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1D X can AF at f/8!

Yakim Peled , Sep 04, 2012; 05:19 p.m.

I kid you not. Look here. What I can't understand is why the 5D3 could not AF in such conditions.

Happy shooting,


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Bob Atkins , Sep 04, 2012; 06:24 p.m.

It's odd. It does it only with the Kenko TC and that in itself is not surprising. I get excellent AF with a Tamron 1.4X TC and a Canon EF 500/4.5L (effective f6.3), whereas AF shuts off with the Canon 1.4x TC (5D and 7D).

What's odd is the difference between the 5D MkIII and the 1D X since they both use the same AF sensors. The 5D MkIII seems to be recognizing the Kenko as a TC, while the 1D-X doesn't seem to know it's there. My 7D doesn't now the Tamron 1.4x is there when I use it with my EF 500/4.5L

Could be a bug in Canon's firmware for the 1D-X. That's more likely than them deliberately enabling AF with a 3rd party TC. Maybe they'll fix the "bug" in the next firmware revision!

Don Baccus , Sep 04, 2012; 06:28 p.m.

It's because the 1.4x converter he's using lies to the camera body, which thinks his 800/5.6 + 1.4x is still a F5.6 lens. This trick has been used before with EOS bodies ...

The real question is why it's not able to lie successfully to the 5D3 ...

Don Baccus , Sep 04, 2012; 06:48 p.m.

This comment by Arthur is weird:

"ISO 400: Evaluative metering -1 stop: 1/640 sec. at f/9. Why -1 stop? The Canon system does not note the loss of one stop of light from the Kenko TC"

Um. how can it not notice? Less light's coming through the lens, older metering systems just measured the light without using max aperture information from the lens! This just seems weird.

Bob Atkins , Sep 04, 2012; 09:13 p.m.

It's possible that metering with the TC may be off. I remember that I used my Tamron 1.4x on my EF 500/4.5L with a previous camera (maybe EOS 20D or 40D) and it did need some exposure compensation dialed in. With the 7D it doesn't.

I remember that the old 7e (the film camera!) need as much as 3 stops of EC when using some manual focus lenses. I think it depends a lot on the optics of the AE sensors.

The reason why the TC needs EC could be the same reason that some manual focus lenses need exposure compensation, i.e. the position of the exit pupil of the lens shifts and that confuses the metering. If it happens with the Canon TCs, they can electronically communicate any compensation required to the body, so it's all transparent to the user. It's also possible that it's not sending some signal to the body that it normally gets from the lens which shifts the metering to compensate for the lens optics. Who knows what the lenses and camera bodies are talking about. I think there's more going on than just a report of an aperture number and driving the AF motor. The body is probably reading data out of a ROM in the lens.

There's something odd going on because the correct focal length and aperture apparently get recorded in the EXIF data with the 1D X. It's almost as though the 1D-X reads that data and ignores it, while the 5D3 reads the data and says "Whoa, that's an f8 lens, I'm turning off the AF". Again, it could possibly be an unintended consequence of a firmware bug.

Sarah Fox , Sep 04, 2012; 09:18 p.m.

It's my understanding that AF capabilities have relatively less to do with the amount of light streaming through the lens and much more to do with the differences in ray path angles. At wider apertures, there's a bigger difference in incident angles from opposite sides of the rear element to the focal plane; thus, the phase detectors can achieve a more accurate focus. It would be similar to comparing the accuracy of focus of two rangefinder cameras, one having a greater distance between the rangefinder windows. The manufacturers decide their phase detectors are sufficiently accurate to a certan cutoff aperture, and that's the aperture at which they disable the AF. But again, it's not as much about light. I can focus some of my cameras in light so dim that I'm not able to focus manually, even without a focus assist beam. (Admittedly it's a bit hit and miss.)

Geoff Francis , Sep 05, 2012; 12:21 a.m.

Sarah's answer would seem to make some sense. An f8 or even f11 lens in bright light will have a much higher EV than an f2.8 lens in a dim room that can still AF.

Bob Atkins , Sep 05, 2012; 01:37 a.m.

That's not the point. It's well known that illumination level isn't the reason for the f5.6 cutoff. Take that as read. Non-issue.

The issue is why the 1D X and 5D MkIII are different since they use identical AF sensors. Their firmware is obviously different. The 5D MkIII acts as you would expect an EOS body to do, the 1D-X doesn't. Whether that is a feature or a bug probably depends on whether you're Canon or a user. If an EOS body with an f5.6 cutoff detects a TC has been mounted it cuts off AF. Since the 1D-X writes correct EXIF data to images shot with the TC (according to the report), it appears to know a TC has been attached, yet it doesn't cut off AF like it should. Canon would not do this deliberately for 3rd party TCs, yet retain the f5.6 limit for their own TCs. Either the 5D3 and 1D-X are looking at different data coming from the lens, or the 1D-X firmware isn't catching the TC like it should ("should" from Canon's viewpoint).

Ed Avis , Sep 05, 2012; 07:10 a.m.

The more interesting question is that if the AF functions successfully under these conditions, why does Canon disable it? A lot of buyers would like to keep f/8 autofocus. Perhaps the Canon 1.4x teleconverter, having different optics to the Kenko one, would not work with the focus points in the new body?

Perhaps a 'sticky tape modification' can be done to the Canon TCs to keep autofocus at f/8 on the newer bodies.

OTOH, I have seen that an older 1Ds body paired with a Sigma 180/3.5 lens and Sigma 2x teleconverter would not autofocus, even though the body is supposed to autofocus up to an effective aperture of f/8. This is with third party optics, of course, so from Canon's point of view all bets are off.

Yakim Peled , Sep 05, 2012; 08:13 a.m.

Whether that is a feature or a bug probably depends on whether you're Canon or a user.

OT: I used to work in IT. As a QA I used to find many bugs and often the reply from the developers was: It's not a bug, it's a feature. As you can guess, we had a lot of quarrels about it. :-)

Happy shooting,

The difference between a bug and a feature...

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