Today’s Question: A question [in Friday’s Ask Tim Grey eNewsletter] suggested photos would have very little noise if captured at the camera’s lowest ISO setting. But aren’t there other factors that cause noise in photos besides the ISO setting?
Tim’s Quick Answer: While a high ISO setting can significantly contribute to noise in a digital photo, there are indeed other factors such as the exposure time, the brightness of the exposure, and heat buildup in the camera.
More Detail: For most digital cameras a relatively low ISO setting will help minimize the level of noise in photos. However, there are other factors that can contribute to noise as well.
Long exposures will exhibit more noise that short exposures, all other factors being equal. If you photograph the exact same scene with the exact same overall exposure, but with different shutter speeds, the image with a longer exposure time will have more noise. This is especially true for particularly long exposures, which is why many cameras include a long exposure noise reduction feature that reduces noise for exposures of around one second or longer.
An under-exposed image will also exhibit more noise than a properly exposed photo. This is why the concept of “expose to the right” is helpful. By capturing an image that is as bright as possible without clipping the highlight detail, you are capturing maximum information (light) and therefore minimizing noise all other things being equal.
Heat buildup in the camera can also contribute significantly to noise in photos. This isn’t generally a problem under typical photographic conditions. However, if you capture a relatively large number of long exposures in a short period of time, heat buildup can be a factor, contributing to more noise in your photos.