The aperture of my Sigma 500 F4.5 APO moves slowly, so I guess that the aperture blades are oily although no oil on the blades is visible. I want to remove the aperture for cleaning. The question is, how to disemble the lens until I can remove the aperure. The lens is the old non-EX version. Lens overview with labled sections
I have started to disassemble the lens from the backside. I have removed the mount, but behind I can not find any screws that are obviously the right ones to remove the next lens section. Lens mount removed
I hope to be able to remove the whole section 1 to access section 2 where the aperture is located.
Does anybody know how to go on? Is it possible at all to remove the aperture from the backside? I have searched the internet but haven't found any useful hints or service manuals.
debuggerus wrote:
OP,
You may try to strengthen the spring (6 o'clock) by finding a stronger spring (or shorten the current one)...
But this operation could result in bigger force needed for the camera to move the iris - which could result in camera damage sooner or later.
I reccomend to search for the cause of the slow iris operation. It could probably be this spring - but I suppose not. There could be grease / oil on one part of the iris mechanics and linkage. Is this lens yours since years? Or didīt you know its history?
On a fresh bought lens it could be even possible that such slow operation is the result of a not correct repair / inside cleaning.
Thank you for all the replies. Strenghtening the spring does not solve the problem. The mechanism controlled by the visible spring is coupled to a lever of a second system, controlled by a second spring (not visible in this status of disassembling). If I decouple the two mechanisms and move the lever of the second mechanism, the aperture still moves slowly. By the way, that property changes. Sometimes the aperture works well, sometimes not.
I still hope that the reason is somwhere in the mechanism before it comes to the aperture itself. The screws for the next step of disassembling are identified now*, but my screwdriver is broken. I have to buy a new one before continuing. Cleaning and giving new grease to the mechanism would be the favourite solution of course.
I bought the lens last automn and got a discount for the aperture problem (which the seller did not notice). I don't know much about it's history.
When I looked at the lens today, I noticed that the transition between section 1 and 2 looked like it is a scrwed on metal sheeting pipe. I gripped it firmly and really could screw it off.
Besides of some dirt, now 6 screws are visible. 3 that hold the tread where the shielding pipe was mounted and 3 that likely have to be removed to detatch that lens section. Unfortunately, two of the 3 screws have damaged heads. Obviously, someone has been working here before. Now I have to find out, how to remove these screws that are mounted firmly / tight. Warming up might cause some grease in the mechanics to flow, so I hesitate to do that. Drilling out the screw and cutting a new thread also sounds not very attractive to me.
How much did you pay for the lens if you don't mind my asking?
You guys that disassemble lenses have my respect. I'd be scared to do that and how would I ever be sure that the optical alignment/collimation would be correct after all the tinkering?
Btw, I have the same lens but I've only used it about twice or so and probably need to get rid of it. (hint, hint)
I originally paid 850 Euro but got a discount of 200 Euro because of the aperture problem, so it was only 650 Euro. I also had the choice to return it.
HopeIsEternal wrote:
how would I ever be sure that the optical alignment/collimation would be correct after all the tinkering?
The rear lens group is usually mounted as one piece in a position that can not be changed. So the risk is not so high. I would not remove any lens in front of the aperture. I also would not tinker on a precious zoom lens, they are mechanically much more complicated.
I had no time to gon on with work on the lens during the past two weeks, but today I did the next little step. I removed the 4 screws that fixed the cover to the next lens section (section 2). Now it is easier to access the screws with the rotten heads. After closing all openings of the lens, I have put e little MOS 2 oil around that srews. Now it has to react for a while. Afterwards, I will try to hammer a slot in the damaged screw heads.
Need the money.