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Message: <blockquote data-quote="aroy" data-source="post: 316108" data-attributes="member: 16090">Low ISO generally gives less noise, and the lowest is the native ISO, 100 in most CMOS sensors. Any lower ISO will be achieved by attenuation of the signal from the sensor. Higher ISO is achieved by amplifying the signal. Many cameras provide the "Lo" range, most do not. For starscapes you go for higher ISO rather than lower ISO. To lower the noise you can do image stacking, that is take a series of shots, say at1 sec interval and then using software add them up. This technique reduces the noise so that it is much less than in an individual frame. Why do it rather than expose for a long time? With digital you increase the noise as the exposure time increases, more so as the sensor heats up (or in hot weather), so you want the noise in each shot to be within a reasonable amount. By stacking a series of shots you are increasing the light (so that the starts are brighter) and at the same time reducing noise (random noise to be precise).To reduce the quantum of light entering the camera, you use ND (Neutral Density) filters. These come in various "strengths" and reduce the light by 1EV to 10EV. Variable ND filters are generally two polarizing filters. as you change the angle of one with respect to the other the amount of light transmitted changes, so you can get an infinite gradations.As is the case every where, good filters cost a lot of money. What do you pay for?. Glass/polymer with uniform transmission across the field.. Better materials for the filter holder. In most cases multilayer surface coating to reduce ghosting and reflections. In some higher priced filters harder glass (less chances of breaking) and anti smudge coating, which ensures that practically nothing sticks to the surface - easier to clean and maintain</blockquote>

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