Why are there limitations to the upper limit of exposure time in digital cameras?

For example, my Canon 550D will only go as far as 30 sec (and then of course the bulb mode).

For measuring exposure, I think all the camera needs is a timer, and I have no doubt they would have to do no extra work to remove this upper limit if it were only for the timer.

What am I missing here? Why do cameras have this upper limit?

Answer

I think there a several reasons that together make sense to limit the shutter speed at about 30 seconds.

• At exposures requiring more than 30 seconds, light is so weak your TTL meter will not be able to measure it correctly.



• 30 seconds is already longer than you'd ever need for any "normal" night scene.


• In a digital camera, sensor heat starts to build up and thermal noise becomes noticeable.


• With film, calculated exposure and actually needed exposure are significantly different thanks to reciprocity failure.


• Bulb mode helps to sell shutter cables and intervalometers to enthusiasts. Accessories are usually more profitable for manufacturers than cameras themselves - the photographer is already "locked" into the system.

30 seconds is a very common, but not an universal limit. For example, Panasonic Lumix DMC-GF2 limits at 60 seconds. Phase One 645DF limits at 60 minutes.