Canon extender tape trick?

[Skyline On Fire]

Can anyone go into more detail about how this "tape the contacts to enable autofocus with 1.4 and 2x extenders" works?

 

[kiko]

you've gotta remember there is still an effect of light loss, so autofocus will work but achieving focus lock can sometime be a pain,. it been about a year since i did it last but i'm pretty sure its these three.

i've got the canon 1.4x tele to use on my dad's 70-200mm f2.8, but got the kenko 2x dg pro 300 so that if i wanted to take any moon shots (or just massive zoom shots) i could stack them together and get 1000mm + (300mm f4 x 1.4 x 2 x 1.3 1dmk2)

now i have my e-p1 adapter to canon i'm waiting for a clear night so that i can effectively shoot 1500mm+

 

[Scotty Pro]

Have a look here, its at the top of the page

 

[HoppyUK]

The taping trick usually doesn't work very well, certainly not as well as normal, and sometimes not at all. Some camera models take to it better than others. There doesn't seem to be any obvious reason why some cameras are better at it - my old 350D would do it just about, but my newer and more expensive 40D would not.

With Canons, the AF system switches off after f/5.6 to ensure reliability. It does this because at higher f/numbers the diameter of the lens aperture, as seen by the AF system, becomes too small.

So by taping the pins, the camera will attempt to AF, but it doesn't make the diameter of the aperture any larger so it will still struggle. Sometimes it struggles successfully, sometimes not.

 

[Roy C]

Luke, 'Hoppy' has summed it up well.I tried both taped Kenko Pro 1.4x and Canon 1.4x tc (not at the same time) with a 400/5.6 for quite a while, it worked reasonable well on the 30D but from the 40D onwards things were not the same (something to do with the AF sensors I believe). Works better on the outer focus points rather than the centre one on the 40D and 7D but still very sluggish. I still used a taped tc on the 40D but always used a tripod where it is easier to revert to manual focus if needed.

Have you tried Live view AF on your 7D Luke? - it is a contrast detect AF and works quite well in my experience but I believe it is somewhat lens dependant and needs good contrast of course. Slower than the normal phase AF but you do not need to tape any pins, worth a try.

BTW, I was playing around with a 7D + 300/2.8 and stacked 2x tc's yesterday (1200mm at effectively f11) and in Live view AF it focussed surprisingly well.
I also use the 7D live view AF with my 7-200/4 and a Canon 2x tc and that also works well.

 

[HoppyUK]

Interesting that the 7D doesn't like the taping trick either, even with its super-dooper AF system.

Normal phase-detect AF works quite well in lower light, but needs a physically wide aperture - it works even better at f/2.8 or lower. Live view's contrast-detect AF cares less about the f/number, but likes decent light and a nice contrasty target to lock on to.