Photography Stack Exchange is a question and answer site for professional, enthusiast and amateur photographers. It only takes a minute to sign up.
Sign up to join this community
I'm about to start exploring the world of long exposure shots, using my Canon EOS M. The camera has a standard max exposure time of 30 seconds, but in bulb mode I can keep the shutter open indefinitely (using a wireless remote to control the shutter.)
How long can I leave the shutter open without running the risk of prolonged exposure to light damaging my sensor?
For the purposes of this question assume I'm shooting in summer daylight, so that we can go by the worst case.
As long as you aren't pointing the camera at the sun, lasers etc. (see this question)
You should be ok, at worst you'll get a completely over exposed image and the camera may give an over heating warning or the battery will run flat.
This is based on the general consensus (google to the rescue):
Almost certainly if your final result is correctly exposed you have nothing to worry about.
If you're shooting in summer daylight with very long exposure times, regardless of whether you damage your sensor or not, you're going to get a completely blown out image, with no recoverable data. If you want very long exposures in bright light, your only real choice is to cut the amount of light going through the lens. For this, you'd normally use a Neutral Density filter of appropriate light-cutting ability.
As with all things photography, the prices of these can vary from a few dollars to many hundreds. When looking at an ND filter, the most important thing to consider is how much light it cuts. See this for information on how ND filters are rated.
There are also variable ND filters of varying quality available. With the cheap ones, using them on a wide-angle lens can result in cross-shaped banding on your image, though this often goes away when shooting at longer focal lengths. The best ones don't suffer from this, but are breathtakingly expensive (a quick search on Amazon shows one selling for $450!).
You can also make a ~10-stop ND filter using welding glass, though these often lend a colour cast to your image, which may or may not be desirable. If you shoot RAW, this is fairly easy to correct (though you'll still lose some colour information).
Sensor damage solely depends on the number of photons that hit an individual pixel on a sensor. Every pixel in a sensor has a well-depth. This is the number of electrons that a pixel can hold. When the well-capacity/depth is exceeded the electrons bleed into other pixels, which causes damage to the sensor.
So, any time you are over-exposing the sensor and see pixels bleeding into other pixels (looks like vertical or horizontal streaks) you are damaging the sensor.
It doesn't matter if it's a laser, the sun, or indirect light, occurs over a millisecond or occurs over 4 days, damage is damage.
That being said, try to damage it as little as possible in terms of electrons and duration.
