Re: Megapixels and noise

"Roger N. Clark (change username to rnclark)" <username@xxxxxxxxx> wrote:
Remember, though, that most of that noise in the shadows
is photon noise, and we need higher QE and deeper full wells
in order to improve that. Or simply bigger pixels. At low
ISOs on current cameras, 16-bit A/Ds would help too for the
very lowest signals.

A 14-bit A/D has a dynamic range of 84 dB. Until you have
a sensor that has more dynamic range than that, using a
16-bit A/D simply is not going to provide any improvement.

I don't know what the current crop of sensors looks like
for dynamic range, but I doubt that it approaches 84 dB
without cooling.

What a 16-bit A/D would produce is either a lot of head
room at the top with no values every used, and the lower
14 bits having exactly the same values they do with a
14-bit conversion; or the signal can be amplified to
provide output using the higher levels, and the lower
bits will be essentially random noise resulting in
exactly the same number of useful levels as a 14-bit
A/D. Either way, the limitation is the sensor not the
A/D converter.

Of course there are other penalties too, such as slower
processing due to the extra bits, and the higher memory
requirements, none of which actually contribute to
useful image improvement. Indeed, that is why 12-bit
A/D's were used until the most recent cameras arrived,
even though the sensors could provide slightly better
dynamic range (by a couple dB) than a 12-bit A/D. The
advantage of going to a 14-bit A/D just was not worth
the cost, at that time. Now, with more parallel
channels from sensors, and 8 Gb vs. 1 Gb CF cards, the
advantages of a 14-bit A/D have a lower cost, and are
viable.

But is there any advantage to a 16-bit A/D at all, to
compensate for the slower speed and higher memory
requirements?

--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd@xxxxxxxxxx
.

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