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Cyclop Coma
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PostPosted: Sun Sep 22, 2019 5:33 pm    Post subject: Cyclop Coma Reply with quote

A friend recently purchased a Cyclop along with its night vision device. It's the correct, original Cyclop 1.5/85 that gained all the attention some years back. When trying it out on his camera, he had difficulty focusing. I checked it out and determined it has nasty coma; the typical teardrop when viewing a small highlight. I've taken it apart, cleaned fully, and reinstalled all optics. It still has coma but the teardrop now points in a different direction, I believe indicating a bad element as opposed to a problem in the lens barrel where it seats.

I'm wondering if this has been seen before and if I could be missing something else that could be the problem. I have taken great care to ensure the lenses were seated properly.


PostPosted: Sun Sep 22, 2019 6:00 pm    Post subject: Reply with quote

If there is coma in the center of the field, this is a sure indication of decentering. Perhaps the machining of the mechanical parts, grinding of the optical elements themselves, or assembly of the parts have been poorly executed.


PostPosted: Sun Sep 22, 2019 6:35 pm    Post subject: Reply with quote

Gerald wrote:
If there is coma in the center of the field, this is a sure indication of decentering. Perhaps the machining of the mechanical parts, grinding of the optical elements themselves, or assembly of the parts have been poorly executed.


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PostPosted: Mon Sep 23, 2019 5:14 pm    Post subject: Reply with quote

Try this chart to test lens for decentering.
STAR CHART DECENTERED LENS CHECKER by anankephi, on Flickr


PostPosted: Mon Sep 23, 2019 7:19 pm    Post subject: Reply with quote

Thanks. What should I expect to see?


PostPosted: Mon Sep 23, 2019 7:38 pm    Post subject: Reply with quote

The way the elements fit into the lens body, there is no room for them to be decentered. Of course, the elements could be improperly ground. Nevertheless, we'll test, using that image. Thank you.


PostPosted: Mon Sep 23, 2019 8:17 pm    Post subject: Reply with quote

woodrim wrote:
Thanks. What should I expect to see?

You need to defocus image - you will see gray rings. It should be round circles, not ellipse.


PostPosted: Mon Sep 23, 2019 9:05 pm    Post subject: Re: Cyclop Coma Reply with quote

woodrim wrote:
I checked it out and determined it has nasty coma; the typical teardrop when viewing a small highlight. I've taken it apart, cleaned fully, and reinstalled all optics.

Either decentering or single element tilt.

woodrim wrote:
It still has coma but the teardrop now points in a different direction, I believe indicating a bad element as opposed to a problem in the lens barrel where it seats.

Pretty sure it's a decentering then.

woodrim wrote:
I'm wondering if this has been seen before and if I could be missing something else that could be the problem. I have taken great care to ensure the lenses were seated properly.

It takes a miniscule amount of movement to fix or introduce decentering.
That's why if you drop a modern lens, the one you've picked up from the ground is not the same as it was before.


PostPosted: Mon Sep 23, 2019 10:46 pm    Post subject: Reply with quote

I'm at a loss as to how it can be decentered. There are ledges for the elements to sit on and rings to be placed over them to secure in place. With the seating of each element, I checked for any slop or tilt. Might it be possible a tilt came from my securing the rings?

The lens is very difficult to focus. It's not just the appearance of coma but a rendering of the lens to be unusable. If I take it apart again, I'll first want to understand what I'm looking to do differently.

This lens came with the night vision device and hasn't shown any indication of having been opened before me.


PostPosted: Tue Sep 24, 2019 4:51 am    Post subject: Reply with quote

woodrim wrote:
I'm at a loss as to how it can be decentered. There are ledges for the elements to sit on and rings to be placed over them to secure in place. With the seating of each element, I checked for any slop or tilt. Might it be possible a tilt came from my securing the rings?

You can't check this by eyeballing.
Tilt won't change after re-tightening, since it's a consequence of the lens barrel machining.

Here is how collimated beam looks on a well centered lens:




Decentered lens:




Same lens, much better centered:




The difference between last two images is I was pressing my finger sideways on lens (uhh, gross) front block when tightening it.


PostPosted: Tue Sep 24, 2019 9:51 am    Post subject: Reply with quote

You can try marking the elements with a sharpie or small bit of tape at 12:00 positions, then try rotating each in turn to see if it follows a particular element or cell.
You might have to shim an element if the mating surface was machined at an angle.


PostPosted: Tue Sep 24, 2019 11:50 am    Post subject: Reply with quote

Lightshow wrote:
You can try marking the elements with a sharpie or small bit of tape at 12:00 positions, then try rotating each in turn to see if it follows a particular element or cell.

Yup, that might just work.

On the internet I've heard stories "I just disassembled and reassembled it and it worked", but I'm not sure how lucky one has to be.
I've had a bit of a reverse luck: reassembled a perfectly centered lens and subtly fucked it up.
Methinks to have a high chance of success it's pretty essential to understand which lens is at fault.

Generally inferior design with individual unscrewable lens blocks comes in handy here:
you can test them individually to avoid messing with lenses in already perfect alignment.

Here the rear lens block (~3 lenses) is telling us "the princess is in another castle":




P.S I don't know how to properly test and correct for internal tilt.
If anyone does, I'd be happy to learn from you.


PostPosted: Tue Sep 24, 2019 2:31 pm    Post subject: Reply with quote

Some great information here. Thank you very much. I have a few things to try. This is very helpful and is great information for anyone in the future that might experience this problem. I'll report back my findings over time as I can get to it.