Re: Canon Rebel XT, Nikon D50 or Pentax K100 best?

On Sat, 02 Sep 2006 20:48:59 +0200, Iordani <somewhere@xxxxxxxxx> wrote:

This is news to me. Can you explain what the ISO setting does?
A pointer to some more info would also be valuable.
I tried google but found nothing appropriate.

The ISO value is a measure of the gain applied to the output amplifier.
In all cases I'm aware of this is done after the sensor is read, but in
principle it could be done inside the sensor, which could provide some
moderate advantage in terms of readout noise.

Astronomical cameras have a gain chosen to provide a good match between
the number of electrons each pixel can store and the number of bits the
A/D converter uses. In the simplest case, you want a full pixel to
produce a full count from the A/D.

In astronomical image, the intrinsic noise is determined by the number
of photons collected (the noise is the square root of the photon count).
When you increase the gain, you increase the signal and the noise the
same amount. That's why unfiltered, high ISO images are noisy. It also
means that any bright sources in your image will saturate much earlier.
High ISO astronomical images may show larger stars because of this. When
you are processing a raw image, it makes no difference if a pixel has a
value of 100 ± 10 or 400 ± 40. You could as well multiply all the pixels
of an ISO 100 image by 4 after the fact as to shoot it at ISO 400.

Well, almost. The difference is that if you can reduce the number of
subframes you need, you can reduce the total contribution of readout
noise. By boosting the gain at the sensor you can pull the lowest level
signal a little higher above the quantization noise.

With the 300D, I've done detailed noise measurements. What I found is
that there is no difference in noise between a 300s exposure at ISO 100
and a 150s exposure at ISO 200. But a 75s exposure at ISO 400 has more
noise, and a 37.5s exposure at ISO 800 has even more, and this trend
continues at ISO 1600 and ISO 3200 (the highest the camera goes).

So clearly the best place to operate this camera is ISO 200. However,
the noise at ISO 400 is only a little higher, so I use that for shots
where a longer exposure might be a problem.

A good camera is an efficient photon counter, recording 50% or more of
all that hit it. From a signal processing standpoint, there is no reason
to artificially multiply that number by anything. You just need to find
the value for a specific camera that gives the best results- probably
between one and two counts per photon.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
.

Relevant Pages

  • Re: Panasonic DMZ-FZ8 versus comparable dSLR
    ... ISO 200, which is the D70's lowest ISO setting, but the noise on the ... D70 at ISO 200 is essentially non-existent while on the FZ5, noise is ... I personally almost always shoot with my FZ8 at ISO 100, ... artifacts from in camera processing. ...
    (rec.photo.digital)
  • Re: Canon 5D stomps Sigma SD14 into the dust
    ... equipped 5D's images. ... ISO 1600 (from what I've seen, things currently turn nasty much before ... There were lots of ISO 800 images, and there was quite a bit of noise; ... personally wouldn't pay so much for this camera. ...
    (rec.photo.digital.slr-systems)
  • Re: Just when you started believe the low noise myth
    ... This is about the maximum latitude in a scene a Canon CMOS digital sensor can cope with before resorting to software enhancement to compensate for noise. ... All the others produce noise in shadows at ISO 100 which 5Ds don't. ... One of the real problems I though this post would highlight, is that reports from sources closely aligned with camera makers - read that as "don't bite the hand that feeds them" - are so distorted as to only present the very best possible report of a product. ... Anyone owning a 20D or a 30D who consistently "exposes for highlights" will sooner or later get pictures like this. ...
    (rec.photo.equipment.35mm)
  • Re: Do you set your camera at high resolution?
    ... of the best for low noise. ... to say about image quality, ISO, noise, etc.: ... Rebel as a low-noise camera is not echoed in my personal ... º Good color, default +1 parameter set means punchy images ...
    (rec.photo.digital)
  • Re: S6500FD/S6000FD owners report
    ... every camera you've ever commented upon. ... take for you to grow disenchanted with the FZ8's noise? ... batteries.Never required or will purchase a spare battery for the F20, ... greatest in the middle of the ISO range. ...
    (rec.photo.digital)
Quantcast