Mir-20 and Zenitar 16mm rear filter, optical design or myth

[seta666]

Hello,
I am interested on this as I bought both lenses planning on using them on a 5D but then I found some mixed opinions regarding mirror clearance problems.
I also found people saying that the filters are a necesary part of the optical design and other saying that without filter there is just a focus shift, that can be corrected reajusting the focus scale.
Regards

[ZoneV]

In converging rays a parallel glas plate (like that filter) produces astigmatism and a focus shift. I think the optical designers that design the Zenitar have corrected that - so you better use that filter.

More examples for the correction with parallel glas plates:
- Microscopic lenses have often a correction for the coverglas on the object
- Lenses for video cameras with 3 CCDs are special corrected for the beamsplitter prism in the camera

Leica Camera developed a very thin infrared filter for the digital M9 to avoid image deterioration of the old lenses which are designed for film (without any glas behind the lens). I think these infrared cut filters, optical low passes and the coverglases on the sensor could be one reason for digital optimized lenses.

[aoleg]

Mir-20 can be used without a filter, and it will NOT hot the mirror no matter what - nothing sticks out of the rear of this lens with an EOS adapter mounted. I never tried it with the filter - there's just no need in them.

Zenitar-16 must be used with the filter. If you use an adapter that's too thin, it *may* hit the mirror; does not happen with mine, but I've read mixed reports as well about this. Mir-20 is guaranteed to be safe though.

[seta666]

Hello,
Thanks for the answers, as far as I know my adapters are of good quality.
The C/Y ones make 1.43mm and the only m42 I have is almost 1.4mm, maybe 1.38mm somthing like that. I will be carefull when trying the zenitar as I also read that mirror hits may be caused because of a bad focusing scale adjusment causing the rear element extend too far. Another cause of mirror hit would be filter not going all the way in beacause of a defective filter thread
Regards

[seta666]

Well, my zenitar just arrived, i tested it and seemed ok but then after one shot I got black viewfinder (mirror hit when going down), the only m42 adapter I have right now is a cheap one, I am waiting for the Big_is one to arrive, I will do farther test when it arrives
Lens new, arrived very soon and was coming from ucrania, 139€ with shipping which was the cheapest price I found for new one