8mm Forum: Focusing using a scope lens

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Posted by Ben Zotto (Member # 6619) on September 18, 2019, 05:03 PM:

Hi all, I have been futzing around with a scope lens occasionally and there's something that puzzles me-- that lens has a focus ring on it (marked distances in meters). I'm not sure when, if ever, or how, to adjust that, and how it relates to the primary focus on the projection lens.

Here's what I do now:
1. Mount/line up the scope lens in the correct orientation (axis) in front of the projector.
2. Twiddle the projector zoom lens outer ring to get the picture size I want.
3. Twiddle the focus on the projector until picture appears sharpest.
4. That's it. I don't mess with the focus ring on the scope lens.

But I have a nagging feeling that I'm probably missing out, or at the very least don't understand the optics interplay.

Can anyone bullet-point me the correct order of operations of setting up and focusing when also using a scope lens? Thanks!

Posted by Nantawat Kittiwarakul (Member # 6050) on September 18, 2019, 07:35 PM:

And here's the only steps you had missed out. [Wink]

3.1 After you get the sharpest focus from the main projector lens esp. horizontal line (look for sharpest possible frame line for example).
3.2 Then swing the scope attachment in. Adjust its focus ring until you get BOTH sharpest possible horizontal and vertical lines.

The scope attachment, in principle, will alter ONLY the vertical sharpness of the projected picture. That's why it is necessary to make the horizontal plane sharpest first.

PS this is probably grossly/overly simplified explanation&process, but should be good enough to get the job done, for now. [Cool]

Posted by Leon Norris (Member # 3151) on September 19, 2019, 09:02 AM:

The best scope lens is the one that fits in the lens barrel! Once its in there just aim and focus! There is no playing around!

Posted by Ben Zotto (Member # 6619) on September 19, 2019, 01:18 PM:

Thank you Nantawat! That is precise and helpful. Will give it a shot next time I set up for scope. Would love to see the high-school-physics-class optics diagrams to show what's going on in this lens combo. Thanks.

(@Leon fair enough maybe I will just hot-glue-gun the lenses together! [Wink]

)

Posted by Leon Norris (Member # 3151) on September 19, 2019, 01:51 PM:

Ben, they did make a scope lens that fitted in the Elmo machines! It worked out a lot better! The films were a lot brighter!!

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