Originally Posted by RobF
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.
But here comes the headache.
This is all irrelevant, because (as I remember mentioning earlier) the true depth of focus of a lens system... IS NOT DETERMINED (directly) BY THE TRUE FOCAL LENGTH! The real depth of focus is only determined by the lens opening, relative to the focal length.
And yes, I know that a 200mm tele at 2.8 gives a much shallower PERCEIVED depth of focus than a 24mm at f2.8, but that is the PERCEIVED depth of focus.
I will see if I can find something that describes this elsewhere, because I really think that this would go way beyond the scope (hmm... pun not intended) of this forum. For us, it is really enough to understand that, the higher the magnification, the shallower the perceived depth of focus, and the easier the rangefinding.
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