• Ed Dozier

Measure Camera Mirror Blackout Time Yourself

Camera mirror blackout is the term for DSLRs when the mirror pivots out of the way for the shutter to open and take the shot. When you’re looking through the viewfinder and take a shot, there’s a brief time when you can’t see anything until the mirror pivots back to its “down” position.


Most of the time, you simply ignore mirror blackout, because it’s really brief. It’s a whole different story if you’re taking a series of shots as fast as your camera allows (continuous shooting). During high-speed shooting, such as 10 frames per second, you really need to be able to track the action and follow your moving subject. Also, your camera needs to see the subject in order to update the focus distance as the subject moves.


If you’re curious about how much time is spent flipping the mirror out of the way so that the camera can take the shot, a great way measure it is with video. Most smart phones and many cameras are capable of taking super slow-motion video at high frame rates. I’m going to show you how I used my Nikon D850 to track mirror activity at 120 frames per second (24p * 5 = 120).


As an additional benefit of this test, you will be able to measure the actual frames-per-second your camera is able to shoot, versus what the manufacturer claims it can do.



D850 filming a D500


In the shot above, you can see a D850 (with a 105mm Micro Nikkor) looking through the viewfinder of a D500. The D850’s105mm lens is focused on infinity, and is seeing what the D500 photographer would see. The lens has to be pretty close to the viewfinder to see very much of the frame.




Video image of the viewfinder


The D850 video, being replayed via its LCD screen, is shown above. The camera multi-selector right-arrow is used to single-step through the video frames to observe when the mirror blocks the viewfinder image during high-speed shooting. With the setup I used, I couldn’t see the entire frame, but that isn’t necessary to conduct a test like this.


I attached a 10-pin remote release to the D500 to make it easier to shoot a sequence of 10 frames per second, while also starting and stopping the video recording on the D850.


After taking the video footage, I just replayed it in the D850. The time duration of each frame of video is 1/120 second or 0.0083 seconds. The results were pretty consistent for each D500 shot that I reviewed. What follows is a summary of the D500 shooting chronology video.


Frame 1: partial image visibility at the top of the viewfinder

Frame 2-7: total image blackout

Frame 8: partial image visibility at the bottom of viewfinder

Frame 9-12: full image visible


The 12-frame sequence above repeated itself throughout the video, with minor variations of how much of each “partial frame” was visible. I consider the partial frames to be the same as “blackout”.


Adding up the frames, it turns out that the blackout time versus visible time is in a proportion of 8 to 4 (0.0667 seconds blacked out and 0.0333 seconds visible). In other words, two-thirds of the time the viewfinder is blacked out while shooting at 10 frames per second.


As a reality check, 0.0667 + 0.0333 = 0.1 seconds per shooting cycle. As expected, each frame then takes 0.1 seconds or 10 frames per second. Nikon is telling the truth about the D500 frame rate, after all.




A partially flipped mirror view


You’d swear while looking through the viewfinder while shooting 10 frames per second that you ‘mostly’ see the subject, and the blackout time is only a fraction as long. Exactly the opposite is true. I think that your brain tries to stitch together each isolated view into a continuous image, just like when you watch a movie.



Summary


Super-slow-motion video can be a great tool for viewing things that are simply too fast for human perception. I showed in a previous article how lens auto-focus speed can be studied and measured in detail.


If you’re interested in comparing cameras, don’t overlook video as a great measurement tool.


It’s rather amazing that the auto-focus system, which only gets to see the subject one third of the time, is able to keep focus on a moving subject while shooting at ten frames per second.


This may sound like a problem that’s been solved by mirrorless cameras, which by definition can’t have any mirror blackout time. Most mirrorless cameras only trade this problem for another, potentially more serious problem, however. The mirrorless camera electronic viewfinders have finite refresh rates, and many of them are slow enough that you can’t see where your fast-moving subject really is. Instead, you only get to see where the subject “used to be”. Whoops.


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