MadSci Network: Computer Science

Re: iso in digital camera's

Date: Wed Jan 10 10:42:53 2001
Posted By: Gareth Evans, Senior Research Associate
Area of science: Computer Science
ID: 978826275.Cs


I'm ready to buy a digital camera, I notice some camera's have a iso of 0 and 
others show an iso of 100, 150, and 200. I can't locate anything on the pro's 
and con's of those spec. and what they do.


Tom:  You ask a straightforward question and not unusually the answer could be 
very complicated.  The question has been asked within the photographic industry 
for a long time while people have tried to compare silver-based photographic 
systems with the CCD or CMOS sensor based digital systems.  The two systems are 
very different and it is not easy.  In fact, the ISO speed of digital still 
cameras ( DSCs ) was only defined a couple of years ago and the rating has only 
recently been used in specifications.  It is still not very meaningful unless 
you know some other facts about the camera.  Even then you can not expect a 
ISO100 DSC to give you the same picture quality as a compact camera with a film 
unless you pay a lot of money.

The people writing the ISO standard tried to make the definitions of ISO 
speed "harmonise" with film ratings.  This means that for a given speed rating 
with the same scene brightness you would expect approximately the same exposure 
defined by the aperture and exposure time. 

The same trade-offs between ISO speed and camera settings and image quality 
apply.  For example, for a given aperture, the ISO rating is an indication of 
the exposure time and the higher the rating the shorter this will be.  Shorter 
times are good for freezing action.  Similarly, for a given exposure time, a 
higher rating will mean a smaller aperture and more of the picture will be in 
focus because of an increased depth of field ( the range of distances from the 
camera where the scene is in focus ).  There is a catch though.  If you are a 
serious creative photographer you may want to control the depth of field so 
that you can defocus part of the scene.  Because the camera sensors are 
normally smaller than a film frame, the depth of field of a DSC is 
normally "better".

The other major trade-off is between speed and image structure.  Here, we need 
to consider another factor, namely the amount of "noise" generated in the 
image. In a CCD sensor, photo-electrons produced by exposure to the image are 
collected and the charge they generate is measured to produce the image 
information.  The image-wise variation in the collected charge is the 
image "signal".  The noise is the charge which accumulates randomly whether 
there is exposure or not.  When the image exposure is low, the signal is small 
and needs to be amplified for image processing, storage and display.  The noise 
is also amplified in this process and the important factor is the signal/noise 
ratio not the absolute signal level.

How low the exposure of the sensor can be before a good image is captured 
depends then on the noise level as well as the efficiency with which a sensor 
converts light into charge.  This is the main criterion which determines the 
ISO rating of a camera.  Some cameras offer several ISO speed settings.  This 
allows additional control over the exposure time and/or aperture settings but 
the higher the ISO setting the lower the exposure will be and the worse the 
signal/noise ratio.
Image quality will of course depend a lot on the number of pixels in the 
sensor.  Let's say we had two image sensors with exactly the same technology 
giving identical noise levels and sensitivities but one was four times the 
area.  The larger one would use a longer focal length lens than the smaller one 
to fill the sensor with the same image at the same aperture.  The smaller 
sensor would see the image at four times the light intensity, thus needing a 
quarter of the exposure time.  This would mean that the camera would be rated 
with four times the ISO speed.  However, it would only have a quarter of the 
number of pixels so high ISO speed is not necessarily good.

In summary, unless you know a lot about the other design parameters it's 
difficult to interpret ISO speeds for DSCs in a way which makes it useful in 
making a choice.  All other things being equal, a high ISO rating implies 
better signal to noise performance but all other things are rarely equal!  A 
camera with variable ISO speed setting will allow some extra control over the 
camera settings.

If you want to look up the ISO standard there is a version published as British 
Standard ISO 12232:1998.  Parts of it are remarkably readable !  Best of luck.  
I hope you enjoy your photography.

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