If you have ever photographed under flickering lights, such as the sodium vapor lamps especially common at sporting venues, you know what a problem that type of lighting can cause. One image is bright and the next is significantly underexposed with a completely different color cast. The bigger problem occurs when using fast/short action-stopping shutter speeds under these lights.
In the top half of the included image are 8 consecutive frames captured from the Canon EOS 7D Mark II in 10 fps burst with a 1/1000 second shutter speed. The subject is a white wall and the lights are fluorescent tubes (I had to go all the way to my basement to find these). All images were identically custom white balanced from the center of an optimally-timed image. What you see is the frame capture frequency synching with the light flicker's frequency to cause a different result in almost every frame.
The killer problem for post processing is that the entire frame is not evenly affected. Correcting this issue is a post processing nightmare. The cause of this problem is that, at fast/short shutter speeds, the flicker happens while the shutter curtain is not fully open.
Because the shutter opens and closes only in the up and down directions (with camera horizontally oriented), the area affected runs through the frame in the long direction regardless of the camera's orientation during capture. When the flicker-effected area is fully contained within the frame, the amount of area affected is narrower at faster shutter speeds and wider with longer shutter speeds.
At significantly longer shutter speeds, the effect from the flickering lights is better averaged in the exposures. At 1/25 second, a reference image I captured during the same test looks very nice.
In this 7D II light flicker test, I shot at 1/500, 1/1000 (shown in the example) and 1/2000 seconds. The 1/500 second test showed approximately 2/3 of the frame severely affected at most, but the 10 frames captured around the most-effected frame had various amounts of one frame edge strongly affected. As you would expect, the 1/2000 second test showed an even narrower band of the flicker's effect running through the image (a smaller slit of fast-moving shutter opening being used), but ... I'm guessing that there are not many venues with flickering-type lighting strong enough to allow use of this shutter speed at a reasonable ISO setting. The 1/500 and 1/1000 settings are more real world settings.
The bottom set of results show off the Canon EOS 7D Mark II's awesome new Anti-flicker mode. The only difference in the capture of the second set of images was that Anti-flicker mode was enabled. These were a random selection of 8 consecutive frames, but the results from all Anti-flicker mode enabled frames were identical regardless of shutter speed tested. I'm not going to say that these results are perfectly-evenly lit, but ... they are dramatically better than the normal captures and you will not see the less-than-perfectly-even lighting in most real world photos without a solid, light-colored background running through the frame.
When enabled (the default is disabled), Flicker Mode adjusts the shutter release timing very slightly so that the dim cycle of the lighting is avoided. In single shot mode, the shutter release lag time is matched to the light flicker cycle's maximum output. In continuous shooting mode, the shutter lag and the frame rate are both altered for peak light output capture. In my tests above, the frame rate was reduced by 1-2 fps and shutter lag can be affected, making the camera feel slightly less responsive.
The 7D II is able to work with light flicker occurring at 100Hz and 120Hz frequencies. When such flicker is detected but flicker mode is not enabled, a flashing FLICKER warning shows in the viewfinder. The FLICKER warning shows solid when a flicker is detected and the camera’s setting is enabled. Flicker detection has been working very well for me. From my own basement to an indoor sports venue to a trade show floor, I've seen the flashing "FLICKER" warning.
Since the viewfinder's metering system is required for flicker detection, this feature is not available in Live View mode (due to the mirror being locked up). The mirror lockup feature is also disabled when Anti-flicker mode is enabled. The owner's manual indicates that Flicker mode is not going to work perfectly in all environments.
In the test I shared in this post, flicker avoidance was perfect 100% of the time. I shot a soccer match at an indoor sporting venue with a complicated economy lighting system. In that shoot, the Anti-flicker mode was successful about 98% of the time in the about-350 images I captured. The post processing work required for this shoot was exponentially lighter than any of my many prior shoots at this venue. Sean's experience shooting an NCAA Division 1 football game under the lights was very good, but perhaps not as good as my 98% experience.
Canon's new Anti-flicker mode is a game changer – it is going to save the day for some events. This feature alone is going to be worth the price of the camera for some photographers.