Aperture Stuck on 4.5?

Zillah

I've had my Sigma 17-70 2.8-4.5 for a few years now. It's been around South America, Canada and a bunch of other places, so I shouldn't be surprised if it is starting to break, but still, I hope I'm just missing something.

I just noticed today that the widest the aperture will go to, even when at 17mm, is 4.5. It doesn't seem to matter what mode I'm in. Other lenses seem fine. If I put pressure on the lens in certain directions the aperture will sometimes drop to 4 or even 3.8, but jumps back to 4.5 the second I let go. It really seems like a mechanical issue. Does this sound familiar to anyone? Is there a simple fix or am I boned? (oh nooo, now I have a legitimate reason to go lens shopping...)

I attached an external flash for the first time shortly before this if that could make a difference, but that has been taken off since and other lenses are fine, as I said.

pixbyjohn

With the lens off the camera can you move the aperture lever at the back of the lens ? By moving this lever you can open/close aperture on lens. Check it is not broken or bent.
See photo

aperture lever by pixbyjohn, on Flickr

PS. Compare the position of the lever with another lens to see if it will engage with the camera in the correct position. Hope this helps

zerohamster

Is it a Nikon or Canon mount?

Zillah

It's a Nikon mount for my D7000. I tested the aperture lever, as far as I can see it seems pefect; no sign of it being bent or damaged and the mechanism seems smooth and functional. I also tried the lens on my old D70s and it's the exact same, it can't go any lower than F4.5.

Further ideas greatly appreciated.

DaireQuinlan

I know the d70 doesn't have the facility, but does the D7000 have the little tab that allows you to use the aperture ring on the actual camera ? If so set the ring to its widest aperture and see what the camera thinks it's set to at both ends of the zoom range. This sounds more as though it could be some sort of electrical screwup than a mechanical one ...

DaireQuinlan

DaireQuinlan wrote: »

I know the d70 doesn't have the facility, but does the D7000 have the little tab that allows you to use the aperture ring on the actual camera ? If so set the ring to its widest aperture and see what the camera thinks it's set to at both ends of the zoom range. This sounds more as though it could be some sort of electrical screwup than a mechanical one ...

...though if the lens doesn't actually HAVE an aperture ring (which a brief google seems to suggest) then this is a moot point ...

Zillah

Yeah, it has no physical ring.

zerohamster

I know this lens is an f/2.8-4.5 lens so when at 17mm it should be at f/2.8 and as you zoom toward the 70mm end the diaphragm will change toward and up to f/4.5.

The problem is definitely a mechanical fault on the lens.

What I speculate has failed, is not catching properly or even just slipped out of place is a small piece on the diaphragm that is connected or being manipulated by the barrel as you zoom to change the aperture value.

If this piece were broken or not functioning properly the diaphragm side of things will be spring loaded toward the f/4.5 side of things which is probably why you are getting stuck on that value.

The system usually operates in a way that as the catch rotates with a lens barrel as you zoom it will catch or just directly rotate part of the diaphragm causing it to contract. As you move back toward the 17mm end of the focal range the catch rotates with the lens barrel and the diaphragm part returns causing the diaphragm to dilate.

Is it a specific direction you have to put pressure on the barrel to get the diaphragm compensation mechanism to catch as you rotate?

Also I'd be happy to have a look for you to confirm.

Zillah

I think you're right. If I put pressure on the lens pushing it to the right the aperture display drops to 3.8. Looking closely at the lens there is a group of four little spring-loaded contact points on the top, but the right-most one doesn't stick out as far as the rest. Pushing the barrel to the right would push this closer to the body contacts, I think. Comparing it to my healthy lenses, it seems like all of their contact points stick out an equal amount. Looks like a faulty spring.

Any idea how fixable that is?

zerohamster

Zillah wrote: »

I think you're right. If I put pressure on the lens pushing it to the right the aperture display drops to 3.8. Looking closely at the lens there is a group of four little spring-loaded contact points on the top, but the right-most one doesn't stick out as far as the rest. Pushing the barrel to the right would push this closer to the body contacts, I think. Comparing it to my healthy lenses, it seems like all of their contact points stick out an equal amount. Looks like a faulty spring.

Any idea how fixable that is?

Hmmmm,

I'm not so sure that one point being slightly further in is actually a fault. Sigma may do this normally as I think I recall seeing other contacts like this.

The 4 contact points would specifically be electronic so unless the diaphragm compensation mechanism is also purely electronic that may not be the cause but it if it was it wouldn't explain why pushing it in contact would specifically only put it to f/3.5. regardless of focal length (f/3.5 even at 17mm?)

If you can run a little test for me I can possibly determine if it is electronic or purely mechanical.
Starting from 70mm and working toward 17mm do the same thing that you were doing to get the lens to change value and note what the value at each step of focal length you test is.
If at 70mm it stays at f/4.5, starts to give reduced value as you test toward the wider end and gives a value of f/2.8 at 17mm then it is the contacts that are causing the problem. If you don't get those results then it's a mechanical issue.

Depending on which is at fault it may be easy or a little more difficult depending on the fault.

Zillah

Yeah, it seems to be mechanical. I get 4.5 at 70mm, and it starts to go down as it approaches ~30mm at which point it shows 3.5, then it starts to climb back up to 4.5 at 17mm. Putting pressure on the barrel sometimes gets an extra stop out of it but that's about it. Where are you based zero?

zerohamster

Ah I see, interesting that that happened. The mechanism inside must catch when you put pressure on the lens but slip after 30mm.
Im based down in Greystones.