AF in Z8 against lens brightness

[Jamidu]

I read that when the Z9 came out 2 years ago. I don't remember where

Tkx

 

[Len Shepherd]

also assumed a larger aperture lens would focus better (going from SLR knowledge),

There are several different aspects to consider.

1/ A faster aperture (set on the lens) transmits more light to the AF system.
In very low light either the viewfinder can mush up with electrical noise or AF stops working sooner with a smaller aperture combination than a faster aperture combination.

HOWEVER even the last of the Nikon DSLR's struggled to AF with combinations marginally smaller than f8 - whereas f13 AF works quite well in decent light - with decent viewfinder brightness with Nikon ML.

This significantly improved ML AF ability has opened up many new optical possibilities - such as "impossible" with DSLR AF with combinations like 600 f6.3 and 800 f6.3 with converters.

Off topic - up to 6 stops ML in lens VR in combination with Nikon Pro body IBIS opens up usefully more safe hand holding opportunities than possible with DSLR's.

2/ From the same focus distance in telephoto work (most wildlife photography) the narrow depth a field transition from unsharp to sharp and unsharp the other side at 200mm f2.8 is close to that at 400mm f8, and again close to that at 800mm f16.

A narrow zone of depth of field can help AF accuracy - with smaller aperture long lenses often having a shallow depth of field advantage over f2.8 lenses.
This helps enable good AF at 600mm and 800mm f6.3 with good ML screen brightness in normal daylight - even when a 1.4x or 2x is added.

Canon have an f8 lens that takes a 2x - with AF at f16.

Whatever any photographers perceptions are of AF limitations with DSLR systems - they are literally thrown in to the waste paper bin of history with ML current systems.

3/ On and off topic - the several stops brighter than DSLR viewfinders of ML combined with the distinct ML display of the zone of depth of field makes manual focus easier - to the extent lenses like the 200 f4 macro that do not AF with ML can easily be manually focussed with good accuracy.
While some bemoan the loss of AF with the 200 f4 on ML - my hands on experience (with this over 30 year old lens design) is the AF is pedestrian and noisy - leading me to use MF at least 80% of the time when I used it on the D850.

Further ML AF improvements will come over time - with manual focus eye AF confirmation already available with the Zf.

4/ Summing up.
While extremely good results remain distinctly possible with DSLR AF, ignoring the cost of good ML bodies and lenses, every few months ML moves further ahead of what is possible with the F mount system.

 

[Ken Miracle]

Okay you have just blown my mind (and thank you for that). My understanding was way off. As soon as I read your post I tested it as you suggest (I mean looking at depth of field in viewfinder). Luckily I had my 70-200 f2.8 lens already attached to my Z8 so I went to my living room and focused on my balcony railing with the condo across from me out of focus behind. Sure enough - when I go from f2.8 to f5.6 the condo behind gets somewhat sharper but when I stop down further it does not get any clearer in the viewfinder. I also assumed a larger aperture lens would focus better (going from SLR knowledge), but apparently that is not the case. A huge thank you for clarification. @Jamidu ignore my reply in post four because my understanding was wrong when it comes to mirrorless cameras.

And remember what @DRwyoming was telling you is that what you are seeing is just in the viewfinder where the f/5.6 is used for what you see in the viewfinder the deeper depth of field is still there in the image captured if you were at say f/8.

 

[ricardo00]

I fully agree, but I am asking about cases when the maximum aperture of the lens is 8 or 11. Will AF work fully or not. Do we know the limitation?

I have used my 800mm PF with the Z 2.0TC (so f/13) photographing black skimmers (not a slow bird) with some success:

https://flic.kr/p/2oPrrqz