greg_the_maker
Posts: 6
Joined: Mon Jun 21, 2021 2:07 pm

Back Focus Adjustment Ring Thread Dimensions

Mon Jun 21, 2021 2:14 pm

As the subject suggest I am looking for the thread information for the Back Focus Adjustment Ring. I have been upable to find this anywhere.

Does anyone know what it is, or where I may be able to find out?

Thanks :)

Image

pidd
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Location: Wirral, UK
Contact: Website

Re: Back Focus Adjustment Ring Thread Dimensions

Mon Jun 21, 2021 3:36 pm

I believe it is CS-Mount so does 1" (25.4mm) 32 tpi (0.794 mmpt) measure about right? (1-32 UN 2A)

greg_the_maker
Posts: 6
Joined: Mon Jun 21, 2021 2:07 pm

Re: Back Focus Adjustment Ring Thread Dimensions

Mon Jun 21, 2021 6:20 pm

Nope. It's larger than that ~28.83mm.

In this image the internal threads are C-Mount which are the 1-32 UN.

Image

pidd
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Location: Wirral, UK
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Re: Back Focus Adjustment Ring Thread Dimensions

Mon Jun 21, 2021 7:22 pm

If you have a C-mount or CS-mount lens you can check if the tpi are the same by placing them against each other, if they are that will narrow the possibilities to imperial threads at least.

EDIT:


The adapter is C-mount to CS-mount, both C-mount and CS-mount are the same diameter and tpi. There must be an internal step in the adapter.

The internal thread of the focus ring is that size.

greg_the_maker
Posts: 6
Joined: Mon Jun 21, 2021 2:07 pm

Re: Back Focus Adjustment Ring Thread Dimensions

Mon Jun 21, 2021 8:56 pm

This isn't the adapter ring. It is the back focus adjustment ring. The adapter ring screws into this part.

The smaller internal threads are C-Mount, the external thread is the unknown one I am asking about.

It's labelled as the back focus adjustment ring in the image below.

Image

pidd
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Joined: Fri May 29, 2020 8:29 pm
Location: Wirral, UK
Contact: Website

Re: Back Focus Adjustment Ring Thread Dimensions

Mon Jun 21, 2021 9:37 pm

Ah, its the adapter and focus ring together?

As I previously struck out - check the external threads of focus ring against the adapters C/CS-mount thread by placing them together, that will tell you if it is also 32tpi or not, they look the same.

Being what looks like ABS its could be any custom size, 28mm filters usually have 0.5mm thread (I think) but there doesn't look that much amount of difference from the C/CS-mount thread.

greg_the_maker
Posts: 6
Joined: Mon Jun 21, 2021 2:07 pm

Re: Back Focus Adjustment Ring Thread Dimensions

Tue Jun 22, 2021 9:24 am

The threads seem to match as you suggest. However, I'm tending towards an M28x0.75 thread as the diameters closer matc a metric thread rather than an imperial one.

pidd
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Joined: Fri May 29, 2020 8:29 pm
Location: Wirral, UK
Contact: Website

Re: Back Focus Adjustment Ring Thread Dimensions

Tue Jun 22, 2021 11:56 am

Its hard to guess without using a proper thread gauge. 1 1/8" would be 28.575mm which is a common engineering size.

If you get a piece of paper and slide it along the threads while pushing hard you get the lines on the paper which may be easier to measure (or compare) but don't forget to measure at the correct angle, not quite perpendicular to the threads.

I used to have a 0.2mm graticule I would photograph next to threads, I lost it when I bought my latest sets of thread gauges which I thought would save me faffing around with a camera - in reality the graticule was the better method by far, even with just three threads exposed I got a convincing result, its not often thread gauges give a confident result with fine pitches on a small number of threads.

greg_the_maker
Posts: 6
Joined: Mon Jun 21, 2021 2:07 pm

Re: Back Focus Adjustment Ring Thread Dimensions

Tue Jun 22, 2021 1:02 pm

Just had it checked against a thread guage. Seems to be 32TPI / 0.0312" or 0.8mm.

Mtkelly04
Posts: 1
Joined: Tue Jun 22, 2021 12:33 am

Re: Back Focus Adjustment Ring Thread Dimensions

Tue Jun 22, 2021 4:52 pm

Yeah I agree that m28 x 0.75 seems reasonable but the 32tpi gauge fit extremely well. I also tried measuring the threads by distance change per revolution. With 7 turns I measured .21"ish which is within the measurement error I had been getting for it to be reasonable as 7/32. Though 0.75mm would be .206" which is also within measurement error but I feel like measuring under the true thread pitch is more likely than over. I'll have to recheck with an 0.75mm gauge but it's likely to be too close to be certain.

Benefit of M28x0.75 is you can buy a thread gauge to check. I wasn't able to find anything on 1-1/8-32 that you could use, and only found one mention of it in use on a laser head

greg_the_maker
Posts: 6
Joined: Mon Jun 21, 2021 2:07 pm

Re: Back Focus Adjustment Ring Thread Dimensions

Tue Jun 22, 2021 9:17 pm

I think we need someone who knows to give the actual information. Short of that it will mean cutting a load of threads and seeing what fits....

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