The number of electrical contacts on the mount of an EF lens

[insanitybeard]

Hello all,
             Sorry if this topic has been covered before but just curiosity on my part. Was having a look at the rear mounts of some of my lenses and noticed some have different numbers of electrical contacts on them. My three non-IS lenses have 7 contacts in total, one group of 5 and one group of two which is slightly more recessed into the mount (and of these one of the two contacts is considerably wider than the other). However, my 70-200 F4 IS has 10 contacts, one group of 8 and the other 2 recessed further same as above. Now initially I assumed that the extra contacts were to do with the IS mechanism but having looked at the corresponding contact pins on my 7D body I see that there are no pins on the body to make contact with the additional 3 pins of the lens (ie, the body only has a total of 8 pins and two of those pins contact the single wider contact on the lens)- therefore it cannot be to do with the IS mechanism and the crop sensor body does not support whatever function the extra contacts are for... Do the full frame bodies have more contact pins and if so what additional functions are they for? 

 Hopefully I have explained myself well enough!
  Thanks for your input! 

[rpt]

Take a look at:
http://en.wikipedia.org/wiki/Canon_EF_lens_mount
I am sure you will find more resources on the net if you search.

[insanitybeard]

Thanks! Problem solved. Perhaps I should have looked there in the first place. Does that mean that even the full frame bodies only have a total of 8 contact pins as well?

[rpt]

Thanks! Problem solved. Perhaps I should have looked there in the first place. Does that mean that even the full frame bodies only have a total of 8 contact pins as well?

Curiosity aroused and here are my findings:
My 5D3 has 8 pins. The 24-105L has 7 pins but one is twice the width - so I count it as 8. Same for my 40mm 2.8. Now my 100-400 has 8 plus 2 (with one as wide as 2) so that totals to 11. As it cant make contact on the camera, I feel they may be connected up on a test-bench in a Canon repair center. I speculate it may be to program the lens... I am sure if we google (or Yahoo or Bing - or whatever) we will get more info.

[insanitybeard]

It's ok, that wikipedia link you provided gave all the answers- the additional pins are for the Canon teleconverters/ close up adapters to allow the change in focal length/ focus distance to be registered by the body for focusing/metering/flash calculations.

[neuroanatomist]

As it cant make contact on the camera, I feel they may be connected up on a test-bench in a Canon repair center. I speculate it may be to program the lens...

I recommend some additional speculation...   

What's different about your 100-400 from your 24-105 and 40/2.8?  The OP mentioned 'extra' contacts on a 70-200/4 IS. What do those two lenses have in common, that lenses like the 24-105 and 40/2.8 don't?  Hint: you'll find 'extra' contacts on L-series prime lenses of 135mm or longer and L-series zooms of 70mm and longer, except the 70-300L.

The 'extra' contacts are for communication with teleconverters, so they are found on lenses that are compatible with Canon extenders (and also on the 50mm compact macro, for communication with the life size adapter). 

[rpt]

As it cant make contact on the camera, I feel they may be connected up on a test-bench in a Canon repair center. I speculate it may be to program the lens...

I recommend some additional speculation...   

What's different about your 100-400 from your 24-105 and 40/2.8?  The OP mentioned 'extra' contacts on a 70-200/4 IS. What do those two lenses have in common, that lenses like the 24-105 and 40/2.8 don't?  Hint: you'll find 'extra' contacts on L-series prime lenses of 135mm or longer and L-series zooms of 70mm and longer, except the 70-300L.

The 'extra' contacts are for communication with teleconverters, so they are found on lenses that are compatible with Canon extenders (and also on the 50mm compact macro, for communication with the life size adapter).

DONG! (bells going of... speculative bells as yet...     )
So when you guys talk about taping contacts on the TCs you are only talking about taping those extra contacts?

[neuroanatomist]

So when you guys talk about taping contacts on the TCs you are only talking about taping those extra contacts?

Ding DING DING.  We have a winner!   

[rpt]

So when you guys talk about taping contacts on the TCs you are only talking about taping those extra contacts?

Ding DING DING.  We have a winner!   

Thanks!

I know Wombat goes on about the Kenko TCs but will a Canon 1.4X III taped work for a 100-400L? Do you know?

[neuroanatomist]

I know Wombat goes on about the Kenko TCs but will a Canon 1.4X III taped work for a 100-400L? Do you know?

Define 'work'.      With the TC pins taped and a 100-400 mounted, the camera will try to AF.  Whether or not it actually succeeds in locking focus depends on the subject (lots of light and a high-contrast subject works ok, although noticeably slower than native), low light/low contrast and AF will fail.

It's worth noting that the f/5.6 limitation applies only to phase detect AF - if you use Live View with contrast detect AF, you can AF without taping pins, even with the 2x TC (but very sllllllloooooooowwwwwwwly).

[rpt]

I know Wombat goes on about the Kenko TCs but will a Canon 1.4X III taped work for a 100-400L? Do you know?

Define 'work'.      With the TC pins taped and a 100-400 mounted, the camera will try to AF.  Whether or not it actually succeeds in locking focus depends on the subject (lots of light and a high-contrast subject works ok, although noticeably slower than native), low light/low contrast and AF will fail.

It's worth noting that the f/5.6 limitation applies only to phase detect AF - if you use Live View with contrast detect AF, you can AF without taping pins, even with the 2x TC (but very sllllllloooooooowwwwwwwly).

Thanks! Unfortunately nobody has a rental where I live so I will have to try it out in the shop. This was very helpful.

insanitybeard, sorry for hijacking your post. Anyway, this too was about contacts so I hope its ok.

[insanitybeard]

No worries as far as I am concerned! Thanks for the input guys!

[Mt Spokane Photography]

AF with those pins taped varies from model to model and body to body.  I taped several lenses and tried TC's on them with my 5D MK III, and they struggled or did not AF at all.  Only one, a old 70-200mm f/2.8L did what I'd call a reasonably fast and accurate autofocus with TC's stacked.  My old 40D would AF on almost any lens with TC at f/8 and usually f/11.  It just is unpredictable.

[rpt]

AF with those pins taped varies from model to model and body to body.  I taped several lenses and tried TC's on them with my 5D MK III, and they struggled or did not AF at all.  Only one, a old 70-200mm f/2.8L did what I'd call a reasonably fast and accurate autofocus with TC's stacked.  My old 40D would AF on almost any lens with TC at f/8 and usually f/11.  It just is unpredictable.

Right. That is why I am not going to buy till I try. I miss the reach of the crop sensor so I figure at least getting to 560mm with a 1.4x will help.