I can use real-world comparatives.
The Olympus E-1 (4/3) has nearly identical pixel density as the Canon
6D. The surface area of the 6D sensor is 4x that of the E-1, so the
pixel count is also 4x that of the E-1. How does that translate to
real-world shooting? If I'm photographing the same subject with the
same angle-of-view (more or less, since the height/width ratios are a
little different), the FINAL Print would have 2x the potential
resolution with the 6D. However, if I use identical focal length
lenses and phototograph the subject from the same distance, the 6D
image will need to be cropped to match the E-1 image. When you do
that, and you "pixel-peep" you will see that the images are a little
different because of the sensor design and differences in the raw
conversion process. But the resolution is the same. The resulting
cropped 6D image and E-1 image are essentially identical.
Sensor design and advances in noise mitigation have greatly changed
how newer sensors respond when the picture is shot at higher than base
ISO. At base ISO, the 6D and E-1 aren't significantly different other
than the 6D has slightly more dynamic range which is a reflection of
the one or two stops improvement in real noise floor between the E-1's
CCD and the 6D's CMOS sensors.
At higher ISOs, you certainly see dramatic improvements with the
newest sensors of ANY pixel density. However, not all high ISOs are
created equal. And not all time-exposures are created equal either.
The 6D is great at high ISO, and also great at long exposures. But NOT
great at both. The extensive noise abatement processing that is going
on in-camera results in large-scale color blotches which cannot be
removed by any means. There are some means around that, but most of it
is baked into the chip design itself.
Generally speaking, the larger the physical sensel itself is, which is
related to the pixel pitch on the sensor, the less noise AND greater
the potential dynamic range. This is physics 101, in that the greater
the capture area for any given pixel, the more photons to electrical
noise ratio you get. But that's very simplistic. CCD sensors are
typically noisier than CMOS sensors (as implemented in digital
cameras), but that isn't always the case. CMOS has power usage
advantages as well as the fact that more processing is done on-chip
than CCD which puts the processing downstream into another chip. I
mention this because it's no longer a clear CCD vs. CMOS world as some
of the latest CMOS designs are actually hybrid chip designs which are
migrating back towards CCD in several ways. But by reversing the chip
design, and going multilayer with the support electronics and
pipelines put below the sensel itself, we are able to achieve a
greater fill factor and increase speed by parallel processing
everything right at the source instead of either at the end of a line
or in the next support chip.
The answer to your second question is that if all else was equal, yes,
a high density chip would be noisier and have less dynamic range than
a lower density chip. But as is shown with the E-1 and 6D example, the
same density yields dramatically different noise patterns and moderate
increase in dynamic range. NOT ALL IS EQUAL. Every generation of chip
design gets improvements in noise mitigation and increases in dynamic
range. It's not just a physics thing, but an engineering thing.
AG Schnozz
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