photo:image_noise
digital image noise
Introduction:
digital sensors produce artefacts called digital noise
each camera type has different noise characteristics at different ISO's, operating temperature and exposure
in normal photography (meaning bright light photography), noise is usually a secondary consideration, since the signal component easily overwhelms it.
astrophotography differs from normal photography by being signal limited - in almost all cases the bottleneck is the available light input. Usually, the signal is of only slightly greater amplitude than the noise (thermal charge carriers, Johnson noise, schot noise, readout noise, bias noise for any EEs that may be reading this). This is the dominating differential of astrophotography from normal photography.
sensitivity in a detector is composed of two components:
sensitivity is a function of the ratio of Signal to Noise. A detector that produces a huge output in response to a small input is not sensitive if it also produces a huge noise component.
image noise is caused by several factors, producing different contributions to the noise:
Noise reduction:
physical techniques:
in-camera processing:
automatic dark frame subtraction to reduce thermal noise in long exposures
choice of ISO setting - the lower the ISO, the lower the image noise
in-camera noise processing
some Nikon dSLR's apply noise reduction even to RAW files hence the need for “mode 3” to disable this for astrophotography.
most other manufacturers only apply NR to jpegs and not to the RAW files.
most cameras give you an option of turning it on or off and in some cases how aggressive it should be (eg. Olympus E510)
post-processing:
photo/image_noise.txt · Last modified: 2011/10/01 03:02 by gary