p.1 #1 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
Hey all,
I have a Z7, D500, and 300mm f/2.8 VR (first version).
On the D500 it racks focus as you'd expect - fast, reliable, and invariant.
On the Z7 w/ FTZ, it racks slowly at f/2.8, faster at f/4, and fastest at f/5.6 and above.
Is there some reason for this? Does anyone know what's goin' on? My understanding is that the focus acquisition speed should more or less be consistent across f-numbers for a given camera/lens pairing. If anything, it should focus *faster* with a larger aperture opening letting more light on the sensor.
p.1 #2 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
Interesting....I am planning to get a Z6 and using the adapter on my Sigma art lenses and not sure about how well or how fast AF would work. The F stop won't change anything because your lens is wide open during exposure and AF...until you hit the shutter and to take the shot...
p.1 #3 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
Actually on the Z’s AF is performed at your selected aperture up to f5.6. above f5.6 and smaller are focused at f5.6 then stopped down just prior to the exposure.
To the original posters question not sure why that would be the case. You would think intuitively it might be the opposite. I don’t think the ultrasonic motors play well with the Z on sensor focus. The Z glass with the stepper motors and lenses like the 70-300 AF-P seem to focus faster and more securely. Maybe they are intentionally slowing the focus at wider apertures so they don’t overshoot. I’ve really noticed a difference in video focus. The F glass gets really chattery when trying to track focus while the AF-P and Z which use steppers are quiet and more secure in tracking.
TooManyShots wrote:
Interesting....I am planning to get a Z6 and using the adapter on my Sigma art lenses and not sure about how well or how fast AF would work. The F stop won't change anything because your lens is wide open during exposure and AF...until you hit the shutter and to take the shot...
p.1 #5 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
jpelt78 wrote:
Actually on the Z’s AF is performed at your selected aperture up to f5.6. above f5.6 and smaller are focused at f5.6 then stopped down just prior to the exposure.
To the original posters question not sure why that would be the case. You would think intuitively it might be the opposite. I don’t think the ultrasonic motors play well with the Z on sensor focus. The Z glass with the stepper motors and lenses like the 70-300 AF-P seem to focus faster and more securely. Maybe they are intentionally slowing the focus at wider apertures so they don’t overshoot. I’ve really noticed a difference in video focus. The F glass gets really chattery when trying to track focus while the AF-P and Z which use steppers are quiet and more secure in tracking. ...Show more →
I think the idea of intentionally slowing down when the plane of focus is smaller seems to make sense to me. I couldn't find anything about this on the 'net, although lots of pros must have tried out their big glass on Z bodies. Does anyone have another 300mm, 400mm, etc. to test out?
p.1 #6 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
Correct me if I'm wrong, but judging from what is visible in the video, your 'test' consists of letting the AF run wild with the cloth lens cap on the lens, i.e. in the complete darkness, and consequently without any kind or subject to focus on
Can't see the relevance of the claimed 'results' based on those criteria.
Normally a test on the AF speed of a lens is done by measuring the AF time between infinity and the closest subject possible
I suggest rather following that procedure to get be able to make a relevant statement
p.1 #7 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
Paul_K wrote:
Correct me if I'm wrong, but judging from what is visible in the video, your 'test' consists of letting the AF run wild with the cloth lens cap on the lens, i.e. in the complete darkness, and consequently without any kind or subject to focus on
Can't see the relevance of the claimed 'results' based on those criteria.
Normally a test on the AF speed of a lens is done by measuring the AF time between infinity and the closest subject possible
I suggest rather following that procedure to get be able to make a relevant statement
Without going through the whole rigmarole of filming another set of focus racks (and measuring their speed again by manual framecount), I can assure you that the AF motor speed doesn't change with respect to the original video when using real world subjects focusing from infinity to minimum and back again.
Regardless of what's in front of the lens, focus mode, or success in acquisition, the AF-S motor seems to scale its speed with f-number in the same pattern - slowly wide open, and faster until plateauing at f/5.6.
p.1 #8 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
What focusing mode are you using?
That behavior (slower for wide apertures) is expected for a CDAF algorithm but not for a PDAF algorithm.
Pinpoint AF is a fully CDAF mode and so if you are using that mode I think what you are seeing is expected. With a narrow DoF the CDAF algorithm has to check more frequently during a focus rack.
