Originally Posted by peterh
Also true. But the EFFECTIVe focal length (determined by the actual mag number) is only one parameter. The opening (the huge, or not-so-huge, chunks of glass at the front) is another, and it is equally important, so it makes no sense to leave that out.
As I said, the combined effects of the parameters are enough to make hardened criminals break into tears. There is so much crud involved, it's not funny. Take two identical scopes, forget to coat a few lens surfaces in one, or economise on the coating of all of them, and the resulting differenceis are nothing short of dramatic.
Peter, the focal length isn't effective or derived or determined by a mag number. It's a physical property of a lens due to it's design. It's the distance the lens focuses at when presented with light with parallel rays. It's thus that which thus determines the mag, not the other way around. And the mag is changed by the ocular's lens in proportion to that.
Here's Celestron's page... ignore the bit about the barlow as that's an additional lens like the lens doublers you can get for rifle scopes. What's important is that you can see the mag is the derived number, not the focal length.
We change ocular lenses in stargazing so we can 'zoom' into narrow field objects such as galaxies or stars, or far planets, or go a lot wider for things like the moon or sun. From memory my scope has a 24mm ocular or a 12mm ocular, but the focal length of the scope's objective is 2000mm... so it's like a 100x or 200x scope depending on what ocular I use. The mag doesn't drive the focal length, the focal length drives the mag.