Measuring thread size.

[Kirk Gittings]

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]

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]

Thanks. Its an old (not super old) B&L Projector lens.

[John Jarosz]

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]

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]

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]

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]

So I get 46.59 on the inside so adding .060 gives me 47mm ish. No such animal out there right?

[Bob Salomon]

Different lens.

[Bob Salomon]

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.