F/8 Autofocusing impressions

[neuroanatomist]

I've used my 1D X with a 600/4 II and 2xIII to track birds in flight, sometimes handheld. It does fine.  However, when shooting small birds in thickets (branches in front of and behind subject), where the bare 600 II locks onto the bird easily, with the 2x it hunts a bit - in that situation, it behaves a lot like the bare 100-400L.

Did you use spot single point or regular single point focus for this test?

I've tried both, spot AF does not offer a significant improvement. It certainly does help for birds in thickets without the TC in place, though.

okay... is the same for the 7D?

The 7D cannot autofocus with an f/8 combo.

[tron]

If I could afford it, I would preorder the 200-400 but that is not an option(My Wife clarified that for me).

Does she have the same hard feelings for EF500 f/4L IS II ? 

[miah]

Someone please explain this f/8 brouhaha to me: my 5D3 + Kenko 1.4x tele-extender + 400 & 70-300 auto-focus (at f/8, pre-firmware update) all day long. I just downloaded the firmware update and will install it later today, but I've been auto-focusing just fine at f/8 for some time. Am I missing something?

[tron]

Someone please explain this f/8 brouhaha to me: my 5D3 + Kenko 1.4x tele-extender + 400 & 70-300 auto-focus (at f/8, pre-firmware update) all day long. I just downloaded the firmware update and will install it later today, but I've been auto-focusing just fine at f/8 for some time. Am I missing something?

You are missing ... Canon teleconverters which report the correct maximum aperture to camera.

When you put kenco with your say 400 f/5.6 lens does it report it as having a maximum f/8 aperture? I guess not! It still reports it as f/5.6!

You have effectively a 1.4X400 = 560mm f/8 lens but the camera thinks it is connected to a f/5.6 maximum aperture lens and tries to autofocus!

[East Wind Photography]

Tried it with my 600 F4 and didnt like the IQ.  So I put the 2XIII in with my 70-200L IS II where it will stay.  F8 AF was what I expected.  Slower than without and IQ did not meet my QA! 

[Tom W]

I haven't given it a good full test yet, but I am enjoying backyard bird photography with the 5D3, 100-400 and the 1.4X II. Focus is reasonably quick, and it is pretty accurate. The image quality is decent, not on par with the big primes, but definitely not bad either. Occasionally, I find some weird pattern to the bokeh, but I think that's more a fault of the 100-400 than anything.

The true test will come with using some of the bigger glass and teleconverter combinations to see how it works for very long distance shooting. I've found, though, that stacking teleconverters does take its toll on IQ so that the results will be slightly less than optimal.

I'm really considering a 500/4 II in the future - somewhat light, and compatible with the 1.4X or the 2X teleconverters to give a variety of very long lens capability without the heft of the 400/2.8.

[JPAZ]

I don't like being an "early adopter" (although this firmware update is not exactly a totally new thing), but reading these comments has convinced me to go ahead and install it.

[miah]

Someone please explain this f/8 brouhaha to me: my 5D3 + Kenko 1.4x tele-extender + 400 & 70-300 auto-focus (at f/8, pre-firmware update) all day long. I just downloaded the firmware update and will install it later today, but I've been auto-focusing just fine at f/8 for some time. Am I missing something?

You are missing ... Canon teleconverters which report the correct maximum aperture to camera.

When you put kenco with your say 400 f/5.6 lens does it report it as having a maximum f/8 aperture? I guess not! It still reports it as f/5.6!

You have effectively a 1.4X400 = 560mm f/8 lens but the camera thinks it is connected to a f/5.6 maximum aperture lens and tries to autofocus!

When my Kenko is attached to my EF 400 f/5.6 lens, the 5D3 tells me that the max aperture available is f/8. It auto-focuses just fine (I used it that way between my last post and this post). The EXIF data in Lightroom lists the lens as the EF 400 f/5.6, but the images display the correct EXIF data: f/8. So, tron, maybe I'm still not seeing something, but this all jives.

[tron]

Someone please explain this f/8 brouhaha to me: my 5D3 + Kenko 1.4x tele-extender + 400 & 70-300 auto-focus (at f/8, pre-firmware update) all day long. I just downloaded the firmware update and will install it later today, but I've been auto-focusing just fine at f/8 for some time. Am I missing something?

You are missing ... Canon teleconverters which report the correct maximum aperture to camera.

When you put kenco with your say 400 f/5.6 lens does it report it as having a maximum f/8 aperture? I guess not! It still reports it as f/5.6!

You have effectively a 1.4X400 = 560mm f/8 lens but the camera thinks it is connected to a f/5.6 maximum aperture lens and tries to autofocus!

When my Kenko is attached to my EF 400 f/5.6 lens, the 5D3 tells me that the max aperture available is f/8. It auto-focuses just fine (I used it that way between my last post and this post). The EXIF data in Lightroom lists the lens as the EF 400 f/5.6, but the images display the correct EXIF data: f/8. So, tron, maybe I'm still not seeing something, but this all jives.

That's interesting for Kenko! However, everyone who had Canon teleconverters could not have this option!

So as I said before it wasn't possible with Canon teleconverters...

[tron]

I don't like being an "early adopter" (although this firmware update is not exactly a totally new thing), but reading these comments has convinced me to go ahead and install it.

Firmware seems fine   Just read the instructions. The part at the end that says to remove the battery for a few seconds seems new to me. I remember members of this forum mentioning the same thing for the 7D upgrade but I do not remember this instruction for the 5DMkII firmware updates.

(Or there was and I simply omitted it and I didn't know it!! )

[Sella174]

... everyone who had Canon tele-converters could not have this option!

Smells like Canon is doing this on purpose ... because what I can't understand is if the 6D can AF down to -3EV, then it should also be able to AF with an f/11 lens (in bright daylight, that is).

[neuroanatomist]

...because what I can't understand is if the 6D can AF down to -3EV, then it should also be able to AF with an f/11 lens (in bright daylight, that is).

Because it doesn't work that way.  The f/5.6 limit isn't all about the amount of light, it's about the incoming rays being phase-separated with enough resolution to determine focus.  An f/2.8 point has a physically wider baseline (separation between the two sensor lines) on the AF sensor than an f/5.6 point. 

[dougkerr]

... everyone who had Canon tele-converters could not have this option!

Smells like Canon is doing this on purpose ... because what I can't understand is if the 6D can AF down to -3EV, then it should also be able to AF with an f/11 lens (in bright daylight, that is).

The AF aperture limit issue that we normally discuss does not evolve light levels. It is based on optical-geometric considerations.

An AF detector is like a rangefinder, with two sensors looking at the scene  through two "virtual sub apertures" located on opposite edges of the real exit pupil.

To attain a certain accuracy, these sub-apertures must be located a certain distance apart. This distance is like the "baseline length" of a classical rangefinder  (I do not mean "rangefinder camera").

The distance between the two sub apertures is built into the optical system of each AF detector pair. If they are farther apart than the diameter of the exit pupil for the maximum aperture of the lens in place, then no light passes to the detector pairs and AF fails. (So typically the Canon cameras block any attempt when the aperture is not "suitable").

But of course the whole situation is more complex than that, and their are various steps that can be taken to "cheat around" the optical-geometric limitations.

Some of the principles are described here:

http://dougkerr.net/pumpkin/articles/Split_Prism.pdf

Best regards,

Doug

[Click]

Thanks Doug. Interesting read.

[Sella174]

OK, and now for my next dumb assumption ... at f/11 the focus needs to be less accurate than at f/2.8 due to the greater depth of field (front & back) of the former over the latter.