View Full Version : Measuring thread size.

Kirk Gittings
29-Mar-2012, 10:49
This is probably a no brainer, but not for me. I have an old B&L petzval that I want to put ND filters on to slow down the exposure. So I'm looking to buy a step up adapter. Do you just put the caliper on the inside measure the inside diameter and the reading you get is the size?

John Jarosz
29-Mar-2012, 11:03
Threads are spec'ed by measuring the outside diameter of a male thread. So if you are measuring the ID of the female thread, then I would add .060 (.030 per side) to your measured diameter. This will give you a starting point. Plus, you don't know the pitch of the thread yet (Threads per inch). Is this an old lens? Maybe if you say what lens it is someone here may already know.

Kirk Gittings
29-Mar-2012, 11:07
Thanks. Its an old (not super old) B&L Projector lens.

John Jarosz
29-Mar-2012, 11:23
It may also be Imperial threads rather than metric, which is what most step up rings are. You may be faced with simply going to a store so you can try on rings and use what fits. Or, you may have to kludge a square filter holder that could be attached to the outside of the barrel.

Brian C. Miller
29-Mar-2012, 12:00
You can make your own filter adapter, if the lens has enough thread on the end of it.

Use a ruler to get a basic measurement. Then buy a cheap filter a bit smaller, and take out the glass. Wrap a bit of plastic or rubber tape on the thread edge, just enough to give the threads on the lens some bite, and gently screw it in. Now you have a standard filter size you can use to screw something else into it.

Bob Salomon
29-Mar-2012, 12:13
Is this the lens brom the B&L that accepted single 2 1/4 slides as well as tray fed 35mm slides? I believe that one had an adjustable diaphragm on it to adjust brightness. But it has been so long since I handled one, late 50s early 60s, that I don't remember the lens having screw threads.

Kirk Gittings
29-Mar-2012, 12:45
Thanks Brian. Thats a great idea.

Bob, hell if I know, but I think its a movie projector lens. it says B&L Cinephor Series 1 on it. It is a chrome barrel and black cellover brass so not that old-uncoated though.

Kirk Gittings
29-Mar-2012, 12:50
So I get 46.59 on the inside so adding .060 gives me 47mm ish. No such animal out there right?

Bob Salomon
29-Mar-2012, 13:57
Different lens.

Bob Salomon
29-Mar-2012, 13:59
Heliopan makes 46 and 48mm filters. Why not measure the outside diameter and see if a 48 or 49mm would slide over it tightly enough? If necessary a layer of tape or Velcro loop material might make it a tight enough fit.

Kirk Gittings
29-Mar-2012, 15:22
I'll try that. Thanks.

Jim Galli
29-Mar-2012, 15:32
Get a 55 - 62mm stepping ring. Remove material until it is a nice tight friction fit on the O. D. of the Cinephor. Tap it on. Add a drop or 2 of super glue if that makes you even happier.

Michael E
29-Mar-2012, 15:40
Add a drop or 2 of super glue if that makes you even happier.

It doesn't look as neat, but hot (melt) glue comes off metal without a trace.


John Jarosz
29-Mar-2012, 16:29
So I get 46.59 on the inside so adding .060 gives me 47mm ish. No such animal out there right?

Hmmm. My bad. I was assuming you were measuring in inches (because it's probably an imperial (inches) thread (but I'm guessing). The height of the thread form is about 3/4 of a mm (.030"), so I'd add 1.5 mm to your measurement which should be more like 48mm.

I like the hot glue idea. It will pop right off when you're done

31-Mar-2012, 10:20
Kirk here is something I keep handy to help me with treads on lenses ,
or I could go the supply house for machine shop equipment and the steel thread indicators

Kirk Gittings
31-Mar-2012, 10:48
That's very cool Lauren.

Leonard Evens
31-Mar-2012, 11:03
I don't know if it will help, but there are tools called thread gauges to measure thread sizes. Try google to see if you find something which might be useful.