Teleconverter contacts and the f8 requirement

[Member69643]

<p>As many of you know, Canon's teleconveters only AF with certain bodies and lens combinations. Enterprising folk have discovered that if you cover one of the teleconverter's contacts with tape, AF is "restored" on those bodies that don't normally allow AF with a teleconverter attached. Does anyone know, is that contact's sole purpose to communicate with the body and jointly decide whether to allow AF? I have the contact on my TC taped over because I like having even limited AF on all my EOS bodies (and it actually works pretty well in good light) but I do have a 7DMkII, which is one of the bodies that supports the TC and AF. I have been using the TC on the 7DMkII with the contact taped over but will that affect AF performance?</p>

<p>Anyone know?</p>

[colin carron]

<p>As far as I know 'taping the pins' in Canon extenders just restores the blocked AF function. I assume Canon officially allow only the most suitable body / extender / lens combination to retain full AF function but at the same time make it so that a lower standard of AF cab be restored to less suitable combinations using the simple 'taping the pins' work-round. Canon seem to be saying 'do that if you want but it is not up to our exacting standards'.<br /> I use a 6D with 100-400 zoom with pins taped and as you say it works reasonably well under most circumstances. I have not noticed any other functions being affected.<br>

Taping the pins :<br>

http://www.michaelfurtman.com/taping_the_pins.htm</p>

[dcstep]

<p>It may impact AF performance when using an EF 2.0x TC-III with a Series II lens. In the Series II lens literature, it talks about improved AF performance when used with Type-III TCs. They don't get specific, but I assume it's quicker acquisition and/or better tracking. You might lose that when taping a contact.</p>

<p>Why don't you try with and without and tell us what you find?</p>

[bobatkins]

<p>Canon TCs slow down AF for better accuracy. In fact if AF isn't slowed (as is the case with some 3rd parts TCs), AF may hunt for focus and never lock.</p>

<p>Do TCs with taped contacts slow down AF? If not, accuracy may suffer.</p>

<p>I would be very surprised if AF conformance on a 7D MkII with taped pins wasn't worse than AF performance with all the contacts in use.</p>

<p>I do not think the pin(s) in question just say "turn off AF". I strongly suspect they tell the camera body that a TC is attached and which TC it is. That then tells the camera how to best adjust focus and exposure parameters.</p>

[steve_dunn2]

<p>The pins actually tell the <em>lens</em> that there's a TC attached, and which magnification it is (1.4x or 2x; the life-size converter for the 50mm macro also uses these pins). The lens then communicates this information to the body.</p>

 

<p>Obviously, the end results include the body knowing the effective focal length and aperture range of the lens+TC, and as mentioned above, AF can slow down for improved accuracy. Another effect, which can be particularly useful in these days of being able to correct for things like distortion and vignetting, is that the body can record the actual lens used in the EXIF data, so (for instance) while a 200/2 + 2x is effectively the same speed and focal length as a 400/4, the body can tell the two apart, and so can your editing software.</p>

 

<p>And just in case anyone isn't clear on why the AF system doesn't work with lenses slower than f/5.6 or f/8 (depending on the body), it's a physical limitation of the way phase-detection AF works. You can engineer a phase-detection AF system to work at slower apertures, but there are tradeoffs in speed and accuracy (which is why bodies that have "high-precision" AF points need faster lenses to activate those points than they do for the rest of the AF points). Canon has chosen f/5.6 for most AF points in most bodies. That doesn't mean that (say) an f/6.3 lens cannot work; there are plenty of third-party zooms that are f/6.3 at their long ends, yet they work, so Canon has engineered a safety margin into the system. And that's what allows some body+lens+TC combinations to AF if you tape the pins: you've gone beyond the official limit but are still (barely) within the limits of what the hardware can do.</p>

 

<p>(So you're probably asking "Why doesn't the body disable AF with a slow third-party lens?" The answer: third-party lenses lie about this and tell the body they're actually f/5.6. It doesn't throw off metering; the body basically thinks the scene is about half a stop darker than it really is, and when the body tells the lens to stop down, the lens compensates for the difference.)</p>

[Marcus Ian]

<p>The phase detection system's limitation doesn't come from the aperture of the lens, it comes from the amount of light striking the sensor. Granted, a smaller max aperture means less light comes through the lens, but on a brightly lit scene, even a cheap AF system can usually effectively focus -even when the lens/TC system is f8 or even f11 (in my experience) especially if you only use the center point- Though the system will often fail in marginal light. </p>

<p>The reason Canon put an f5.6 limitation is because they didn't want to put a more sensitive (expensive) system in, and didn't want a bunch of newb photogs returning cameras because their "lens won't focus right!" in low light situations...</p>

 

[steve_dunn2]

<p>The amount of light coming in does indeed affect the AF system, but that's not where the f/5.6 (for most bodies) limit comes from. If it was, then an autofocus system that would work at f/5.6 at dusk would work with even ridiculously slow lenses as long as there was bright sunlight - but if you try it, you'll find it doesn't work. You might get an extra stop or two, but after that, you can add all the light you want and the AF system simply will not work.</p>

 

<p>The reason is that the light being sent to the two halves of the AF sensor comes from the outer parts of the cone of light that the lens sends into the body. Have a look at <a href="http://www.clarkvision.com/articles/understanding.autofocus/" target="_blank">this page</a> for an explanation, complete with diagrams, some of which illustrate what happens if the aperture is smaller than what the engineers set as a requirement. You can make the AF system more accurate by sampling light from farther apart in the cone (which is what the high-precision f/2.8 sensors do), but that requires a wider cone of light, which is only possible with a faster lens. Likewise, you could make an AF system that will work with as slow a lens as you like (as long as the scene is well illuminated) but accuracy will suffer.</p>

 

<p>Some manufacturers, such as Canon, choose to turn off the AF system if the effective aperture of the lens (including any TC) is slower than a certain limit; others choose to allow the AF system to try to do its thing, even if that may be slow or inaccurate. The advantage of Canon's choice is that it (as you put it) prevents newbs from returning their supposedly defective cameras. The advantage of doing it the other way is that you don't arbitrarily lose AF in situations in which it might actually work.</p>