Repairing a Sigma 70-210 f/2.8

Well, 'repairing' may be too strong a word, but the lens is usable and I only ended up with one extra screw and a piece of extra glass... ;)



I bought a Sigma 70-210 f/2.8 APO from a Dyxumer for about a hundred EUR. It had trouble with focusing whereby the focus mechanism would jam and sound very rough. In essence, AF was completely useless and even MF was barely usable.



I opened it up a long way from the rear (mount) side, removed the mount, the rear barrel, the rear group with the diaphragm and electronics, some AF gearing and spent a whole lot of time removing the Zoom ring mechanism, but still couldn't gain access to the front of the lens.



I didn't see any obvious methods of opening the front besides a filter ring -type thin ring that held the front elements in place. When I tried to remove that ring, it got horribly stuck and I had to cut it with a dremel to get it off (did it quite nicely actually). After doing that I noticed to my humiliation that it wasn't even supposed to be possible to remove it that way because the threads it was on were recessed beyond the filter threads slightly. Yet, I still couldn't get the lens open so it was left collecting dust.



Then one evening, just a few days ago, I was getting ready to go to sleep and found a neat screwdriver that would let me try and open some screws in a very difficult to reach place inside the lens. That turned out to be a false hope since the screws were glued tight. I took the lens in my hand and really started to ponder how the outer shell could be removed. I twisted and turned everything that I had already twisted and turned a dozen times before and suddenly a threaded ring came loose near the focusing ring and it was all obvious. The front barrel came off and I could access the focus threads and clean them thoroughly, applying just a dab of grease in the process. After cleaning the front elements well (they had got dirty during all this) I started to put the lens back together. This was at about 3 AM so I was just a wee bit tired. I dropped the lower front element in the lens barrel where it was supposed to be dropped, but did so carelessly and to my horror a piece of glass chipped off the edge of the element! I assessed the damage and came to the conclusion that although that wasn't a Good Thing(TM), it was insignificant enough to not cause major problems with the light transmission. The pics you'll see will show what the chip looks like.



Well, I finally got the lens reassembled and went to bed. The next day it seemed to work quite ok before the AF motor suddenly went "Whrrrrrrrr!" The AF motor axle had decoupled from the focusing mechanism somehow and I could hear a rattle inside the lens. At this point I was more amused than annoyed so I opened the back of the lens again to find that one of the gears from the AF gearing had popped off. The gear, although it stayed quite nicely in its place at rest, became twisted when torque was applied because its axis was loose from the mounting plate. I managed to reattach it with a spike and a hammer and now it should stay in place quite fine. In retrospect, this loose gear was probably the main reason that the AF jammed, although it had probably become loose because of the rough focusing threads.



Although a lot less clunky than prior to the gear popping, the focus still seemed a bit heavy so I opened it up from the front again and wiped off all the grease from the bottom of the threads, removing a lot of minute metal dust in the process. I tried a different grease, but ultimately wiped that off too. I left it with a very thin layer of residue grease and now it seems to move relatively smoothly - well enough for me to be happy with it, although the focus mechanism was probably a lot smoother when it was new. During all this I managed to re-smudge the front elements so I cleaned them again, but I took short cuts and some grease was left on the edges. I'll probably take it open once more and clean those off too. There are probably some small scratches on the glass as well from the handling. I managed to light it nicely to show and exaggarate all the ugliness in one of the pictures I'll link to later.



The autofocus mechanism works now and doesn't make me fear of overheating the focus motor, but it seems to do strong hunting. In other words, it overshoots, then compensates and overshoots in the other direction, going back and forth across the proper focus distance. I'm hoping that cleaning the front elements will reduce this behaviour. I will also try blocking light from entering the lens through the chipped part, just in case that causes havoc with the focusing.



I'll post some pictures at the address below. See the 'Large' directory for full res shots.

Pictures!



I'll probably keep this baby, but if someone wants to buy it, make me an offer.



Thanks for reading all the way through! Feel free to ask questions.

Edited by Gubbe - 08 December 2016 at 18:37

Ahh, the saga of dis-assembley without instructions... very often the removal of the broad rubber focus/zoom bands reveals a way in. I don't think the chip will do much damage to contrast - the grease is probably much more of a problem. Your story highlights one of the things I've found from taking lenses apart - the focusing linkages on some of the third-party lenses (chiefly cheap Tokinas in my case) can leave a little to be desired. Even the cheap Minolta lenses (such as the 35-80 with the integrated lens cover) are impressively built inside.... That reminds me, I've still to find a way into that 28-105 xi zoom with the sticky aperture...

Bob

Who cares if fixing lenses with a hammer doesn't work that well? It's fun!
The gear axle is actually made of very soft metal and its attached into the plate by expansion of the end, done by striking a pin into the dimple at the end of the axle using a hammer. Unfortunately I've mangled the axle by using "suboptimal" equipment and decided to try different methods.
The epoxy is currently curing on the gear axle. I'm hoping that will hold it in place. If it doesn't then this thing will probably become a manual focus lens.
I'm wondering if that loose gear, assuming it came loose right away back when I last assembled the thing, could be the culprit in the AF hunting.
I'll have to test it again after the epoxy cures.

Any chance you could get a spare bit from another Sigma lens?

Bob

Thanks for the tips! I'm sure I could use a spare, but finding a donor lens might be really difficult.
I'll have to start looking if this epoxy thing fails.

I would investigate CKsam's idea too, but there are no electronics in the focusing part. Only the zoom ring has a cable coming out from it and even then it's only for the focal length data which I understand is of no concern to the AF system. If you think I'm missing something, please inform me.
Just to be on the safe side though, I'll be sure to test that the focal length data is correct by taking shots at different zoom settings and looking at the EXIF info.

Great news! Now for another project, eh?

Sam.

Re 70-210 f2.8 APO,

Gubbe. It's been awhile but thought I would ask. I am trying to deal with a rough focus as well, took the rear off up to the gears but stuck at that point. When looking inside the front, I noticed a plastic ring floating around so something has loosened up. I don't quite understand the front removal procedure. Did you say the front barrel unscrewed (the front element along with it)? This is on tight and I want to be sure before I try it again.

Thanks.