Larry, Read through this, it may help your understanding (or it may confuse the hell out of you):
http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/noise-p3.html#ETTR And the entire document: http://theory.uchicago.edu/~ejm/pix/20d/tests/noise/index.html On Sun, Apr 10, 2011 at 8:27 PM, Larry Colen <[email protected]> wrote: > In my typical geeky fashion, I'm trying to wrap my head around all of the > ramifications of adjusting sensitivity (ISO) on my camera. Please correct the > errors in my understanding. > > In the simplest form, it is a measure of how many LSBs per photon (or tens, > thousands or millions of photons). > > If the base is 10 photons, per LSB at ISO 100, 1,000 photons hitting a sensor > site gives the value 100, at ISO 200, it'll give 200, and at 6,400 1,000 > photons will cause the raw fill to read 6,400 for that sensor site. > > In a similar vein, if we have 14 bits of data, and at base ISO (100), 14 > stops of dynamic range. That gives us a maximum data value of 16,384 which > means at the above sensitivity, 163,840 photons will cause the sensor to read > maximum value, and any more than that cause data to be clipped. > If we then increase the gain by a factor of two, then at ISO 200, we are > expressing 13 stops of dynamic range in 14 bits of data, 5 photons will cause > an LSB worth of change in the data, but it will only take 8,192 photons to > clip in the data path. > > Since the data width is constant, every time we double the sensitivity, we > force ourselves to only use the lower, noisier, half of the signal. The > benefit of this is that we can read that narrow band of the the sensor with > more resolution so at ISO 12,800, rather than only having 7 bits of data for > the lower 7 stops of dynamic range, we have 14 bits of data to work with. > This is why using high ISOs give us more contrast. > > It seems to me that if we are shooting a low contrast situation, such as > clouds on a grey sky, or with a mediocre, low contrast lens, we could > compensate by using a higher ISO to spread the fewer stops of dynamic range > in the input out over more bits of data, at the cost of more noise, because > we're constraining ourselves to the lower, noisier portion of the signal. > > Is this basically accurate? > > -- > Larry Colen [email protected] sent from i4est > > > > > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and follow > the directions. > -- David Parsons Photography http://www.davidparsonsphoto.com Aloha Photographer Photoblog http://alohaphotog.blogspot.com/ -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
