Noise in digital photgraphs is generally insignificant until you start shooting at higher ISO values with longer exposures or make very short exposures. Increasing ISO to provide an accpetable shutter speed in low light can result in ‘noisy’ images, caused by chromatic, luminous or shot noise.
Sources of noise:
Doubling ISO say from 800 to 1600 doubles the sensitivy to light by increasing the sensor voltage. The downside is that individual pixels are prone to influence from their neighbours and the sensor temperature increases hence the expression ‘hot pixels’.
Very long exposures will contribute to noise as a charcteristic of sensor design and unfortunately this can only be improved by changing to a later camera with less noise.
Later camera designs generally produce less noisy images, although as pixel density increases it can result in sensor noise from interference. The D810 with its 36Mp sensor produces excellent extremely detailed images and highly sought after by landscape photographers. However, the higher density pixeled D810 produces fractionally more noise than the D750 24Mp sensor, but is easily removed in post processing.
Noise is also generated by the random nature of light photons, it increases as the signal increases, but not by the same amount. More signal – longer shutter speed, should overcome to some degree the unwanted noise.
Clearly you cannot just rack up the ISO without considering shutter speed, it’s important to optimise the signal to noise ratio. Slower shutter = more signal = less noise, however there is a point where the sensor heats up which introduces noise. Selecting the correct ISO to provide a reasonable shutter speed and ‘ETTR’ exposing to the right will help reduce noise. It’s a matter of experimenting with a bit of guesswork.
Keep your camera cool: a warm camera means a warmer sensor which induces more noise. Even on a cool night a camera can get warm from repeated long exposures.
Use a tripod: A tripod will allow slower speeds at lower ISOs.
Stacking: I’m no great fan of extensive post processing but multiple images stacked will produce an image with reduced noise.
Post Processing: Photoshop, Aperture and other post processing programs have noise reduction, also plug-ins such as Noise Ninja can be added to reduce or remove noise.
In camera noise reduction: I’ve found these (Nikon) seldom work correctly. The camera makes a guess at the noise reduction at the expense of fine detail and in doing so destroys the image. Turn it off and remove noise in post processing. There are two types & this is my understanding:
Long exposure noise reduction (LENR): A second exposure is taken after the first, this potentially captures the worst noise and subtracts it from the first. However if you are taking a series of images, turn this off to avoid excessive sensor heating.
High ISO noise reduction: This type of noise reduction just softens your image, this is best corrected in post processing.
Any comments welcome