If you thought the Canon 5D Mark II’s black dot problem was obscure, here is an even stranger defect in some Canon cameras. The issue (red flare on multi-minute exposures) was originally attributed to the a problem with an LED on the Canon 5D Mark II – but this proved to be wrong. In January 2009 a DPReview forum user known as “darksbrother” posted:
Ok, I’ve got a conundrum. I need to find a way to disable the blinking red LED to the right of the wheel on the 5D Mark II. I’m doing long night exposures and that LED keeps creating a red streak on sufficiently long enough exposures.
Firstly, apparently that LED on the back of the camera is on during long exposures. Which is in itself surprising, and lead to some discussion because users were not aware of this. Secondly, the original poster cared about that red LED only because it seemed to cause a red spot in the lower part of dark images during multi-minute exposures. In other words, the poster thought that light from the LED somehow leaking via the internals of the camera and managed to reach the exposed CMOS sensor. Within a few days, it was announced on the DPReview forum by its_RKM that this was a problem with the Canon 24-104mm f/4L lens rather than with the camera body: infrared light from an infrared LED inside the lens (used to detect motion for the USM mechanism) leaked into the camera at certain zoom ranges (notably 24 mm). The Canon 5D Mark II sensor understandably registered this infrared light as red light – despite the infrared absorbing filter that is the light needs to pass through before reaching the sensor. If you expose long enough (e.g. 15 minutes) and set the camera to automatically subtract a 2nd image (also 15 minutes!) taken with the shutter closed, the resulting noise-reduced image will show red “flare” as shown in the image
On March 31st 2009, the original poster summarized the forum thread as
Thanks to extensive testing by RKM, we can in fact conclude it’s the lens. I have tried the test with the body cap and have gotten no red streak. What was concluded was that it was an internal infrared LED within the lens which helps calibrate focus/read information on the position of the lens that was leaking through into the sensor area causing the red streak.
but Canon technical support is so far unwilling/unable to confirm the situation. Technically interesting questions are:
- Does this show up with all SLR cameras using the same lens? So far, it has been seen on a 5D and 5D Mark II. The problem may be less visible on an APS-C sensor as the worst parts would be cropped off anyway.
- Does the problem still occur if you disable autofocus? Manual focus may turn the LED off (although that’s less likely).
- How much infrared does the IR cut filter actually block (this link suggests at least 5 stops for 800 nm light)? An ancient Canon 10D might be sensitive to it because it said to have a weak IR cut filter (on the other hand, the sensor is noisy by current standards anyway).
- Are there other Canon USM lenses with the same problem? Most high-end lenses have a similar USM focus technology.