I'm Struggling To Understand What Focal Length Is ?

[KingAndromeda]

The title pretty much says it all, I'm trying to give a definition of it and compare the focal length of a refractor f/7.5 and reflector f/5... could someone help me out?

[Bizibilder]

The focal length is the distance between a lens and the image it forms (or mirror and image it forms). Aparture has no bearing on it. A lens of 1000mm focal length produces an image 1000mm away.

If it happens to be a lens of 100mm aparture then the focal ratio is 1000/100 = 10. We call that an f/10 lens. An f/7.5 refractor of 100mm aparture produces an image 750mm from the lens. An f/5 refractor of 100mm aparture produces an image 500mm from the lens. Scopes of these sizes would have tubes about 700mm and 450mm long respectively. This is a bit shorter than the focal length so the image is formed inside the focusser. The eyepiece itself focusses on this image.

[Steve Ward]

Try this . . . http://en.wikipedia.org/wiki/Optical_telescope#Focal_length_and_f-ratio

[JamesF]

Given a focal ratio, the "f" represents the focal length, so given an f/7.5 scope the aperture is f, the focal length, divided by 7.5 whereas for an f/5 the aperture is the focal length divided by 5. The other way around, the focal length is the aperture multiplied by 7.5 in the first case, or 5 in the second.

In real terms, with a simple system involving only one optical component (lens or mirror), the focal length is the distance from that optical component to the point at which light from it comes to a focus.

James