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05-29-2012, 11:44 AM   #1
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Join Date: May 2012
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Tokina 100-300mm F4 conversion issue

Hi guys, hopefully you can help me out here.

ok was going to ask how to adjust the focusing on this lens as it is wrong but just had a thought which for some reason didn't occur till now.

basically got this lens a little while back and it had been converted from nikon to pentax in the past, quite poorly tbh.
Initially iq was horrendous but I quickly found that to be down to the fact the rear element retaining ring was screwed in wonky which let the element sit at an angle but after fixing that I'm pretty sure its image quality should still be a bit better than it is.

at the moment infinity at 100mm is at 5m and at 300mm is 20m also closest focusing should be 2m all the way through and is a 300mm but is more like 3m at 100mm.

any way as said just had a thought as writing this and that was that given the poor quality conversion could the lens have had too much removed and the distance from the lens mount to sensor be wrong as well and so quickly tested that by moving the lens away from the pk adapter and seeing if it helped and it did at around 4mm away from the adapter the lens hit infinity at both ends when focused at the infinity symbol so now I'm wondering how to best go about shimming the lens/adapter to make up so this shortfall or if indeed it will have any effect on image quality or not? the main issue with the lens is it shows more purple fringing than I believe it should and I still think it should be sharper than it is.

Ps I'm using this with a pk adapter on an NEX 5n

at the moment I'm thinking the easiest solution would be to shim the pk-nex adapter as you can see it should be a simple case of adding a ring under the mount and drilling a few holes for the screws unless I'm mistaken, but as said will this improve image quality at all?


adapter with mount removed


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