limited AF area R5 + RF600mm F/11

[Leeuw]

Having both RF 600mm F/11 and EF 600mm F/4L II lenses it is surprising and disappointing to see that the max AF area of the RF lens is much smaller than the almost full sensor AF area when the EF lens is used.

The difference is quite big as can be seen in the pictures I took of the R5 flip screen for both situations. Especially for BIF using face detection + tracking this is quite a drawback. But also for spot AF when the subject is outside this ‘small’ AF square.
There is no way to have the AF point beyond this square.

What would be the reason for this?
I guess it has to do with the slow F/11 (the RF800 shows the same issue?) but I can not think of a technical explanation why the AF of the R5 must be limited to this square.

Am I missing something? Has this been reported and discussed before?

R5 with RF600mm :







R5 with EF600mm :

 

[Alastair Norcross]

Having both RF 600mm F/11 and EF 600mm F/4L II lenses it is surprising and disappointing to see that the max AF area of the RF lens is much smaller than the almost full sensor AF area when the EF lens is used.

The difference is quite big as can be seen in the pictures I took of the R5 flip screen for both situations. Especially for BIF using face detection + tracking this is quite a drawback. But also for spot AF when the subject is outside this ‘small’ AF square.
There is no way to have the AF point beyond this square.

What would be the reason for this?
I guess it has to do with the slow F/11 (the RF800 shows the same issue?) but I can not think of a technical explanation why the AF of the R5 must be limited to this square.

Am I missing something? Has this been reported and discussed before?

Yes. Basically, with the 600 and 800 F11 lenses, you are restricted to the kind of AF coverage spread that professionals had to put up with on the flagship DSLRs of the dark ages of 2018 or so.

 

[JustUs7]

Disclosed in the lens specs. Discussed and demonstrated in several reviews, both YouTube and otherwise. Talked about here.

Pretty well known spec or feature of the lens.

Please don’t read tone into this that isn’t intended as it comes from a place of curiosity. I’m always surprised when I read reviews or things like this about glass or other gear that people drop hundreds or thousands of dollars on and they don’t already know things like this. Did you read 600mm and $700 and just bought it without reading anything else? Or is it more of a read it, saw it, but didn’t fully register what it meant until I put it on my camera? Some of us are more experiential learners, so all good either way.

The ones that get me most are the negative reviews about RF glass because it doesn’t work on their DSLR. Your situation isn’t too surprising, but curious to me.

Of course you’ve got a $4,000 camera and a separate $13,000 lens. Probably don’t have to do much homework on $700 lens. . I’d have to run it through the household budget and get my wife’s blessing. I better be able to justify it.

 

[Leeuw]

Thanks for sharing your view on it and your interpretation of my motivations.
For sure I did not dig very deep into the specs of it but also for sure the limited AF area is not one of the first aspects that you read about. The reviews and results I have seen on this board (and no it was certainly not 600mm and $700 = buy) convinced me, and to that respect I am not disappointed in the lens as a whole at all. Although the AF area limitation is a bit disappointing (especially because I can’t understand the technical reason, do you?) I am very happy with this lens and would buy it again for sure. It is wonderful sharp, light weighted and small. At my age the EF600mm is getting quite heavy during the longer hikes so this is an ideal replacement for those situations.
I have also posted this finding with a technical interest to understand the reason why, not to complain.

 

[SteveinLouisville]

I have also posted this finding with a technical interest to understand the reason why, not to complain.

I have an "assumption" that f8 is probably the max aperture that the RF system can use full AF coverage. The RF 24-105 STM (4-7.1) can focus across the frame at 7.1, and my presumption is the new RF 100-400 f5.6-8.0 can too.

Anyone seen a spec on this later presumption of mine? Anything in the literature released with the lens as to whether the RF 100-400 can cover the full frame with AF at f8.0?

I read about the limited AF area for the f11 lenses when they first came out. Kind of amazing it can AF at all at that fixed aperture.

Edit to add: ah ha! Maybe it is because the aperture is fixed at f11? Maybe the others AF wide open, and then the lens stops down when you shoot the picture? True? False? IDK.

