[photolady95]

Several months ago I bought the Sigma 400mm f5.6 from a forum member. Yesterday and today it has stopped working in AF. I've checked all my settings, and nothing has changed. The camera is a K-50 but it works fine with my other lenses. Even my other Sigma lens.
Yesterday I thought it might be the K-50 so I mounted the lens on my K-30 and I get the same result.
I've cleaned the contacts on both the camera and the lens.
I also had a Sigma 1.4x TC on it so thought that was the problem but removing the TC didn't change anything. Do lenses just stop working in AF because I've never had this problem before.

This is the lens in question:

Sigma 400mm F5.6 (AF) Lens Reviews - Sigma Lenses - Pentax Lens Review Database

Any help would be appreciated!

[WestBug]

Manually turn the focus ring and see if that brings it back to life.

[photolady95]

Manually turn the focus ring and see if that brings it back to life.

Nope. When it stopped AF, I changed to manual and was turning the focus ring. Still not AF. Thanks anyway.

[Des]

I have the later model Sigma 400 f5.6 "Tele Macro". The model I have is known to have a propensity for AF failure, which is caused by the AF gear assembly coming away from the base plate. Some years ago a PF member from France posted a detailed description of how they fixed the problem in the sibling of the 400, the 300mm f4 Tele Macro: [French] AF repair: SIGMA 1:4 300 mm APO Tele Macro - PentaxForums.com The decription which is linked in this thread is in French, but even if you don't speak French, Google Translate will give you the general idea. It involves fixing the plate with Araldite.

See also this discussion: https://www.dpreview.com/forums/thread/3620995

Of course this might be a complete red herring for the issue with your lens, but it's a start.

[photolady95]

Thanks Des. I don't have Google Translate to fix this but I think I can figure out how to take the lens apart just looking at the pictures he posted.

[photolady95]

I opened it in Chrome and it was translated with that. I don't get that pop up in FF.

[UncleVanya]

My Pentax 80-320 died in a similar way. Worked one day, then it didn't. My FA 35 suffered a fall and after realignment and fix it refuses to work in AF or Auto Aperture. The electronics inside the lens can die. It can also be a more mechanical issue as pointed out above.

The 400mm Sigma I had was reliable up until I sold it. If I recall it was not long after you bought yours that I sold mine. I don't know how it is getting along now but I hope it is still plucking away. They aren't super sharp but they are light and fun to play with.

[photolady95]

My Pentax 80-320 died in a similar way. Worked one day, then it didn't. My FA 35 suffered a fall and after realignment and fix it refuses to work in AF or Auto Aperture. The electronics inside the lens can die. It can also be a more mechanical issue as pointed out above.

The 400mm Sigma I had was reliable up until I sold it. If I recall it was not long after you bought yours that I sold mine. I don't know how it is getting along now but I hope it is still plucking away. They aren't super sharp but they are light and fun to play with.

Thanks Brad.

I don't know if I'm up to trying to fix it or not as bad as my tremors are, I'm not sure I want to attempt this repair. I only paid $100 for it and I still have my Vivitar M-200mm prime and a Sigma 70-300mm Macro. I can still manual focus the 400. For now anyway.

[UncleVanya]

Thanks Brad.

I don't know if I'm up to trying to fix it or not as bad as my tremors are, I'm not sure I want to attempt this repair. I only paid $100 for it and I still have my Vivitar M-200mm prime and a Sigma 70-300mm Macro. I can still manual focus the 400. For now anyway.

I understand that. I see them on Ebay for $200 or less all the time. Mine sold for $150 on the forums but it had been CLA'd recently which may have made it a bit more desireable despite some coating loss from fungus previously.

[UncleVanya]

Wait - does that lens have a clutch that you engage for manual by pushing the focus ring forward or backward? I can't remember. If so check to make sure that hasn't happened.

[photolady95]

Wait - does that lens have a clutch that you engage for manual by pushing the focus ring forward or backward? I can't remember. If so check to make sure that hasn't happened.

Not that I know of. I just put the camera switch in MF and turn the focus ring. I just tried to move the focus ring and nothing, so my guess it doesn't have to be pushed or pulled to be manual focus. I gave you a link in the first post to the lens here.

[photolady95]

AF is working again. I took this off the K-50 and put my Sigma 70-300mm on, then today I saw an Osprey in the neighbor's tall pine tree. Took camera, tripod and the 400 with 1.4x tc outside. I shot the Osprey with MF and decided to go to the backyard and there was a Yellow-bellied Sapsucker in the bird bath, just for curiosity sake, I turned the camera to AF.S and lo and behold the lens is autofocusing again.

[Des]

Good to hear it's working again. Hope it continues.

[photolady95]

Good to hear it's working again. Hope it continues.

Short lived. It stopped working about mid morning when a Red Shouldered Hawk showed up in my backyard this morning. Fortunately, I'm still pretty handy at manual focusing. Yes, I got the shot/s.

[Des]

Here are some options to consider:
1. Get the lens fixed. Probably $50-$80, at a guess.
2. Keep the lens as a MF lens only.
3. Sell the lens "as is" for $50 and look for another 400 f5.6 AF lens of the same standard or better for about $150, like the one @UncleVanya sold.
4. Sell the lens "as is" for $50 and sell the Sigma 70-300 as well for whatever you can get for it (presumably <$100). Get a DA-L or original DA 55-300 instead for <$150 (or a WR model if your budget will run to it). Use it with your TC when the light is really good; otherwise just crop.

Personally I would go with option 4. Even with my Sigma 400 f5.6 tele macro, which is said to be about the best of the breed, there often isn't a lot of difference between images with the Sigma and images with the DA-L 55-300 cropped (especially when the DA-L is stopped down to f8), at least on the K-3. I suspect the gap, if any, would be narrower between the earlier Sigma 400s and the 55-300.