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Is the 1.25" EP filter thread 30.5mm?

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Hi All, as per the title - is the 1.25" EP filter thread 30.5mm?

The reason I'm asking is I have a photo Tamron 500mm f/8 mirror lens (a something-Cassegrain derived design) and a 10mm 45 deg angled prism EP adapter (the EP is built-into the adapter and can't be swapped). The lens has a rear filter thread (inside the mount) of 30.5mm (I don't know the thread pitch) and comes with a selection of filters (red, green, blue for monochrome film, and a Neutral Density) - I understand a filter must be used to maintain the optical path correctly - there's a Skylight filter in there at the moment.

On the assumption the manufacturer would have picked a commercially available thread size - I presume it must be pretty close. It would be good to experiment with Astro EP filters in the lens but access is rather fiddly and I don't want to risk cross-threading anything unless I'm convinced it will work. There is a photo filter thread on the front, but at 82mm, good filters are very expensive. As the lens has no photo aperture control, the moon is very bright and being able to use a variety of lunar filters would help.

I have considered trying to attach an astro 1.25" EP holder & star diagonal to the lens mount, but the very short register of about 45mm suggests there isn't enough room for infinity focus.

Any suggestions gratefully received.


Edited by johnha
Title corrected

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Unfortunately no.

the 1.25" filter thread is a 28.5 x?? Different suppliers seem to have slightly different thread pitches- around 0.5 mm or an imperial equivalent.


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Thanks Guys,

The link is really helpful - I'm using Pentax lenses (the Tamron has a swappable 'Adaptall-2' mount - perhaps allowing slightly longer lens register). The Kenko 'Lens2Scope' is the one I have for Pentax - nice robust piece of kit but lacks the ability to change the EP or use filters etc. I can 'Barlow' the lens by adding a 1.4x or 2x photo tele-converter between the two.

So given I have a lens register of about 45mm, in order to keep infinity focus, I believe I need the EP lens at (or closer) than that (i.e. this has to include the distance through the diagonal) - does this sound right?

Thanks for the suggestion RE the 20.5mm step-up - I'll have a think about that.


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