 

[JustUs7]

I have an "assumption" that f8 is probably the max aperture that the RF system can use full AF coverage. The RF 24-105 STM (4-7.1) can focus across the frame at 7.1, and my presumption is the new RF 100-400 f5.6-8.0 can too.

That’s probably a safe assumption as lenses, to my understanding, always acquire focus wide open regardless of chosen settings for the shot. Since the f/11’s don’t open any wider than that, there is no acquiring focus at f/7.1 then closing to the desire aperture to shoot. It’s just always f/11 and perhaps less reliable outside the center so they just chose to limit it for consistent results. My best guess anyway.

 

[JustUs7]

The reviews and results I have seen on this board (and no it was certainly not 600mm and $700 = buy) convinced me, and to that respect I am not disappointed in the lens as a whole at all.

Makes sense, and thanks for not letting me have it for asking. Fair enough that it’s not widely discussed in the user experiences and sample images posted on these boards anyway.



They are a lot of fun. I rented one for a Florida trip and used it on my RP. For the cost and weight, it’s an incredible piece of equipment.

 

[Leeuw]

One would think if focussing on centre pixels is possible at native F/11 why not on outer pixels that ‘see‘ the same F/11. Those pixels aren’t any different are they? I wonder what would make it less consistent there. That’s what I like to understand. Maybe focus acquisition is somewhat slower at F/11 and with more pixels to judge (greater area) it slows down to much…….. But that doesn’t hold true for spot AF where only a few pixels are to be taken into account and even then the AF area is limited to the small square.

 

[JustUs7]

That’s getting way outside my pay grade. My finance and accounting education won’t be of any benefit. I’ll let the technical experts take over.



I’m more of a, this is how it works and I accept that, kind of person.

 

[PicPocket]

I have also posted this finding with a technical interest to understand the reason why, not to complain.

I have an "assumption" that f8 is probably the max aperture that the RF system can use full AF coverage.

...

Edit to add: ah ha! Maybe it is because the aperture is fixed at f11? Maybe the others AF wide open, and then the lens stops down when you shoot the picture? True? False? IDK.

I think it is not just aperture, perhaps the optics too. Edge sharpness, vignetting, whatever

I see plenty of fast EF lenses that don't go 100%, while some slower counterparts do

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www.canon-europe.com

If I had to draw some conclusion from that list, I would probably say how clear the lens can resolve at the edges. It's either slow lenses, or older optics where edges drop off quite a bit

Just a guess. I'm curious too

 

[Leeuw]

Beautiful shots! Very inspiring. I own the lens for only a few weeks and looking forward to my first trip with it to France soon (living in The Netherlands). The EF600 will not join the party since it is not a birding trip but I hope to spot several though

 

[tkbslc]

Yes this is a moderately annoying limitation. But it can help keep things in perspective if you consider that the smaller f11 lens box is comparable to the full AF spread on FF DSLR cameras. Yes it is worse than the faster lenses on the R5. But it is not worse than the best coverage SLR photographers ever got.

Here's all the AF points for the 5D4, which is among the best SLRs for AF coverage for FF.

As for a reason, my guess is Canon found the AF was not accurate at the sides due to either the aperture, light path or optical formula and didn't want people getting out of focus pics.

 

[Karl_Guttag]

Thanks for sharing your view on it and your interpretation of my motivations.
For sure I did not dig very deep into the specs of it but also for sure the limited AF area is not one of the first aspects that you read about. The reviews and results I have seen on this board (and no it was certainly not 600mm and $700 = buy) convinced me, and to that respect I am not disappointed in the lens as a whole at all. Although the AF area limitation is a bit disappointing (especially because I can’t understand the technical reason, do you?) I am very happy with this lens and would buy it again for sure. It is wonderful sharp, light weighted and small. At my age the EF600mm is getting quite heavy during the longer hikes so this is an ideal replacement for those situations.
I have also posted this finding with a technical interest to understand the reason why, not to complain.

People sometimes miss things. Beyond the obvious limitation of the fixed f11 aperture, the limited region of AF is well known and mentioned in multiple reviews. The other well-known big limitation is the minimum focus distance of about 15 feet with the RF600 and 20 feet with the RF800.

