Problem with Tamron 200-500mm on EOS 5D

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Richard Black
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Firstly, happy saturday morning.

A lad at work has a 5D hes recently upgraded from a 350D, on the 350D he had a 200-500mm Tamron EF lens. Its deffintly not a EF-S lens so fits fine.

When he tried it on his 5D last night he found it did not auto focus at all.

Is there a setting he is missing on his camera? or is it simply not compatible? Although it should be as its a EF lens.


Cheers
 
The 5D is bound by the same constraints as the 350D as far as AF is concerned...namely an aperture value of f/5.6 or better.

I'm assuming this Tamron is the 200-500/5.6-6.3...(DI or not)

If the lens is used at the short end then it should AF on any Canon body but is outside the spec for AF when used at the long end...it would be f/6.3 at 500mm.

Bob
 
He says it focus'd perfectly at all focal lengths on the 350D?
 
I have a 350D and it autofocuses quite happily with my Sigma 18-200mm f/3.5-6.3, even at the long end of the zoom where it's f/6.3. The 5D was introduced 6 months later than the 350D, so you'd think it would be at least as good here.

I think this f/5.6 limitation might be a misunderstanding or a myth. But it seems to be very difficult to pin down. The best source I've come across is an article by Rob Galbraith, where he says: "The centre AF point acts as a cross-type sensor with lenses whose maximum aperture are f/5.6 or faster." To me that implies that the autofocus will still function, albeit not so effectively, when the maximum aperture is slower than f/5.6, which is consistent with my observations. I don't know whethere there's any absolute limit, though.
 
Update: Maybe its not a myth, but it does look like I've misunderstood something. I've seen reports that the Sigma 18-200mm f/3.5-6.3 manages to autofocus even at the long end where it's f/6.3 because it "lies" to the camera's AF system and claims it's f/5.6. Then the AF system doesn't get into a huff and it tries to focus. It might not work very well in poor light, but at least it will try. Apparently the Sigma 170-500mm f/5-6.3 and the "Bigma" 50-500mm f/4-6.3 perform the same trick.
 
Another possibility is that Canon have ever so slightly changed the interface between camera and lens. Unfortunately Canon doesn't release the spec of the interface so Tamron & Sigma have to reverse engineer it.
 
You can fool the Canon electronics like the sigma lenses do by taping over a couple of the electrical contacts on the lens. The problem is that if there isn't sufficient light for the AF sensors to work then the lens hunts, trying to get a lock.It doesn't always give you a good focus point.

Here's a link to the Fred Miranda site that shows the pins covered.

http://www.fredmiranda.com/TipsPage/example.html
 
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