--- "Mr. Bill" <[EMAIL PROTECTED]> wrote:
> Robert Meier wrote:
> >
> > But you don't consider that so far mainly CCDs
> have
> > been used for professional digital cameras. CMOS,
> > despite the size, have many advantages over CCDs.
> To
> > name just a few:
>
> Ah, but you don't mention the flip side. If
> everything you said was
> true, every digital camera would be using CMOS
> sensors. What are the
> downsides to CMOS sensors:
> 1) Significantly less sensitive to light
> 2) Significantly more electronic noise generated by
> the chip
> 3) Resulting in significantly less shadow
> detail/dynamic range
CCD is a relative old technology that performce almost
at its best possible. Therefore, besides resolution
CCDs have not advanced that much anymore.
CMOS sensors in contrast are relative new. Huge
advantages have been made in the last few years. The
main concern was mainly random as well as fixed noise.
New technologies have eliminated both to a very high
degree that make CMOS amost equal or for some
applications even better then CCDs. To answer your
concerns:
1) Low sensitivity is mainly a problem of active CMOS
because they need additional transistors. This can/is
fixed in two ways:
a) Smaller geometries allow smaller transistors. This
increase the active area considerably.
b) As with CCDs micro lenses can be used that increase
the effective fill-factor to almost 100%
By the way, if you want anti-blooming for CCDs the
sensitivity of the CCD will be decreased.
2) CCDs transfer (and destroy) the charge of all
pixels to ONE amplifier. Therfore, this amplifier has
to work at a much higher bandwidth which increases
noise. This problem increases exponential with the
increase in pixels. Active CMOS sensors in contrast
have an amplifier for each pixel. Therfore, these
amplifiers only have to work at the rate that lines
are read-out. This is MUCH lower then the pixel rate
and therefore reduces random read-out noise a lot. Now
the fixed noise pattern comes from different
characteristic of the pixel amplifier. This becomes
more a problem the smaller the transistors become.
Nevertheless, because this is a fixed characteristic
the sensor can be calibrated and the fixed noise can
be reduced to a very high degree.
3) The advances mentioned above have improved the
dynamic range to a very high degree, for some
applications even better then CCDs.
Now, if you expect Canon to put old technology in
their digital cameras I agree with you. Otherwise CMOS
sensors are very good nowadays. Proof is the D30. I
haven't heard any significant amount of complaints
about noise, etc. which proofs that CMOS is up for the
task. And again, while CCD does not improve a lot
anymore CMOS is still advancing fast. So you probably
can expect even better performance for the next
digital CMOS camera by Canon then the D30.
Robert
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