How to Measure Nikon ‘Z’ Lens Auto-Focus Speed

Finding out how fast your mirrorless Nikon Z lens can auto-focus isn’t as simple as you might think. I use super slo-motion video, typically 120 frames per second, to observe the details of lens focusing. Most lenses that Nikon made in the past had exterior focus scales on them, which could be monitored (and filmed) to evaluate auto-focus behavior. That’s no longer the case.

Nikon has a feature in their Z cameras to display a ‘focus distance indicator’ while using the manual-focus ring on their Z lenses. You won’t see this feature when using adapted F-mount lenses, unfortunately. This distance indicator is the key to accurately measuring focus speed. The focus distance scale will appear through the viewfinder and also on the rear LCD screen, assuming you have configured your camera to have both the viewfinder and the monitor active. My cameras are configured to switch between the viewfinder display and the monitor display by detecting my eye at the viewfinder.

I like to (manually) set my lens under test to its minimum focus distance, and then let the camera auto-focus on a subject that’s far enough away to force the lens to focus on infinity. When I test a zoom lens, I will zoom in to use the maximum focal length, too. This will force a worst-case focus scenario for the given lighting conditions.

Since I use a DLSR or another mirrorless camera to take the slow-motion video, I use two tripods while testing: one will support the camera under test and the other tripod supports the camera taking the video. I pre-focus the video camera on the rear LCD of the camera/lens being tested (a macro lens is helpful here).

You can use any video device that you prefer to take the video, but it needs to support a way to review the video footage by stepping one frame at a time. The focus measurement timing will be within about a single-frame time duration, which at 120fps becomes 1/120 second, or 0.0083 seconds.

To perform the timing test, you need to first set up the mirrorless Nikon to point it at a distant object. Next, manually focus the lens up close with the rear LCD monitor activated. The rear LCD should display a focus distance scale, showing the focused distance. The ‘Setup’ (wrench) menu of my Z8 lets me select distance units of meters or feet.

Reviewing the video of a Nikon Z8 LCD monitor

In the shot above, I’m looking at a video that was shot looking at the rear LCD of a Nikon Z8.  There are two things of note in the shot shown above. The first thing to notice is the distance scale showing “4.27 ft”, which is the currently-focused distance for the lens under test. The second thing to notice is the square focus indicator, which is colored red. The LCD screen (and the viewfinder) displays the focus indicator in red until the subject gets into focus. After proper focus is achieved, the focus indicator will turn green. Different focus-modes will have different versions of the final focus indicator, but the color will be green.

You might see the in-focus indicator turn from dim to brighter green for a few frames. Use the first frame that you see the green in-focus indicator as the ‘done’ frame.

Perform the focus video-capture a few times to get an idea of the ‘typical’ focus time, and take note of the lighting level, too.

To capture the video of the focus action, you need to begin by pressing the ‘Record’ button of the video (I use my right hand for this). Next, you need to press the focus button on the camera/lens under test (I use my left hand for this operation). After the subject is in focus, you can let go of the camera focus button. Finally, hit the ‘Record’ button again to stop capturing video.

Now, it’s time to review the captured slow-mo video of the focus operation. After starting to play the video, I can use the ‘down’ arrow to pause, and the ‘right’/’left’ arrows to advance or reverse the video frames.

In order to know which video frame indicates the beginning of the focus action, pay attention to the white ‘focus distance’ scale. As soon as the auto-focus action starts, this scale will start to disappear from the monitor. This is the ‘zero’ marker in the video, where you can start counting frames. It takes a few frames for this scale to fully disappear from view, but the first frame where it starts to fade is the actual start of the auto-focus action.

Next, single-step through the video frames and begin counting. When the subject is in focus, notice that the focus indicator will turn green. This the end frame that indicates focusing has completed.

The ‘in focus’ indicator has turned green. Focus is done.

You now have the information to calculate how long the lens takes to complete the auto-focus operation. Just count the number of video frames and multiply it by the frame duration number (0.0083 seconds for 120fps).

In the video for this example, I counted 90 frames for my Nikkor 28-400mm zoom at 400mm to focus from the minimum distance to infinity. (90 X .0083) = 0.75 seconds.

Summary

I wish Nikon included external focus distance markers on every Nikkor lens barrel for tests like these, because you could also evaluate focus-hunting and focus-chatter behavior. At least it’s still possible to get accurate start-to-finish timing measurements using the procedures shown above for the Z-mount lenses.