In the past week it has come out that there is an issue with LCD on the 5D Mark III. At first it seemed like the cause was the Internet feeding frenzy moving on from obsessing over whether the 5D Mark III’s movie mode is sharp enough to a new touchstone but Canon has acknowledged the issue.
Described as a light leak, it’s important to note what the issue is and what it is not. It is not a light leak which affects image capture or video footage. It will not fog either. It is a light leak caused by the backlight of the top LCD screen, which under extremely dark situations can affect the camera’s meter reading. But only if that LCD backlight is on. What’s that mean in day to day use? Honestly, not much.
It has to be very dark and, again, you have to have illuminated the LCD. Additionally, the LCD light stays on for only ~five seconds. Assuming you illuminated the LCD for a reason, to check a setting, then the window of time where this might affect a picture is only a couple of seconds. And you can always turn the light off. That’s not to pooh pooh the issue, Canon needs to fix it, but the odds of running into it while shooting are very slim.
I tested one of my 5D Mark IIIs with the focus set to manual, camera in Av mode, and with the *viewfinder cap on. Here’s what I found:
5DM3 at ISO 25,600
f/1.2 @ 1/8 sec., turning on the LCD backlight changes the exposure to 1/10 or 1/13 (it bounces a bit between them.)
f/1.2 @ 1/30 sec., turning on the LCD backlight has no effect on the exposure.
5DM3 at ISO 6,400
f/1.2 @ 1/5 sec., turning on the LCD backlight changes the exposure to 1/6.
f/1.2 @ 1/15 sec., turning on the LCD backlight has no effect on the exposure.
ISO 3,200 is similar to above, exposure has to be at f/1.2 and longer than 1/10 sec. for the LCD backlight to affect the exposure reading.
As you go down in ISO the tipping point where the LCD backlight affects the exposure reading becomes longer in terms of exposure. e.g. ISO 160, f/1.2 @ 2.5″ -> backlight has no affect on exposure, ISO 160, f/1.2 @ 4″ -> backlight changes the exposure to 3.2″.
Manual metering mode is also affected in terms of the camera indicating correct exposure, e.g. if it was correct at ISO 6,400, f/1.2 & 1/5 then turning on the LCD will change the reading to 1/3 stop overexposed (e.g. the same difference as Av mode dropping the exposure from 1/5 to 1/6 sec.)
You probably get the idea – this needs to be corrected but it is highly unlikely that it will be problem.
One site, Petapixel, is also reporting that a strong external light on the top LCD will affect the meter. They are running a video which they say shows that shading the top LCD from the sun will change the exposure. I tried that but saw no change in the meter reading. I also tried shining a bright light (iPhone camera light) within 1/4″ of the top LCD but it had no effect.
*The viewfinder cap needs to be on because light entering the viewfinder can affect exposure.