Re: Megapixels and noise

Floyd L. Davidson wrote:
"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.

A useful way to look at this is that as you add more bits you should be able to get the noise performance of higher ISOs but with the upper end range of the lower ISOs. As an example if I use a 14 bit converter I should get the same shadow detail at ISO 100 as I would if I used a 12 bit converter at ISO 400, but I would have a lot more head room at ISO 100. It is easy to do a test keeping the aperture and shutter speed the same and see that as you increase the ISO you can more shadow detail, on the 20D you begin to run out of room at around ISO 800. IOW going from ISO 400 to 800 show more shadow detail but from 800 to 1600 only shows a small improvement. Unless the 20D does not come close to using the upper
end of the A/D this shows that there would be benefits going to at least 15 bits on the A/D. With other cameras that have larger pixels I would imagine that 16 bit would be of value.

Scott


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.
Most cameras that are using 12 bit A/D are suffering from not having enough bits. If this was not the case there should be no improvement in shadow detail when increasing the ISO. I would say that there is a lot more then a couple of dB range being lost in the camera using 12 bit converters.



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.

There was always the option of allowing the user to store the raw file in a lossy format, I note Canon is now adding this file format.

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

A/D get faster, if a 16 bit A/D can be made fast enough then speed is not an issue. As for memory requirements, why not let the user decide what bit depth they want to store the images to? I really don't believe that even with the 20D 14 bits is enough to get all the range there is.

Scott
.

Relevant Pages

  • Re: Megapixels and noise
    ... 16-bit A/D simply is not going to provide any improvement. ... for dynamic range, but I doubt that it approaches 84 dB ... the limitation is the sensor not the ... should get the same shadow detail at ISO 100 as I would ...
    (rec.photo.digital.slr-systems)
  • Re: Do you like an A/D converter with 150dB dynamic range?
    ... few hundred points can go as low as a few nano-volts. ... kind of A/D converter? ... We do not talk about theoretical dynamic range. ... For a 24bit A/D ...
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  • Re: ISO 200000 ?
    ... At ISO 1600, only the bottom 1/16th of the full well is digitized, so the dynamic range drops to ~50,000/16/read noise. ... One would need a 16-bit A/D to digitize that range. ...
    (rec.photo.digital)
  • Re: Adventures in RAW
    ... The camera most likely sets the highest A/D output to some relative fraction of the sensor exposure. ... This way you get a smooth gradation over the ISO 200 exposure range. ... If this is done after the fact you may end up with 'missing counts' since the A/D full count would have been set for 'full well' sensor values and you are stretching it out after the fact. ...
    (rec.photo.digital)
  • Re: 21MP 1Ds Mark III Announced
    ... Why, then, is the blackframe noise almost perfectly proportional to ISO in so many non-Canon cameras? ... I posted a link the Analog Devices A/D specs. ... If you have a 12-bit converter with 70 dB specified noise on a 12-bit ...
    (rec.photo.digital)
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