8mm Forum: Focusing using a scope lens

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Topic: Focusing using a scope lens

Ben Zotto
Film Handler

Posts: 33
From: San Francisco, CA, USA
Registered: Sep 2018

posted 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!

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Nantawat Kittiwarakul
Master Film Handler

Posts: 280
From: Rajburana, Bangkok, Thailand
Registered: Aug 2017

posted 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]

--------------------
Just a lone collector from a faraway land...

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Leon Norris
Jedi Master Film Handler

Posts: 958
From: Elkins Park, PA, USA
Registered: Jun 2012

posted 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!

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Ben Zotto
Film Handler

Posts: 33
From: San Francisco, CA, USA
Registered: Sep 2018

posted 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]

)

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Leon Norris
Jedi Master Film Handler

Posts: 958
From: Elkins Park, PA, USA
Registered: Jun 2012

posted 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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