On 29/04/2015 20:30, Dennis@home wrote:
On 29/04/2015 16:56, Fredxxx wrote:
On 29/04/2015 16:32, Dennis@home wrote:

snip

Digital doesn't produce the same sort of noise, its just one pixel being
more sensitive to heat than another and you can see it in low light.
Of course if you are going to do low light stuff with digital you could
always go the amateur way and cool the sensor to reduce the noise
effects. You can find youtube videos on how to fit cold fingers to stuff
like D50s if you want to try it.

For thermally generated currents becoming significant you need long
exposure times such as those used in astronomy.

Where do you think the noise comes from in image sensors then?

Most of it these days is readout noise and a small amount of thermal
Johnson noise which only becomes significant on long exposures. Early
CCDs had problems with a warm corner near the readout amplifier and
stray IR photons producing patterns on a 16s uncooled exposure. eg

http://www.nezumi.demon.co.uk/astro/dc120/orion.htm

Modern CCDs have much higher efficiency and lower noise than this early
Kodak chip in the Dc120 which was amongst the first megapixel digicams.

How long exposure times are you worrying about. I think I would be
worrying about image stability first!


snip

Its apparent that something is happening with lens designs as camera
manufacturers are removing the filters from the sensors to get better
resolutions, or at least they are doing so on compact system cameras.

They are doing it because sharpness sells and people don't seem to care
about image quality beyond naive lines per millimetre benchmarks.

What filters are these?

The ones they fit to some DSLRs to try and work better with old design
lenses.

They are the infrared hot mirror blocking filter and anti aliasing
filters to control the sharpness and make the Bayer demosaicing
algorithm better behaved. If you remove the former you will end up with
an out of focus mess since CCDs are sensitive out to around 1000nm and
the optics will pass enough of it to put a blurry IR mess round points.

Yes they do have power, but usually considered negligible, or can be
taken into account in the lens design.

Well yes they can be but they weren't on all those old lenses so you had
better make sure you buy new lenses when you buy your DSLR because the
old ones don't know about the differences in image sensors and film.

Rubbish. Old 35mm family lenses work perfectly well with modern CCDs as
the paraxial approximation is better on the smaller sensor.

Many modern digital lenses will not illuminate a full 35mm frame (and
use insane 35mm equivalent focal length marketing notation).

--
Regards,
Martin Brown