5DM3 works at f/8...sort of...

Got my 5DM3 today and have been running it through its paces. Just for kicks, I threw on a Tamron non-reporting 1.4x TC on my 600mm with another 1.4X giving me a focal length of 1176mm @ f/5.6 (at least that's what the 5DM3 thinks. It's actually f/8, but since the non-reporting teleconverter isn't seen by the 5DM3, it thinks it's f/5.6) And???

It autofocuses w/o hunting like crazy. It's a tad slower and jitters on occasion, but it's definitely useable. I have yet to try it on BIF, but I'm quite certain it should work reasonably well.

Besides, the 600mm @ /f.8, I tried the non-reporting TC 1.4x on the Canon 400mm f/5.6 and the Canon 100-400 f/5.6. Both are useable!

Here's a sample I took at the peanut feeder this afternoon under dark and gloomy conditions (Canon 600mm + 1.4x + 1.4x, Canon 5D Mark III, ISO 1600, 1/640 sec):

The 5DM3 autofocus is excellent!

Alan

It's actually 1200mm since a 1.4x converter is actually a 1.414x converter

What's a few mm amongst friends? (I'll remember that for future reference.)

Sorry, you must be mistaken. It has been stated, by experts, that the nature of the new af system makes it physically impossible for it to auto focus at f/8.

Hi Neil,

It will work by tricking the camera into thinking that it does not have the teleconverter installed between the camera body and lens. There are two ways of accomplishing this:

1. Use a TC that does not have all the pins necessary to electronically report to the camera that there is a TC attached or...

2. Tape the pins of the TC preventing the electronic reporting of the TC.

This trick worked "so-so" for earlier camera models like the 20D that also would not autofocus with f/8 combinations. With the 7D, it didn't work at all for me. But with the new 5DM3, it's the best I've experienced out of any Canon camera that would not allow f/8 combinations to work.

This doesn't mean that I will use it extensively as I never was a fan for stacking TC's or using a 2x with my 600mm since it does cut down on the sharpness, but there are occasions in which you want that extra reach for helping to ID a distant bird for example.

One of the major complaints of the new pro-body 1DX is that it won't autofocus at f/8... since the 5DM3 and 1DX share similar autofocus systems, maybe there is a way to get around this problem by tricking the camera.

Alan

Excellent intel. I wonder if with the 1.4x III you could do the same... I have to try that when I get mine today.

Hey Alan, I was just poking fun at some of the rumours and theories that have been going around in relation to these new bodies, e.g. "The phase detecting AF becomes more difficult at smaller apertures, not because there is less light but due to the angle of incident light. At f/8 the angle of incident light is not steep enough for precision phase-detect. 1DX uses new AF sensors and the split signal at f/8 is probably below its angular detection threshold. Again nothing to do with low-light AF sensitivity."
You have shown that it is not impossible for it to AF at f/8 (and regardless of what the camera thinks it is, you have an f/8 optic in your configuration), but that it just might not be as reliable as some would like. This means that if a 5DIII or 1DX (as they essentially share AF systems) won't AF at f/8 it is because it is programmed to not even try. It could just as easily be told to "have a go at it", e.g. via a custom function.

Neil Fitzgerald wrote:Hey Alan, I was just poking fun at some of the rumours and theories that have been going around in relation to these new bodies, e.g. "The phase detecting AF becomes more difficult at smaller apertures, not because there is less light but due to the angle of incident light. At f/8 the angle of incident light is not steep enough for precision phase-detect. 1DX uses new AF sensors and the split signal at f/8 is probably below its angular detection threshold. Again nothing to do with low-light AF sensitivity."
You have shown that it is not impossible for it to AF at f/8 (and regardless of what the camera thinks it is, you have an f/8 optic in your configuration), but that it just might not be as reliable as some would like. This means that if a 5DIII or 1DX (as they essentially share AF systems) won't AF at f/8 it is because it is programmed to not even try. It could just as easily be told to "have a go at it", e.g. via a custom function.

Neil, now that you picked up my comments:

That is not my theory, that is how phase detect AF works, it's all about the phase difference between the split images. Please read a little bit on the subject before commenting.

Also please do show me where I said "it was impossible" for the camera to focus at f/8. I said it was more difficult. Please show me where I used the word "impossible" so I can retract my comments, Furthermore I will reconsider contributing to your site.

Thank you

Arash, I picked that up as one example how we all theorise (me too) about what might be going on. I don't think I said you said it was impossible. If I inadvertently implied it, I'm sorry.
You said "the split signal at f/8 is probably below its angular detection threshold." The use of the word "probably" suggests to me it was a theory. I never questioned what you said about how phase detection works.

Basically, there have been two schools of thought on this f/8 AF issue; either it can't, or it won't. These are different things (as I tell my son often), and the evidence Alan provides here is that it is not that they can't.
I'm not sure what site you are referring to.

Canon cameras have been fooled by off brand TC's since AF was invented. The fact that Canon does not "support" AF above a certain minimum aperture is because it's simply not reliable. I would stick to the limitations of what the manufacturer designs the system to operate within.

P.S. to ahazeghi If Neil misquoted you I am sure it was an honest mistake. You have contributed a lot to the site with over 4000 posts. I would not allow an honest mistake or misquote deter you from continuing further.

Has anyone tried this with a 2XTC taped as my 7D will allow me to use 2x 1.4 TCs but not a taped 2X TC on my 500 f4 IS.....weird huh?