S-AF is a PDAF mode with a CDAF fine tune at the end. So it might slow down a bit as you achieve focus but I wouldn't expect the focus rack to slow down.
The other AF modes are pure PDAF and so I wouldn't expect to see the behavior you are showing.
Do you happen to be in Pinpoint AF? Is so then this behavior is expected and if you switch to a different mode it will probably disappear.
p.1 #9 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
kwalsh wrote:
What focusing mode are you using?
That behavior (slower for wide apertures) is expected for a CDAF algorithm but not for a PDAF algorithm.
Pinpoint AF is a fully CDAF mode and so if you are using that mode I think what you are seeing is expected. With a narrow DoF the CDAF algorithm has to check more frequently during a focus rack.
S-AF is a PDAF mode with a CDAF fine tune at the end. So it might slow down a bit as you achieve focus but I wouldn't expect the focus rack to slow down.
The other AF modes are pure PDAF and so I wouldn't expect to see the behavior you are showing.
Do you happen to be in Pinpoint AF? Is so then this behavior is expected and if you switch to a different mode it will probably disappear....Show more →
This happens to this lens/camera combination in all focus modes: AF-S Pin/Single/Area and AF-C Single/Dynamic/Area. It even happens in Movie/AF-F.
I don't have access to another Z camera or big AF-S telephoto - if anyone out there has a set, I'd really appreciate your take on things!
p.1 #10 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
To me it sounds like the lens isn't talking to the camera correctly, probably as it is an older AF-S VR and may need a firmware update for the lens to work correctly on the Z cameras. The AF instructions to the lens are different on Z cameras compared to DSLR's.
p.1 #11 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
Lance B wrote:
To me it sounds like the lens isn't talking to the camera correctly, probably as it is an older AF-S VR and may need a firmware update for the lens to work correctly on the Z cameras. The AF instructions to the lens are different on Z cameras compared to DSLR's.
I've read elsewhere (I believe through Roger over at Lens Rentals) that the Canon RF adapter is kind of an engineering miracle: it has to deal with a mountain of different motor varieties, communications protocols, and power delivery quirks that accumulated over the EF-Mount years yet somehow works transparently to the end user. I'm sure the FTZ has the same hurdles to jump adapting various AF-S motors to the Z AF system so it wouldn't be a surprise to see some strange behavior on the fringes.
Just to be sure I updated the Z and FTZ to their latest posted firmwares (3.1 and 1.1 respectively). Alas, no change. I don't believe the 300mm 2.8 VR has any way to update the lens through software but I'd be happy to be wrong.
p.1 #12 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
blackadde wrote:
I've read elsewhere (I believe through Roger over at Lens Rentals) that the Canon RF adapter is kind of an engineering miracle: it has to deal with a mountain of different motor varieties, communications protocols, and power delivery quirks that accumulated over the EF-Mount years yet somehow works transparently to the end user. I'm sure the FTZ has the same hurdles to jump adapting various AF-S motors to the Z AF system so it wouldn't be a surprise to see some strange behavior on the fringes.
Just to be sure I updated the Z and FTZ to their latest posted firmwares (3.1 and 1.1 respectively). Alas, no change. I don't believe the 300mm 2.8 VR has any way to update the lens through software but I'd be happy to be wrong. ...Show more →
I'm sure the FTZ is not the issue as it basically just transfers AF data to and from the lens and the camera controls the AF, direction and speed etc. The only thing the FTZ controls is the aperture of the lens via a small motor, but that motor is controlled by the camera and your aperture setting. I would have thought the Canon version would act the same, just a conduit for the AF drivers in the camera to control the AF motors in the lens and like the Nikon FTZ, the aperture is operated by a motor in the adaptor but also controlled by the camera and the aperture setting.
I am sure there are firmware tweaks that can be intered into the lens on board memory that will be able to be controlled correctly by the Z AF drivers. It just seems to me that the camera and lens aren't talking to each other correctly to garner the correct direction to AF or how fast that shoud be happening. The AF system in the Z is different from that of a DSLR due to the way the phase detect is on the sensor rather than of a dedicated AF module as in a DSLR. The AF of a Nikon DSLR is done at f5.6 not at f2.8 and the DSLR needs to judge for that using algorithms that the camera has on board. Does this affect the Z camera in some way that is causing it to slow up, trying to be more accurate? Just speculating. I guess the other thing to check is that the aperture is being controlled correctly by the camera as this may be affecting the AF speed? Is the AF accurate? Whatever the case, I think a trip to Nikon needs to be done.
p.1 #13 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
To the poster above: the AF is accurate, and the aperture stops down correctly when set.
