Nikon Z Cameras Fix Spherical Aberration Focus Shift
Most camera manufacturers designed their mirrorless cameras to focus with their apertures wide open. Nikon doesn’t do this with their Z cameras; they autofocus at the shooting aperture instead. Who’s right?
Huge spherical aberration: Nikkor 85mm f/1.4 AF-S
All DSLRs autofocus with the lens aperture wide open, because the partially-silvered mirror over their focus sensors causes really dim light. Dim light causes slower or failed focus. Wide-open apertures give a camera the best chance for fast or successful autofocus. Why wouldn’t all manufacturers always autofocus this way? Read on.
Most high-speed lenses (f/1.4, f/1.2 or faster) suffer from something called spherical aberration. With this type of lens, the focus will shift when the aperture changes. This focus shift ruins shots, particularly at close focus distances.
As shown above, the best focus happens at the location of what’s called the “circle of least confusion”. This circle location shifts as the lens aperture changes, blocking light from the outer fringes of the lens. The circle of least confusion typically doesn’t shift much after the aperture is stopped down beyond roughly f/5.6.
The Nikon Z (mirrorless) cameras stop the lens down to the shooting aperture to autofocus, up through f/5.6. They don’t stop down the aperture beyond f/5.6 while focusing, in order to retain acceptable focus speed. They of course stop down to the requested aperture when the shot is captured.
Because of the way the Nikon Z cameras autofocus, focus is always correct when using lenses that have spherical aberration, no matter which aperture is selected. At apertures beyond f/5.6 (such as f/8) any additional focus shift gets “repaired” due to the large depth of focus that masks the focus-shifting error.
It turns out that focus speed is not a problem with a stopped-down lens until the ambient light levels get really dim. Nikon has determined that this is a good trade-off, especially since most photographers will open up their lens apertures in dim light anyway.
I have the Nikkor 85mm f/1.4 AF-S lens, which has pretty severe spherical aberration, and therefore severe focus shift problems. I had nearly abandoned using the 85mm when I would want to shoot at any aperture other than f/1.4. The pictures were always out of focus at other apertures, since I had calibrated focus at f/1.4 on my DSLRs. I actually carried around notes that indicated what calibration values to use for which apertures on which cameras! Very, very irritating. Using my Z cameras, focus is nailed every single time at any aperture.
I have never had any complaints about autofocus getting sluggish at any aperture with my Nikon Z8 or Z9, until the ambient conditions get really dim. Since I use wide apertures in dim light anyway, Nikon’s choice to focus at the shooting aperture is optimal for me. I recognize that there is a theoretical advantage of always focusing with the lens wide open, but for me Nikon’s method is the preferred design choice.
A side benefit of always having the lens stopped down to the shooting aperture (again, through f/5.6) is that I always view the actual depth of focus in the viewfinder as well.
I can’t see ever using my DSLRs with my fast lenses again. They’re fine for many shooting applications, but this definitely isn’t one of them.