F/8 Autofocusing impressions

[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).

 

[Loswr]

...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.

 

[Loswr]

Further to what Doug stated, it's always been possible to 'trick' Canon's f/5.6 AF points into focusing with an f/8 max aperture, with pin taping or a TC that doesn't properly report. Canon did not allow the functionality because it wasn't completely reliable. Their recent firmware, possible coupled with higher density line sensors on the 1D X/5DIII (which is how the past 1-series bodies achieved f/8 AF), allowed them to enable it with confidence.

I also suspect they built the capability into the 1D X/5DIII sensor, but left it disabled in firmware until later.

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.

Correct. The smaller the baseline of the sensor, the less accurate. But, the narrower the selected/available the aperture, the deeper the DoF and the more inaccuracy tolerable in terms of focus.

 

[Tom W]

Further to what Doug stated, it's always been possible to 'trick' Canon's f/5.6 AF points into focusing with an f/8 max aperture, with pin taping or a TC that doesn't properly report. Canon did not allow the functionality because it wasn't completely reliable. Their recent firmware, possible coupled with higher density line sensors on the 1D X/5DIII (which is how the past 1-series bodies achieved f/8 AF), allowed them to enable it with confidence.

For the record, utilizing stacked teleconverters or third-party telecons (like the Kenko), I have been able to reliably autofocus with great results on the 5D3 (center region AF points), fair results (center point only) on the 5D2 and 5D, and "hit-and-miss" with the 7D.

Basically, the results I got with the 5D3 were so good that I was surprised that Canon hadn't included f/8 AF on that body right from the start. Perhaps they needed to iron out some issues first, I don't know.

 

[tron]

I was surprised that Canon hadn't included f/8 AF on that body right from the start. Perhaps they needed to iron out some issues first, I don't know.

+1 Same with the 1DX. As it was replacing 1D4 (at least!) it was very strange that they omitted this feature initially.

 

[tron]

Finally I found time to test my 5D3 with a 300mm f/4 (non-iS) lens with EF2X II teleconverter.
The test was inside my house and not much light.

It worked fine

 

[tron]

Finally I found time to test my 5D3 with a 300mm f/4 (non-iS) lens with EF2X II teleconverter.
The test was inside my house and not much light.

It worked fine

More information about "not much light"

Shutter: 1/50 Aperture: f/8 ISO: 25600

According to an EV calculator I found it worked fine to 3.6EV.

Next step would probably be to use half the lights I used...

 

[AlanF]

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?

The Kenko locks up the 5DIII when used with the latest generation telephotos, and you have to remove the battery to restart.

 

[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?

The Kenko locks up the 5DIII when used with the latest generation telephotos, and you have to remove the battery to restart.

I found this to be true ONLY when AFMA is enabled. Otherwise, Kenko TC's autofocus fine at f/8 and did so prior to the 5D3 firmware update.

 

[Loswr]

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?

The Kenko locks up the 5DIII when used with the latest generation telephotos, and you have to remove the battery to restart.

I found this to be true ONLY when AFMA is enabled. Otherwise, Kenko TC's autofocus fine at f/8 and did so prior to the 5D3 firmware update.

My car runs fine, except when I try to back up while turning - then the steering wheel locks up. But otherwise, it works fine. Maybe that fits some people's definition of 'works fine' - not mine.

But hey, if locking up with AFMA means the Kenko TC 'works' then you could also say the Canon 1.4x 'works' with the 70-300L. If you zoom the lens out to the 250-300mm range, the 1.4x TC can be mounted. It will AF on the 5DIII, you can AFMA the combo, and your camera won't lock up. Retracting the zoom will bump the rear lens element into the rubber ring on the TC, long as you keep the zoom extended, it 'works' just fine.

 

[miah]

My car runs fine, except when I try to back up while turning - then the steering wheel locks up. But otherwise, it works fine. Maybe that fits some people's definition of 'works fine' - not mine.

But hey, if locking up with AFMA means the Kenko TC 'works' then you could also say the Canon 1.4x 'works' with the 70-300L. If you zoom the lens out to the 250-300mm range, the 1.4x TC can be mounted. It will AF on the 5DIII, you can AFMA the combo, and your camera won't lock up. Retracting the zoom will bump the rear lens element into the rubber ring on the TC, long as you keep the zoom extended, it 'works' just fine.

Touche, Neuro, I clearly should have said it works fine--provided AFMA is not required. As stated in my other thread, I'm not happy about not being able to AFMA the lens + Kenko TC combo (although I'm still playing with possible work-arounds). That said, I'm getting well auto-focused shots from my various lenses + Kenko TC combos, including at f/8, and they are quite sharp.

 

[Tom W]

The Kenko locks up the 5DIII when used with the latest generation telephotos, and you have to remove the battery to restart.

I found this to be true ONLY when AFMA is enabled. Otherwise, Kenko TC's autofocus fine at f/8 and did so prior to the 5D3 firmware update.

Agreed - I had that issue prior to the new firmware also. Could not micro-adjust the Kenko-lens combination because the camera would lock up. The solution was to delete the MA setting for that lens and not use AFMA with my Kenko (which I only used for F/8 AF situations anyway).