|MadSci Network: Computer Science|
Question: 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. Response: 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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