measuring for filter thread

[abruzzi]

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.

[BrianShaw]

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

[Keith Pitman]

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

[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.

[abruzzi]

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.

[BrianShaw]

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.

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”.

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[BrianShaw]

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

[BrianShaw]

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.

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