Guenther54
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 p.94 #18 · Front-End Filter Improves Corner Smearing | |
Can reading glasses reduce corner smearing in vintage RF wide angle lenses on Sony A7R?
Thanks to HaruhikoT and candreyo for their description how to remove corner smearing in rangefinder wide angle lenses like Contax G and Zeiss ZM types. There is now much experience with planconvex lenses with foval lengths between 6000 and 1500 mm. These lenses are not everywhere available and cost about 100 $ each.
For vintage lenses like Jupiter 12 they may be helpful. As very good discussed there the J12 like his father (ore mother) Carl Zeiss Jena Biogon 2,8/3,5 cm is in the center very sharp, but suffers in the corners from smearing.
Is there another solution especially for these cheap RF Lenses Jupiter 12 2,8/35 mm, which is available in the traditional old Contax/Kiev RF Mount and also m39 (LTM) Mount?
I started a simple test by using spectacle lenses for eye glasses. These are menisci ore convex-concav not planconvex lenses. The lenses are aivailable in the interesting range of
+0,25 diopt. = 4000 mm focal length
+0.50 diopt. = 2000 mm focal length
+0,75 diopt. = 1334 mm focal length
+1,00 diopt. = 1000 mm focal length
and are ophtamological used for longsightedness (=hyperopia, hypermetropia)
These lenses are cheaper than the known planconvex lenses, have diameters between 55 and 65 mm and are available super multi coated for few money more. Mostly made of plastic, there are still more scratch resistent mineralic glasses available.
I purchased three types between 0,25 an 0,75 diopt. and tested it on the Jupiter 12 and a Sony A7R.
The best image quality is with a combination of Jupiter 12 and 0,5 diopt. in reversed position. The concave surface of the lense points to the object, the convex surface to the camera.
The diameter of the J12 filter ring is 40,5 mm.
There is a vintage close up lens from Voigtländer Focar 0 f=2m 342/41 AR that fits perfectly on the filter ring. Unfortunatly we need a reversed position of the convex-concave lens (less concave than eye glasses). But with some expirience I could change the direction without destroying the filter holder.
There is now a little problem. Combining a lens with 35 mm and a close up lens with 2000 mm focal length the total focal length is about 34,45 mm (1000/35 = 28,57 diopt. + 0,50 diopt. = 29,07 diopt. =34,45 mm) . The filter lens combination needs a reduction of the flange back of about 0,55 mm for adjusting to infinity.
You can shim the lens by opening it and removing spacers. But I dislike this solution, because the integrity of the lens is destroyed and when using the lens like me furthermore with classical rangefinder cameras it doesn't work.
I decided for these handicraft work:
Sony E-Mount LTM adapters are cheap and in some types you can change the flange distance by removing about 0,6 mm (better a little more than less) from the planarity. But be awere to buy a LTM adapter with a prominent thread part like
this.
If you take an adapter with no difference between the Sony E-Mount part and the LTM thread you have to remove 0,6mm material in two positions. If the flange distance is to wide, you can adjust it with thin sheets of material you like (I used baking paper ore packing foil with alu core).
These test shots are jpegs with auto white balance and without correcting color shift and vignetting.
My next step is buying a 0,5 diopt. multicoated spectacle lens from mineralic glas (the test glas is a little cheaper organic glas) and let it fit by an optometrist in a 40,5 mm filter ring. If you buy a cheap one from chinese ebay seller be aware that you order a filter with a screw socket. My test +0,5 diopt. spectacle lens was fitted in a 46 mm ring.
Unfortunatly with a step up ring from 40,5 to 46 mm the distance placement stops at 2,5 m because of the moving the outer tube of the J12. The Voigtländer close up lens is in this case uncomplicated. But microcontrast is less (lacking multi coating?) and there is vignetting in the corners (prominent filter ring in combination with more angle of field because of a little less focal distance?)
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