measuring for filter thread

I have an older lens (Goerz Dogmar 16.5cm) that I'd like to figure out the filter thread for (and a couple of others as well.) I have a set of digital calipers so I can measure the diameter of the inside of the thread. Is that all there is to it, or do I need to account for the depth of the thread? If so is there a common depth that I can just add to the measurement? Sorry for the really simple question, but a search didn't find the actual filter size, or how to do the measurement reliably. Its around 40/41mm, and I don't have anything existing in that range to try screwing on.

There’s a different approach that is often both easier and more correct: slip-on series filter adapter. Have you considered that? Measurement would be the OD of the front lens element

Following on Brian’s suggestion, many old lenses don’t take screw in filters. The “threads” you see may be simply concentric grooves.

the OD of the front of the lens is 42mm. I assume they make slip-on to threaded adapters? How do the slip-ons "fit"? i.e. are they tight enough to support a square filter holder? Do they use rubber or cloth or something else to ensure a good grip? Aside from push-on LF lens caps, I have zero experience with push on filters.

I feel kind of dumb, but it just occurred to me to measure a lens with a known filter size. The closest lens was a Nikkor-H 85mm ƒ1.8 F-mount lens. It should have a 52mm filter thread, and the digital calipers read 51mm, so it sounds like the thread is .5mm deep? I don't know if this is consistent across different sizes, but it seems likely. So, at least so far, it sounds like the trick is to measure then add 1mm.

Quote:

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Originally Posted by abruzzi

the OD of the front of the lens is 42mm. I assume they make slip-on to threaded adapters? How do the slip-ons "fit"? i.e. are they tight enough to support a square filter holder? Do they use rubber or cloth or something else to ensure a good grip? Aside from push-on LF lens caps, I have zero experience with push on filters.

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The series adapters are aluminum and grip the lens with “fingers”. I’d recommend a series 6 or 7 size. The adapter can be a mm or so oversized and the fingers can be bent to fit the lens.

Best source of adapters, filters and shades is Tim Rice:

https://filterfind.net/Series_7_A.html

https://filterfind.net/series_6_A.html

This is a series 6 adapter I’m using on a 1906 Kodak. You can see the “fingers”.

Attachment 215934

Attachment 215935

The filters are unthreaded. They seat in the adapter and are locked in place with either a retaining ring or a shade.

Here’s another example. This is an old Gundlach Radar that isn’t set up for screw-in filters. I couldn’t get exact fit so went about 2 mm larger and bent the tabs for a perfect fit. Kodak recommended +|- 1 mm of bending but, if careful, a bit more can be done without breaking tabs.

Attachment 215936