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I was travelling on a trip with my camera. One day I was using my Sigma DC 18-250mm Macro HSM lens on Canon 70D in autofocus mode to take a picture. When I pressed my shutter button halfway to focus it wouldn't lock the focus on the object. It keeps jumping back and forth on focus, so it doesnt let me click the picture. I can take pictures easily in manual focus mode. This is a weird problem. I've never had this before. I do find there are a very few times when using the same lens between 18-50mm I can lock the focus and take pictures, but not at longer focal lengths.

I tried using my friend's lens on my camera and the autofocus mode works well. So I think it's lens fault. I tried dismounting and cleaning the contact points. I also tried removing the front UV filter, but no solution so far.

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It sounds like you've already done almost all you can to diagnose the issue yourself. The only other obvious thing you might try is using your 18-250mm lens on your friend's camera.

You can also rule out the possible user errors that can cause a lens to not autofocus listed in this answer to a related question.

Assuming the issue is present when the lens is used on the other camera, it's time to send the lens to a service center for diagnosis and repair (unless the repair will cost more than the lens is worth).

When electrical connections work at zoom lens focal lengths when the lens is shortened and do not work at other focal lengths when the lens is extended it usually means either a ribbon cable is failing or the connection on one end or the other is working loose.

Unless you know your way around the inside of a lens, including what to touch, what NOT to touch, which screws can be moved/removed, and which ones should not be moved unless you have the proper diagnostic equipment and expertise to realign the lens during reassembly, you're probably not going to be successful at repairing the lens. Taking it apart yourself (and not marking certain alignements as it is disassembled) can make it much more difficult (and more expensive) if not impossible for a professional lens repairer to put it back together properly.

Quite frankly, if you have that kind of skill and knowledge, you wouldn't need to ask us what is wrong with a lens that is showing the classic signs of a ribbon cable issue. You'd already recognize the likely cause of the problem.

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Let me ask the obvious questions first:

  • What was the distance between the lens and the subject? Were you within the minimum focusing distance of your lens? Note that minimum focusing distance increases with focal length. If you exceed the minimum focusing distance of the lens, it will fail to focus.

  • How much natural contrast was there on the subject? If you try to focus on a single-colour subject that has little surface texture, your camera may find too little contrast to focus and start hunting (or failing to focus altogether).

  • How much ambient light was there? With little light available (and corresponding high ISO values), contrast starts to deteriorate, again leading to the aforementioned problem.

Only if all of the above can be ruled out should you start to think about a technical defect in the lens.

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It's a lubrication problem, they don't put enough lubricant into the optical tube mechanism and it wears off or gets stuck. the lenses are still usable in case you would like to open the lens and lube it up properly or you simply can support the lens while autofocusing, with your finger. The lubricant amount doesn't fit the physical requirements (kinetic powers, gravity, friction) and it simply stops to work.

It's nothing electrical or anything complicated, this answer is valid for basically all of these:

"Hey, I finally found THE REASON and a temporary workaround: I bought this lens recently once and was facing the same problem - the autofocus wouldn't work. As my piece had some fungus and I'm quite of a noob into optical gear, I simply thought the fungus is hindering the autofocus at corresponding with the light correctly, so I took it apart using only a screwdriver and actually wrecked this whole thing but after looking at the optical tube inside which felt very dry I came to the conclusion that it would be stuck because there wouldn't be enough lubricant and the power delivered by the camera simply wouldn't be enough to make it move correctly, the angle of the optical part towards the sensor/film wouldn't be exactly 90° anymore. So, yesterday I have bought the Exakta counterpart which is basically the same lens, I bought it without a lens hood but I still would have the hood from the Sigma - without hood, the autofocus works perfectly. Today I tried it out with the hood, and it showed the same symptomes and it just clicked - the hood applies weight on the optical tube so it kind of gets stuck because it's becoming too heavy (and the angle isn't correct anymore), this can be circumvented by supporting the optical tube, which drives out of the lens when zooming, slightly with your finger. I would be very happy if anyone would be able to find a permanent solution for this, I'm fine with taking the lens hood with me and only putting it on when necessary and then focusing with the slight support of my finger but out of convenience I would prefer to make it work normally because this is a very banal problem as it seems. Probably taking it apart and lubing the optical tube again is the solution but i'm not going to take this thing apart again, lol. Are there lens hoods for this thing which would be slightly lighter or something like that? This product wasn't going through a long term usage test in complete configuration, obviously."

(Sigma 70-300mm)

You can give this hint a repair-man (lol) and he should make it work again, also you can try to install some kind of a supporting bar which would relieve the optical tube.

The thing with these lenses that they completely lack ball bearings and that makes these ridiculously prone to such faulties, it's basically the tube starting to "hang loose" because there would be too much function relying on a completely outdated mechanical principle while using sub par materials.

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