If you’re curious about how fast a lens can focus, a great way to measure it is to use slow-motion video. I previously used my smartphone video to do this job, but now I can use my Nikon D850.

The D850 is capable of 120 fps video in DX crop-mode. This will allow you to time events down to a resolution of .0083 seconds. This is plenty accurate to measure your lens focus speed (lens is mounted on another camera body).

D850 setup for slow-motion 120 fps video

The overall testing scenario goes as follows. Mount the camera/lens you want to measure on a tripod, and set the focus ring to minimum focus. Make sure the lighting level is set how you want it; dim illumination will of course result in slower-focusing rates. Mount the D850 on another tripod (or hand-held if you’re careful), with its lens focused on the lens focus scale under test. Start the (120 fps) video recording, and then initiate focus on the target camera. Stop recording after the target lens is focused.

Make sure the target camera/lens is pointed at something at a long distance, so that its lens will have to move from minimum focus to infinity. I’d recommend the target camera have “AF-ON” programmed onto a button, so you can just press that button to start focus.

The recorded video should capture the entire focus sequence, so that you can watch the lens focus scale while it changes from minimum-focus to infinity. The slow-motion video can also capture any focus hesitation or focus “chatter” problems that your un-aided eye cannot detect. Not all lenses have focus scales, of course, so you might have to improvise on tracking what constitutes focus activity.

The D850 video doesn’t need to be transferred onto a computer for analysis. You can play back the video recording in-camera, using its multi-selector button.

In-camera Video Controls in Video Playback Mode

Multi-selector center button: play or resume play after a pause

The center button is typically used to play/resume your slow-motion video at the configured frames per second

Pause video. Use “forward” or “rewind” while paused for single-frame mode.

Rewind the video (back up a frame if “pause” is active)

Fast-forward (2X, 4X, 8X, 16X per press) or one-frame advance the paused video

Start slow-motion playback if the video is already paused

Monitor “beginning of video” indicator (top right of monitor)

Last Frame indicator (top right of monitor)

Evaluate the video

Use the keys shown above to navigate around your video. Locate when the lens distance scale first starts to move in the video. Step through the video to locate when the distance scale reaches the infinity mark and stops moving. Knowing the number of frames (or right-arrow clicks), you can now easily determine how long it took for the lens to focus.

At 120 frames per second, that equals 1/120 or 0.0083 seconds per frame. For instance, it takes 30 right-arrow clicks for 0.25 seconds. The math would simply be (30 * .0083) = 0.25 seconds.

Review 120 fps video of a stopwatch running on a smartphone

In the shot above, I paused the camera video at the frame showing 19.0 seconds, and then clicked the right-arrow on the multi-selector and counted the clicks until the smartphone stopwatch reading was 19.25 seconds. It took 30 clicks, just as expected. It's always good to double-check your work.

I used to include a high-resolution timer display in my videos, which shows the elapsed time directly. Using the video navigation controls in the D850 make this additional complexity unnecessary.

I did this test on my Sigma 70-200 f/2.8 Sport zoom at 200mm in fairly good light outdoors (the sun was at a low angle), and it took 43 clicks (frames) to go from minimum focus to infinity, or 0.358 seconds. I had previously measured this lens in really bright sunlight (using a stopwatch in the video), and it took 0.36 seconds. Pretty darn close. By the way, this current focus speed test was done using a D500. My previous tests were done using a D850. They’re supposed to have equivalent focusing capability, and this proves that claim to be true.

Summary

Not many Nikons can manage 120 fps video yet, but the options open up considerably for 60 fps video. Even 1/60 second resolution is pretty good for measuring focus speed, although you might miss some nuances involving focus hunting or focus chatter.

Other camera models probably differ in how to review in-camera video, but the discussion above will hopefully give you enough detail to enable you to try it yourself on whatever camera you're using. You may have to transfer the video to a computer and analyze it there, if your camera doesn't include the necessary controls to review it in-camera.

This in-camera frame-counting technique makes it really simple to determine focus speed. You could, of course time anything you want using this same technique. Happy testing.

#howto