Difference between long exposure in a dim room Vs. short exposure in a well-lit room (using a tripod, ISO 100)

Question

I'm trying to photograph a painting without using any lighting equipment, just the natural light in the room on a cloudy day (i.e, little or no direct sunlight). I am shooting straight on with a tripod and DSLR using Av mode with ISO set to 100. (By the way, I am also shooting RAW).

The goal is to reduce glare as much as possible and so it is important to have the painting as evenly lit as I can without using lighting equipment.

My question is this: what's the difference between shooting a long exposure in a dark room versus shooting in a well lit room, when the only difference in the two photographs is the shutter speed and all other settings remain the same? In theory, I would hazard to guess that it should produce the same image, but in practice I don't think this is true. In my tests, the long exposure is definitely not as sharp when viewed at 100% while the normal exposure appears to have more pronounced highlights and shadows (due to the fact that there is a stronger light source?). Hopefully someone can shed some light (no pun intended) on the difference here.

Just to clarify: in the "dark room" scenario, the shades are closed over the window but there is still some light entering the room. In the "well lit room" scenerio, the shades are open but there is no direct light entering the room.

Kind of an odd question I suppose, but thanks in advance.

Edit: I just realized I had forgotten to turn off image stabilization on my lens, which may account for the lack of sharpness in the long exposure.

Answer 1

This is a complex answer, so feel free to ask more questions.

Basically, one major difference between the images is that your noise sources in the image is vastly different. Because the noise is different and because its contribution is different with regard to exposure, the obtained images are quite different.

Think of your camera detector as a well that measures electrons. That well captures either light (photons) which is converted to "good" electrons or "bad" parasitic electrons that are caused by system noise (camera).

1. when shooting in long exposure in low light scenario the dominant noise source is usually the "dark current" of the detector. you can read about it more on wikipedia but its basically noise that is caused by the detector itself being active. dark current increases in a linear manner with the exposure. ergo, the total measured current in an exposure is I_dark_current*t_exposure.
2. With the room lit, the light signal is vastly greater than the dark current, your system dominant noise is a different noise source called shot noise. Its very interesting, and its root is related to quantum mechanics and if you have the time and curiosity, you can also read about it in wikipedia.

The main difference, with regard to your question, is that the shot noise is proportional to the square root of the input signal. so for example, if you increase your exposure by 2 and capture twice the "good" electrons, your noise would increase by the square root of 2. while in a dark room, increasing the signal by 2 would increase the noise by 2 as well! This means that for low light, once you are shot noise dominated, increasing the exposure would increase the signal more than the noise, which is desired if you want to capture the actual details in the scene.

This is a very important distinction and for this reason, a camera designer would always want his system to be shot noise limited when possible in order to obtain the best signal to noise ratio.

Hope this shed some light on things :)

Answer 2

A long exposure increases the possibility of blur due to camera movement (even tiny vibrations). You're also more likely to get noise in the image from the sensor if the exposure is very long (seconds rather than some fraction of a second).