For example (linked to the point this issue is discussed in the four reviews below):

Once a limitation that is "well known," they don't get discussed much on the forums. Thus it is not discussed much here.

BTW, it is technically not a lens limitation but a camera limitation. In the olden days of DSLR (I don't know when DSLRs started supporting more than f5.6), cameras would not even try to AF with a lens with more than f5.6, which was a problem when using TCs.

Note that there are also issues with certain EF lenses and issued caused when using teleconverters.

On the R5, the AF zone will start creeping in other cases. For example, with the RF100-500 plus 1.4x TC, the AF zone creeps a little, and this is true even at 300mm x 1.4 with a max aperture of f6.3.

 

[Ray Chen]

I put on my 100-400L IS II with a 2x converter, and my R5 was still using the full focusing area when zoomed in for f/11. Eye/face detection works all the way out to the marked perimeter as well. Just for shizz and giggles, I mounted the same combo on my 5D4. Again, the combo will focus to the edge in the liveview mode, provided at 10x slower speed.

Maybe there is a technical reason that Canon thinks the perimeter focusing (outside of 40% horizontal x 60% vertical) is not reliable enough on these RF telephotos. Regardless, I myself have no reason to focus beyond the thirds, but YMMV. Lastly, this was added via firmware upgrade, so it might be one of those Canon crippling hammer doings.

--
Ray

 

[Leeuw]

Yes, good to mention this. The R5 AF is a big step forwards compared to a DSLR to every aspect. Although I have to say that my 7D MkII has a better horizontal spread of AF points compared to the 5D4 (and the R5 with RF600) but its not a FF. And with a F/11 lens the 7D probably would struggle massively. Its impressive what the R5/R6 are capable of.

 

[tkbslc]

Yes, good to mention this. The R5 AF is a big step forwards compared to a DSLR to every aspect. Although I have to say that my 7D MkII has a better horizontal spread of AF points compared to the 5D4 (and the R5 with RF600) but its not a FF. And with a F/11 lens the 7D probably would struggle massively. Its impressive what the R5/R6 are capable of.

Yes, basically the AF point size limit was more an absolute size limit, not relative to the sensor size. Because the focusing did not take place on the sensor. So for crop, that meant almost covering the whole frame, and for FF, only the middle 50% or so.

That was always one of the let downs when moving to FF on SLR. (That and how short all your tele lenses became)

 

[SteveinLouisville]

Is the AF zone affected by a 1.4x or 2x TC? Will the camera have AF at all?

 

[Leeuw]

Thanks for the links. I was aware of the min focusing distance but somehow the limited focussing box of the RF600 (on R5) escaped from my attention.
Yes, I have experience with focus limitations with certain combinations like the 1.4x and 2x extenders on 500mm and 600mm slowing down the lens and limiting the AF points to even only one in the centre. It is amazing what the R5 is capable of.

 

[Alastair Norcross]

Yes, good to mention this. The R5 AF is a big step forwards compared to a DSLR to every aspect. Although I have to say that my 7D MkII has a better horizontal spread of AF points compared to the 5D4 (and the R5 with RF600) but its not a FF.

Yes, the 7DII has just about the best coverage of any DSLR. But, as others have pointed out, crop cameras were always capable of wider coverage than FF. For example, the 6DII and the 80D have pretty much the same AF modules, but on the 80D the coverage is a lot wider, because the sensor is smaller.

And with a F/11 lens the 7D probably would struggle massively.

Not just struggle. It wouldn't focus at all. The 7DII is limited to F5.6 for all points except the center point (and surrounding helper points, I think).

Its impressive what the R5/R6 are capable of.

 

[Leeuw]

Nice experiment. Maybe in low light situations the R5 perimeter focusing for some reason is less reliable in combination with a F/11 lens and Canon want to prevent having troubles there.
An example where I could have used a more perimeter AF point for the composition and have the big sunflower a little bit more to the right when using servo AF in windy condition. Not that this is a nice composition, just as an example.