and zoom lenses also have an adjustable focal length).
What differs is the common usage of a single measurement to describe a camera lens versus a telescope. A 200 mm camera lens has a
focal length of 200 mm; if it is f/4, this implies a maximum wide-open aperture of 50 mm. A 200 mm telescope has an
aperture of 200 mm, and will typically have a focal length somewhere between 1000 mm (e.g. an f/5 Dob) to 2000 mm (e.g. an f/10 SCT).