Eyepieces - physical size

[bombassa]

Yes I'm a total beginner and my english is not too good so please have patience with me.

My wife gave me a cheap telescope this christmas and even though its very low quality i'm still enjoying myself.

But my question - it came with two eyepieces marked 'k' - kellner i suppose. They are marked with the size 6mm and 25 mm. Last night i was cleaning the 6 mm and thought 'thats actually some large 6 mm' and got my ruler and yes - the lens is physically 12 mm. The eyepieces marked 25 mm is 18 mm. So I suppose Im not getting it. I thought the size of the eyepiece was equal to the size of the lens but no. So what does the size actually relate to?

- J

[Chris H]

Hello and welcome to the forum. I hope I have this correct the 6mm and 25mm sizes of the eyepieces are the focal lengths of the eyepieces rather than the actual diameter of the glass.

[bombassa]

Ah so its the distance between lens elements in the eyepiece?

[Breakintheclouds]

No, it's the distance at which the lens brings light to a focus. It's easiest to see with a diagram - something like this: http://images.trustedreviews.com/images/article/inline/3973-FocalLength_diag.gif

If you shone a torch through your eyepieces and used them to focus the light onto a piece of paper as a dot, for the one you should find you're holding the paper 6mm from the lens and for the other, 25mm.

HTH

[bombassa]

Ok I learn everyday.

Thank you for your replies

- J