Another member contacted me via PM with some testing on a range of lenses with their Z6 + FTZ:
200-500 5.6E VR
5.6 and 11 are the same (as expected, due to 5.6 being the max stop down point)
400 2.8G VR
2.8 is obviously slower than 5.6
200 2G VR
2.0 is obviously slower than 5.6
85 1.4G and 105 1.4E
1.4 is ever so slightly slower than 5.6 (really subtle)
Since I don't have enough posts to reply to their PM directly, I figured I'd do it here:
Hey,
Thank you so much for taking the time to investigate this on your end. Having so many other data points (different Z model, another FTZ, and a range of F-mount glass) really solidifies in my mind that this is likely an intentional design decision on Nikon's part to limit focus speed; as you said, perhaps to provide more time for the AF algorithm to process shallow depth of field.
I know the secret sauce of each manufacturer's AF system is more or less under wraps, but the basics of on-sensor PDAF rough target -> CDAF fine tune suggest that this may be an attempt to prevent overshoot and subsequent chatter when the plane of focus is thin and the motor is relatively beefy, as you find on the 200/300/400 VRs.
There's an interview on IR that vaguely talks about how it works:
"... both Phase and Contrast-Detect AF are in fact used when attaching an F-mount lens to the Z series cameras via the Mount Adapter FTZ."
Whether that's an issue of available processing power or some other intrinsic limitation to their on-sensor PDAF / CDAF implementation, I have no idea. I hope it's just the former because that seems like an easier problem to fix. Just throw more CPU cycles at the problem until it goes away.
I tried to find some other mention of this behavior on the web but came up empty. Pretty crazy, given the system has been in the wild since 2018!
At least now I can be sure that it's not some quirk of my own camera system or lens. Whew....Show more →
p.1 #14 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
Any other thoughts? Perhaps a third poster could chime in with one of the faster-focusing F-mount lenses and see if it's reproducible on their end as well?
p.1 #15 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
blackadde wrote:
Any other thoughts? Perhaps a third poster could chime in with one of the faster-focusing F-mount lenses and see if it's reproducible on their end as well?
By golly, you're right! I had never noticed this previously, but on my Z7 my 400 f2.8E FL VR is slower to focus at f2.8 than it is at f5.6! I then tried my 70-200 f2.8E FL VR and it also is slightly slower to focus at f2.8 than it is at f5.6, the difference is not as marked as the 400 f2.8, though. This is very interesting! I have rarely used the 400 f2.8 on my Z7 and when I did it was usually at f4 and maybe with the 1.4x TCIII but I don't think I ever shot it at f2.8 on my Z7. This means I would never have noticed the slightly slower AF at f4 than at f5.6 and unless you do it under the same circumstances you would just put it down to different light and subject matter. I have used the 70-200 f2.8E FL VR on my Z7 many hundreds of times but never noticed the slower focus at f2.8 simply because the difference is less noticeable. Let ye not despair! I tried the new Z 70-200 f2.8S and low and behold, it too is slower to focus at f2.8 than it is at f5.6. I then tried my Z 24-70 f2.8S and I think it is slightly faster at f5.6 than at f2.8 as well, although it is marginal.
I set the lens at the same minimum focus distance (1.5mts for the 70-200 f2.8E FL VR) for both apertures and then let the camera focus on a tree about 20mts away. I did this for each lens using an arbitary minimum focus and the same for each aperture and the same tree and in every case the lens was faster at f5.6 than at f2.8.
p.1 #19 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
With a telephoto, focusing on minimum focus distance at f/2.8 means that anything at infinity is a huge blob, and vice versa. Even phase data will be difficult to interpret, as the phase data related to a single object will show up at different AF points, and each AF point will have to deal with a mixture of phase data.
Thus, maybe at larger apertures, the focus speed is deliberately slowed down to reduce the risk of missing the focus plane while adjusting the focus. Maybe this is just newly implemented, or mirrorless / Z cameras are more at risk of missing the focus plane due to the integration of AF into the image sensor.
More relevant that MFD to infinity would be the speed of adjusting focus from, say 20 meters to 40 meters distance.
