Help needed for zoom lens infinity focus adjustment

riverfour · Joined: 21 Jan 2011 Posts: 32 Location: Plano, TX

I have an old lens Mitakon MC 28-135mm f3.8-5.2 bought in 1994. I found that the focus to infinity changes while zooming. I managed to make it focus to infinity at 135mm. When I zoomed back to 28mm, the focus to infinity is lost. I can adjust the front foucs group to achieve focus to infinity for a specific zoom, but cannot achieve focus to infinity from 28mm to 135mm when the focus barrel staying at infinity. I have some knowledge of disassembling zoom lenses, but didn't encounter this problem so far.

BTW, the macro is on at 135mm. There is no macro ring near the aperature ring. I believe many other brands such as Vivitar has similar zoom lenses.

Any help is appreciated. Please see two pictures:

wupdigoj · Joined: 18 Mar 2010 Posts: 85

Hello. I don't know this specific lens, but the collimation process is quite similar in many zooms. You have to adjust separately the short and the long extremes of the zoom.

Usually (as you already know) you adjust the long extreme by adjusting the focusing group, i.e., the frontal group. This will have little effect in the short extreme. Then you have to change the short extreme. To do so, you have usually to adjust the rear group of the lens, that is to say, move this group backward or forward. Many zooms have some means to do so (typically some screws in grooves), but usually you have to dissasemble the lens to some degree.

Try to remove the cilindrical cover with the 28-35-45,etc lettering (i am sorry, english is not my language), by unscrewing some little screws. Perhaps you will have to remove the mount, but sometimes is not necesary, as the "cover" just slip upwards revealing the adjusting screws. Move the rear group and check the effect in the short extreme of the zoom. When you are satisfied, fix it. Then you have to check again the long extreme and adjust it if neccesary. One or two iteractions are usually enough. If the zoom is well designed all the intermediate focal lengths will be spot on. Hope this helps

Javier

riverfour · Joined: 21 Jan 2011 Posts: 32 Location: Plano, TX

Thank you for the suggestion. I also figured it out by googling some sites. You are correct - I have to adjust the rear lens group. What I did was almost disassembled the whole lens and found that the rear lens group focus adjustment was designed to be simpler. There is no need to disassemble the lens, not even the rear part. Basically unscrew/loose the 3 lock screws on the barrel with zoom marks (28mm|35mm|...|135mm), pushed it upward a bit, there is a lock screw again near the lens mount. Take the lock screw out and save it in a safe place. This basically release the lock to the rear spacer adjustment mechanism. Push the zoom mark barrel down to original position and tighten the 3 lock screws. Mount the lens on a DSLR preferrably with Live View (LV). Now zoom to the maximum range, for this lens 135mm, focus to a remote object as far as possible (>200 meter, moon or star is the best) to simulate a focus to infinity. Rotate the barrel with zoom marks gentelly and slowly by picking a direction. Pull the zoom back to widest zoom (in this case 28mm). View from LCD in LV mode by enlarging picture by 5x or 10x from DSLR camera and see whether it is clearer than before. If worse, rotate to the different direction to check. If better, keep rotating and check. If overshooted, rotate back to check. Repeat the above process to reach a point that the focus to infinity kept from pulling from 135mm (focus to infinity achieved) to 28mm. It requires a bit patience to do this. If you can mount the camera on a tripod, that might save some efforts. I had mine done handheld. After this is done, unlock the 3 screws, push the barrel with zoom mark upward again. Install the rear djustment lock screw that was saved previously to the original position. Push the barrel down, gentely rotate the barrel with zoom marks to align the infinity marks on the barrel with the focus grip. That is the foucs grip at infinity should stop at infinity mark on the barrel with zoom mark. Tighten the 3 lock screws on the barrel. Then the whole job is completed. I will upload a couple of picutures showing the rear focus adjustment lock screw.

What I found is that this process is very similar to those lens with macro rings. For the zoom with macros rings, the rear/short foucs is done through loosening/tightening the lock crews on macro ring and rotating the macro ring. There is a article here for zoom lens with macro ring infinity adjustment.

http://www.kyphoto.com/classics/kironlens.html

riverfour · Joined: 21 Jan 2011 Posts: 32 Location: Plano, TX