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I have recently purchased a used (but remarkable condition) Tokina AT-X 400mm f/5.6 with an EF mount. This works on my 6D, allowing for aperture and focus control from the body.

However, my 70D doesn't register it at all: the display shows an aperture of 0.0 and selects manual focus in drive menu. While the lens is functional, it would be nice for it to work with the higher frame rate of the 70D. I also have the AT-X 300mm f/4 which works perfectly on both (hence this purchase).

What should I check / examine / investigate to get this working on my 70D? Has anyone experienced this or similar (old lens → new body)?

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What should I check / examine / investigate to get this working on my 70D?

I'd start by cleaning the contacts on the lens. You know that the lens works because it works on a 6D. While it's certainly possible that the lens is completely incompatible with the 70D, one would expect at least some basic functionality if the lens is working at all. Dirty contacts, though, could prevent the lens from getting power or making the proper data connection to the camera, either of which could easily explain the behavior you're seeing.

To clean the contacts, use a soft cloth slightly dampened with some isopropyl alcohol. Let the contacts dry completely (this shouldn't take long if the cloth was just damp) and try the lens. If that doesn't work, you could get a little more aggressive and use a pencil eraser, but if you do you should take steps to ensure that none of the eraser dust gets into the lens. I'd completely seal off the back of the lens with some painter's tape or similar before using the eraser.

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I'm surprised AF works with the 6D! That lens was discontinued at least a decade ago. Being that it does work on the 6D, though, it is disappointing that it doesn't also work on the 70D. Both the 6D and 70D use the Digic 5+ processor and were released about 9 months apart in 2012-13 around ten years after the Tokina AT-X 400 AF was last made. It would be interesting to know if the lens would work on the 7D. The 7D is very similar to the 70D in terms of still photo performance and the AF sensor design but was released in 2009 with dual Digic 4 processors.

If the 70D at least recognized the lens and allowed you to control the aperture I would say the problems focusing are likely due to the fairly narrow aperture of the lens and the narrower baseline of the APS-C 70D's AF sensor. With f/4 lenses and a third party 2X TC I have had more (limited) success regarding AF using one of my FF bodies than using an APS-C body with the same lens/TC combo. But since the 70D won't recognize the lens at all I highly doubt that is the issue.

That's the risk you take buying third party lenses, though. It may be that you are experiencing what is often the downside of using an older third party lens on a newer camera body: AF incompatibility. Lens makers such as Tokina do not license technology from camera makers such as Canon to insure compatibility. Instead they reverse engineer the technology in order to design compatible lenses. When a manufacturer such as Canon then introduces a newer model that was not available when the third party lens was designed they can make subtle changes in the protocol that renders the older third party lens incompatible.

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Because Canon doesn't license its mount or give the internal details of the electronic communication to third parties, nearly everyone has to reverse engineer the mount communication signals. So this is the main danger of going with a 3rd party lens (particularly an old one)--that when the lens is new, it will communicate properly, but that with older cameras or future ones, that compatibility could be compromised.

When Canon updated their camera mount to support EF-S digital lenses/bodies, a lot of film-era 3rd-party lenses--most particularly in autofocus function--were compromised. Makers like Sigma, Tokina, and Tamron would often offer to rechip the lens to bring back function, but only for a limited period of time. It's unlikely that Tokina will rechip your lens at this late date (and the fact that AF works on the 6D indicates that it may have already been rechipped).

The incompatibility you ran into is likely why someone felt they wanted to sell it, and could be why you got a bargain price. It's also why saving up the pennies for an EF 400/5.6L might be worth it in the long run.

It is odd that the lens works on a full frame but not a crop body. I would suggest looking closely at the physical coupling for the mount against that of an EF-S lens, and seeing if there's something on the mount that blocks/prevents it from mounting fully/properly on a crop body, and keeping the chip contacts from mating properly with the pins on the camera body (the behavior you describe is what happens when the camera can't sense any lens at all on the mount). And then, depending on how daring you are, consider getting out a dremel tool :D, returning the lens and saving up pennies for a Canon 400/5.6, or just getting used to cropping 6D shots with it.

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