Nice article on why to shoot RAW...

Started Oct 11, 2007 | Discussions thread
Bill Janes Senior Member • Posts: 2,025
Re: Absolutely Correct

RomanJohnston wrote:

ISO 340 isnt optimal...just where you want to stop cranking up the
iso because EV adjustments in RAW match or do better than shooting
the higher ISO.

100 is still optimal.

The main source of noise in digital images is due to shot noise, which is inherent in the collection of photons by the sensor and not a limitation of the sensor. Shot noise follows a Poison distribution and the standard deviation of the noise is equal to the number of photons collected. At base ISO, when you expose to the right, you collect the maximum number of photons and get the best signal to noise ratio.

When you expose above base ISO, the electron wells in the sensor are not filled, and the signal to noise ratio is determined mainly by the exposure (which determines the number of photons collected) and not by the ISO. The other main source of noise is read noise, which is noise introduced by transferring the collected electrons (or charge) from the wells of the pixels to the analog to digital converter (ADC) for conversion to a data number and the noise introduced by the ADC itself. Read noise is predominant in the deep shadows. It turns out that read noise is highest at base ISO and decreases ad ISO is increased. Therefore, if exposure is limited by shutter or aperture constraints, it is best to use a higher ISO to reduce read noise. You also get better quantization, since the full range of the ADC is used.

When 1 photo-electron corresponds to one data number (unity gain), then you have captured all the information, and increasing ISO further will not help. These considerations are explained in an article by Roger Clark on his web site. He gives the unity gain for various cameras. It is about ISO 1500 for the D50, 1050 for the D70, and 800 for the D200. He does not give the unity gain for the D2x, but I would expect it to be slightly less than for the D200.

If you expose at unity gain, rather than a higher ISO, then you can increase exposure in the raw converter when needed and get the same results as exposing at higher ISO, but you will have more highlight headroom as Roman points out.

Bill Janes

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