[Konvas] Lomo 20-120

[Bruce Taylor]

Hi Shane,
 
I went on a hunt awhile ago. The front element is so large that in standard sizes a 6.6x6.6 square or 138mm round are the only ones big enough to fit, and they are very expensive if you walk into a store. So I looked into rear mount glass filters. I spent some time on it, and came up with little. The only one I was actually able to get my hands on was a 30.5mm (I think) ND6. I found filters that would (sorta) screw into the rear filter adapter-- maybe 37mm, I don't remember, but they are not correct. The thread pitch is different, and they do not screw in properly. Also, they protrude from the rear, and if you're mounting this on a Kinor (as I am) you need to be very careful with rear clearance or you'll hit the mirror.
 
The only workable one I was actually able to get my hands on was a 30.5mm (I think) ND6. This would drop into the holder, and I was able to secure it properly with the locking ring. There were no other cine related filters I was able to find in this size, and I did not test it for vignetting, as it has the standard metal ring surrounding the filter glass.
 
One other issue is the depth of focus. A glass filter changes that, so the point of focus at the film plane will shift. I came across a formula that related rear filter thickness to depth of focus changes on cinematography.com, as well as a few discussions of lenses that did or did not need to compensate for this focus shift. Tests would be in order to determine this for yourself if you go for the rear glass filter.
 
The simplest solutions I have found are big filters in front or gel filters in the rear filter holder-- they are so thin that the depth of focus change is not significant. I had a client that took the rear gel route with some NDs and it worked great. The only problem was getting them. None of the rental/supply houses (and we're in LA!) knew what he was asking for. I found a great selection of 4x4's at the photo supply shop Freestyle http://www.freestylephoto.biz/ (search "gel"). He cut them out with a pair of scissors and all was well.
 
Lael, sorry to hear of your mishap with the OPF18, I hope everyone was okay. I have a spare OPF18 for parts, but the same inner elements are smashed, so unfortunately no help there.
 
Good luck with the rear filters, please let us know what ends up working for you. 
 
Best,
 
Bruce Taylor
www.Indi35.com

--- On Thu, 1/22/09, Shane Kelly <cinematography at earthlink.net> wrote


Hi all,
does anyone with a lomo 20-120 zoom (opf 18-1 ) use the rear filters? I have one that I'd like to use some ND's in the rear filter stage. The size is a little strange though, about 39.5mm, so it's between series 5.5 and series 6. Does anyone have any suggestions on where I can get a filter to fit? 
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