Zoom lens infinity adjustment of the rear lens group

PhantomLord · Joined: 08 Apr 2013 Posts: 476 Location: Szczecin, Poland

Hi,

I bought Tokina AT-X 28-85mm f/3.5-4.5 and encounter weird problem. On 85mm when I set focus at infninity mark, the lens is focusing on infinity, however when I turn zoom to 28mm and would like to achieve focus on distant objects near infinity I need to turn focusing ring all the way down to 1.3m which is almost at the mfd of 0.9m.

Well I know it's a varifocal lens, but I don't thing such differences in focus is normal.

When I read about the problem I found out that probably lens has separate infinity adjustments for wide and for long end. Adjusting long end from the front of the lens is piece of cake, but I honestly don't know how to adjust infinity at 28mm by moving rear lens group.

I tried to disassemble the lens and find something out, but without success. I only destroyed spring that was under Close Focus button Wink

(which would be easy to repair).
What's more I think I had similar problem with Vivitar Series 1 70-210/3.5 (Tokina made) which in the other hand is zoom lens, not varifocal and should keep focus while zooming.

Any thoughts would be appreciated.
Mateusz

Gardener · Joined: 22 Sep 2013 Posts: 950 Location: USA

Your adapter is too thin.

PhantomLord · Joined: 08 Apr 2013 Posts: 476 Location: Szczecin, Poland

Well, I cannot agree, because if it were to thin, there would be problem at both ends of the zoom. If it would be thicker there will be no problem at 28mm I assume, but it would not be possible to reach infinity on 85mm on the other hand. The problem is uneven infinity setting at long and short end of the zoom.

Lloydy · Expire: 2022-01-01

My ATX SD 80-2-- / 2.8 wouldn't hit infinity, I adjusted the focusing barrel by removing the rubber grip - slacking the barrel - found infinity - tightened the barrel.

But, that might be different to your lens?

PhantomLord · Joined: 08 Apr 2013 Posts: 476 Location: Szczecin, Poland

Hi Lloydy
I think you're missing the point Wink

.
I did adjusted infinity on 28 but then I lost infinity at 85 :/ and that's the problem - I couldn't adjust infinity to be at similar point at 28 and 85. Ren� (TrueLoveOne) who has the same lens told me he could set focus on point at 85, than change focal length to 28 and the focus stays at the same point, while in my lens focus is way way different.

Mateusz

Gardener · Joined: 22 Sep 2013 Posts: 950 Location: USA

sammo · Joined: 04 Jan 2012 Posts: 223 Location: CH and SI

Yes, adapter is too thin. Set the infinity focus at the longest focus by adjusting the focusing ring. Then, set infinity focus at low end by screwing in the rear element group (to move it away from the sensor). Long focus shouldn't change much, but in any case check it and recalibrate if needed. Repeat until you have the infinity focus at both ends. I don't know if that procedure is correct with your lens, I perfectly calibrated a parfocal zoom by doing that.

PhantomLord · Joined: 08 Apr 2013 Posts: 476 Location: Szczecin, Poland

Hi,

Adapter is all right, as today another OM mount lens arrived and it works perfectly. What's more I just received Olympus OM-10 as well and Tokina behaves the same way as on my Canon with adapter.

I think I need to spend some time with this Tokina and try to move the rear lens group as you mentioned.

Cheers
Mateusz