Re: Why not make the sensor larger?

Alfred Molon wrote:
In article <46720DC9.9090603@xxxxxxxxx>, username@xxxxxxxxx says...
Alfred Molon wrote:
In article <46712A6F.6070300@xxxxxxxxx>, username@xxxxxxxxx says...
Actually, it's brighter. For each doubling of pixel size,
you get 4 times the light (keeping megapixel count the same),
thus gaining 2 stops. They key is what is the aperture
diameter, not the f/ratio. Larger apertures deliver more
light. Thinking common f/ratios deliver the same photons
is a common misconception.
But doubling the pixel size (i.e. increasing the area 4 times) will only reduce the thermal noise by a factor of 2, while it will increase the weight and volume of the lens by a factor of 8!
Thermal noise is negligible in most situations.
Noise in an image is:

N = (P + r^2 + t^2)^0.5,

Where N = total noise in electrons, P = number of photons,
r = read noise in electrons, and
t = thermal noise in electrons.
Noise from a stream of photons, is the square root
of the number of photons.

Dark current is a small fraction of an electron per second,
and thermal noise is square root of the dark current.

Read noise is a few electrons, so most noise in digital
camera images is from the random arrival of photons.

For more information, see:
Night and Low Light Photography with Digital Cameras
http://www.clarkvision.com/photoinfo/night.and.low.light.photography

Do you have any reference for this noise equation (other than your own)?

And by the way you are adding photons with electrons!

1) 1 photon captured = 1 electron

2) try googling: CCD thermal noise.

Here is one variant of the above equation:
http://www.astrophys-assist.com/educate/noise/noise.htm
(also includes in their equation sky background for
astronomical imaging, and averaging pixels).

Roger
.

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