Focus Speed Slowdown at Different Lens Apertures

All of the Nikon Z cameras perform autofocus while stopped down to the shooting aperture, up through f/5.6.  This is different than almost all other mirrorless camera manufacturers, which focus the lens at the widest aperture. Is this a smart or a dumb strategy for Nikon? Let’s find out.

Nikon Z9 with 85mm f/1.4 lens focusing at f/5.6

There are, of course, the internet fanboys that smugly claim how ignorant Nikon is for performing autofocus with the lens at the shooting aperture. I’ll ignore the performance issue of having to open the aperture, focus, stop down the aperture, and then shoot. The main advantage of stopping down during autofocus is to eliminate focus errors when using lenses that exhibit focus shift at different apertures.

It’s true that the lens will focus more slowly in less light, so you should always want to perform autofocus at the maximum aperture (if focus accuracy is of secondary importance). But how much does a lens really slow down focusing when the aperture is stopped down? That’s what I’m going to explore.

My preferred method of measuring focus speed is by taking a slow-motion video of the lens focus scale while it focuses. I can count the number of video frames to get a very good measurement of the time to change focus from one distance to another. I shoot these videos at 120 frames per second, so each frame lasts for 0.0083 seconds. This method of course breaks down with lenses that don’t have a focus scale on them…

Most lenses are focus-motor limited in their ability to focus. The camera tells the lens what to do and has to wait for it to finish. Portrait lenses with bright apertures are typically slow to focus, both because it’s hard to nail paper-thin focus zones and because the lens moves a lot of heavy glass during focus. It’s not really fair to compare different lenses for absolute focus speed; this article just concerns itself with how much a lens slows down focusing as the aperture changes.

It isn’t realistic to measure focus speed by having the lens travel through its entire focus range. Macro lenses would always lose any focusing contest. I’m measuring focus speed this way because it’s the most straightforward and repeatable way to do it. Just don’t interpret longer focus times to always mean ‘worse’ lenses.

Nikkor 85mm f/1.4 AF-S Lens

I shot this lens in fairly bright conditions (EV 13.2 to be exact).

The focus action was recorded at 120 fps video. The lens was mounted on a Nikon Z9, using the FTZII adapter.

f/1.4 through f/4.0      focus 0.51 sec. (3m to infinity: 61 frames)

f/5.6                              focus 0.55 sec. (3m to infinity: 66 frames)

As shown above, the focus time didn’t change until stopping down to f/5.6, and then it was only slightly slower to focus. The focus speed changed about 8%.

Sigma 70-200 f/2.8 Sport at 200mm

I shot this lens under the same bright conditions at EV 13.2. I also shot using my Nikon Z9 and the FTZII adapter.

f/2.8 through f/4.0      focus 0.37 sec. (1.2m to infinity: 44 frames)

f/5.6                              focus 0.40 sec. (1.2m to infinity: 48 frames)

Once again, the focus time was consistent with f/2.8 through f/4.0 and got just slightly slower at f/5.6. Again, focus slowed by about 8%.

Summary

I am glad that Nikon made the design decision to focus at the shooting aperture, up through f/5.6. When I shoot action in dim lighting, where focus would start getting slower, I open up my aperture to keep a sufficient shutter speed. For landscapes (f/8 usually), I really don’t care how long it takes to focus.

I no longer have to concern myself with missed focus when I use fast lenses that have spherical aberration and therefore suffer from focus shift problems. I appreciate actually seeing the real depth of focus as I stop the lens down, too, especially in close-up portraits.

I can’t help but think that the Nikon engineers did some careful testing before making the design decision to focus at the shooting aperture. All of the Nikon DSLRs focus with the lens wide-open instead, because there’s really no choice when considering the dim conditions that those focus systems have to operate in.

It would be optimal if there was a firmware feature that allowed photographers to choose which kind of focus method to use, but I’m not holding my breath on that one.