p.2 #2 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
I just tried this on my Z6 + 70-200 f/4, comparing f/4 to f/5.6. At 70mm I don't notice much if any difference. At 200mm I see f/4 is noticeably slower than f/5.6, both when focusing with the lens cap on and when focusing on a bright unfocusable subject (bright lightbulb). I also see the difference when focusing on a focusable object, provided I rack focus to MFD first.
What's curious is how this occurs with the lens cap on. Normally the Z's will focus at their shooting aperture, up to f/5.6. However, in low-light scenarios, like with the lens cap on, the Z's open the aperture during AF up to help get more light onto the AF points. For my 70-200 that means the camera opens the aperture to f/4 from f/5.6 - I can hear the click of the aperture while focusing. Even in that scenario the camera still takes longer to focus @ f/4 vs f/5.6, even though the actual aperture the camera is focusing with is identical. This implies the speed difference is not a technical function of the PDAF points being better or worse at a given aperture but instead a decision by the camera to intentionally slow down the AF when the shooting aperture is larger.
Since the speed delta correlates to focal length I'm guessing it's a function of focal length plus DOF. It's understood that on-sensor PDAF is less adept at measuring phase differentials at longer focal lengths than shorter, as a function of the limited geometry of sensor phase pixels vs traditional PDAF sensor arrays. The aperture-specific AF performance delta might be the camera accommodating that reality by more carefully sampling the range of focus distances, accomplished by slowing down the focus rack so that more granular phase samples can be acquired.
p.2 #4 · Nikon Z w/ FTZ - Autofocus speed varies with f-number?
Billphoto1 wrote:
Do other mirrorless systems work like this?
I'm not sure if other systems will focus slower at larger apertures. The Z's are a bit unique in that they don't usually perform a CDAF confirmation cycle after acquisition like other systems do, particularly Sony, so that may be a reason for the conservatism on the focus speed in this scenario.
@blackadde, Can you repeat your experiment on the Z7 but using the pin-point AF mode in AF-S? That uses CDAF only.
p.2 #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.
***Maybe they are intentionally slowing the focus at wider apertures so they don’t overshoot.***
This ^^^ Less margin for error at wider apertures. Also, many lenses are "less sharp" at the more open apertures, so there is less contrast for the AF pixels to latch onto.
