Objective focal lengths

I purchased an old Astro Optical scope off Ebay. It came with two Danchyka objectives. One was a bit damaged and the other is in better shape and apparently a replacement. Although both are marked with a focal length of 910mm (which is the fl of the optical tube), the newer objective will not come to focus. The older, damaged one will. When I hold the objectives at arms length together, the new one has more magnification factor by about 2x. Is this a longer or shorter focal length than 910?

Thanks
David E

Just measure it.

For shorter FLs I simply use a yard stick with a sheet of white paper stuck to the "bottom" end. I point the "top end" of the stick at the sun and move the lens along the stick until I get a sharp image of the sun on the paper. I then write down the number of inches from some mechanical feature on the cell that I'll use as a reference. When that's done I put out the paper fire.

Lately though, there has been no sun visible to point at.

Jeff

Maybe I don't quite get what your experimental situation is, here, but if you are holding two eyepieces at arm's lenth (about 910mm?) and they both come to focus, then they are both 910 mm focal length.

And, yes, the easiest way to tell is to measure it.

Turn a lamp on. Remove the lampshade if you like. Get about a yard or two away from a wall on the other side of the room, and hold your lens up. YOu should see a focused image of the lamp (or bulb) on the wall behind you. If it is not focused, move the lens forward and backward against the wall. When you get focus, measure the distance between the lens and the wall. THat is your focal length.

(Note that this is not an exact measurement, because the lamp is not a star at infinity, and things focus closer when they are at infinity. But, it gets you in the ball park.)

Alex