Samuli Vahonen
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HaruhikoT wrote:
So I think that Optosigma's PCX having 50mm diameter should be usable for ZM18 if you can make optimal casing for it.
Blank 58mm filter and a custom made retention ring that holds the PCX without vignetting should be required.
Maybe 3D printer helps to make the ring but I'm not sure about 3D modeling and printing.
Hi Haruhiko, yes the lens itself is OK, but in princible one would need to get the PCX-lens ~1.1mm off from lens front plate (front lens element goes further than front plate) and then hold it in place with retention ring inner diameter of 49.95mm (largest I'm able to hold OptoSigma lenses in place firmly) and have no thickness on the retention ring. Even any fraction of millimeter is critical on thickness due to very wide angle.
Therefore in order to hold OptoSigma lens in place one would need to create some kind of device which has outer thread same as 58mm filters, and somehow holds the lens in place by holding it from top, bottom, left and right but not from corners. This would be possible with 3D printing the "OptoSigma holder" from metal (because of the thread). Alternatively this kind of "OptoSigma holder" could be made from plastic and clued to inside of 58mm filter, and then just file off metal from the filter to avoid adding any vignetting from the filter frame.
I have also tried 58-67mm -> Hoya 67mm +1 and it also would need filing due to hard vignetting in corners. Due to positioning of the lens further away from front lens the corners are somewhat worse than they are with Hoya 58mm +1 filter.
PS. With this lens even adding normal 58mm filter generates more darkening to corners. As can be seen from lens PDF sheet the corners are at f/8 <40% illumination on film, which can take light at any angle. Digital sensors don't produce same amount of lightness when light hits extreme angle to CFA compared to same amount of light coming in to CFA without angle ---> any added vignetting, optical or mechanical is troublesome for this lens. After all we are miss-using lens designed for film cameras by trying to use them with digital cameras. If one simply wishes to get good 18mm I would recommend to try their luck in Batis quality control lottery, much more easier option.
Personally I love rendering style of ZM18 - I don't even need 18mm lens, it's way too wide for any real photography in Finland, but I love the rendering style and challenge of getting this lens to work. Maybe one day I learn to use ultra wide lens in a way wide FOV is adding value to photo, not the opposite. Thou I don't see it happening very soon, as 99% of ultra wide photos I see here and other places, taken by very talented photographers fail due to unnatural FOV and simply would be better if shoot 25-35mm range. Really great ultra wide photos I remember are uhoh7 ZM18 photos (reason why I wanted to get ZM18, real good demonstration of this lens rendering style) and one German guy architecture photos (sorry don't remember the name) where he shoot upwards stairways etc. architectural features - I don't have mountains like uhoh7 or don't shoot architecture...maybe one day I figure proper usage for the lens in my photography 
Samuli
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but considering adding a Leica 75AA at some stage. How are you going?