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canon20D extension

Hi Chieh

I bought your book - which I think is great - then visited your web site. Although I'm technically challenged you explained things pretty well.

I have an canon EF100-400 4.5-5.6 L IS lens and a 2x extender on my 20D. I heard from someone and found a web site
D30/D60 Tips and Techniques
that shows how to get this set-up to AF by sticking scotch tape on three pins. I tried this and the camera goes into hunt mode but does not stop at the focus.

Is this the right way to do it?
Am I doing something not quite right?
What is your opinion - I trust you - about this fix?

I'm going on a birding trip next Wednesday so would appreciate a timely reply.

Thanks
Gill
PS It's about time someone fixed up the primitive software on even expensive cameras! Keep up the good work

Gill Arden
Thu, 16 Mar 2006 08:06:32 -0800

Gill,

Thanks for your encouragement on my book. I really do appreciate feedback.

You performed the hack correctly. The problem is not that you taped the pins wrong, the problem is in the fundamental way AF works. On the entire Canon EOS SLR cameras line-up (with the exception of a few camera bodies), the camera AF algorithm can only auto-focus with a lens aperture opening of f/5.6 or bigger. This is because the AF sensors need to gather a certain amount of light (to auto-focus on) before failing completely.

I don't know exactly what the three extra pins on the teleconverter (TC) do, and I don't have a EOS SLR/lens handy to play around with right now. But I believe the pins are the teleconverter signals to tell the camera that it is attached and it has decreased the aperture by 2 stops. Remember a 2x teleconverter causes 2-stops of light loss (why? Read my "Stacking Teleconverters" article elsewhere on this site). Therefore, your Canon EF 100-400 f/4.5-5.6 L IS Lens with 2x TC becomes a 200-800 f/9.5-11 lens (see "F-Stop Quick Reference").

The camera's AF algorithm cannot auto-focus at f/9.5 or f/11 and normally wouldn't, because the TC would have told it so. But by covering the three electrical pins, you've stopped that communication. So the camera think the lens aperture is still f/4.5-5.6 and attempts to AF, resulting in AF hunt. Have you tried to AF (with no TC) in the dark or in a dim room? It's the same concept. I only know of one Canon EOS cameras that could auto-focus at f/8, which is the Canon EOS 3 camera. And it could only AF at f/8 using its center AF sensor.

So why do people provide tips to trick your camera to auto-focus. Because on a bright and sunny days (especially in noon time) in an open field with no shades, there could still be marginally enough light hitting the sensor to achieve auto-focus.

You see, most people using this trick are using a f/4 lens with a 2x TC; resulting in a f/8 lens. Sometimes, on a bright and sunny day, a sharp f/8 lens can still AF, very slowly, with a bit of hunt. The f/5.6 AF cut-off is an artificial limit set by the AF algorithm design. For example, you can still AF at f/6.3 with 75% (made-up number) success rate, AF at f/6.7 with 50% (made-up number) success rate, and AF at f/8 with 10% (made-up number) success rate, etc. I think you get my drift.

Test this trick out on your lens on a bright sunny day at noon at an open field. If AF works, then you have a chance of using it in the field. And if it doesn't then you are out of luck. If your birding trip involves trees and jungles (shades), then I wouldn't bother counting on this trick. Instead, spend the time learning to manual focus quickly, such as memorizing which way to turn the focus ring to focus closer. Manual focusing is a good skill to have anyway, even if you are lucky enough to AF at f/9 or f/11, because as my example and your experience show, the AF success rate at small aperture is extremely low.

Chieh Cheng
Thu, 16 Mar 2006 17:20:40 -0800

Chieh
Thanks for your super-prompt reply. It's pretty annoying when the camera manufacturers put so little expertise in the software! The lenses are critical but as you show in your book the software could do with a hefty dose of improvement.

I'll try the things you suggested on the weekend and I'll get back to you. I will also recommend your book to other photographers.

Thanks
your friend
Gill

Gill Arden
Fri, 17 Mar 2006 00:35:39 -0800

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