What is ISO? When to increase sensitivity, types of noise and more
If you’re new to photography you may have asked yourself, ‘What is ISO?’
Back in the days before digital, film came in a variety of different speeds. The ‘faster’ the film, the more sensitive it was to light – allowing you to use faster shutter speeds than with ‘slower’ film.
Using these higher-sensitivity film emulsions was useful for moving subjects – and particularly so in low light. This film speed was measured using a number of different scales – with two of the best known, the American ASA and German DIN scales, eventually being brought together to give us the standardized ISO system.
ISO 1000 – 1/15sec at f/5.6
Digital cameras, of course, do not use film – but the same ISO scale is now used to measure the camera’s sensitivity to light. Although the camera’s imaging chip cannot be changed to suit the subject (unlike film), its sensitivity can effectively be boosted by the camera’s circuitry.
This is done with the ISO control. Think of ISO as being like the volume control on your radio. If the signal is weak, you crank it up to compensate.
The signal from the sensor is simply amplified – and this helps you get the fast enough shutter speed you want in low light.
Click on the image to see the larger version. The crops inset are shown in actual pixels.
The advantage of digital over film is that the ISO can be altered for each individual shot. This makes ISO a powerful tool for the photographer, helping you to get sharp shots in a variety of lighting conditions.
ISO is the name of the International Organization of Standardization: a body that creates thousands of agreed standards for a huge range of products, procedures and practices.
For the photographer, ISO is simply a set of numbers. The base sensitivity of all current Canon EOS SLRs is ISO 100.
But this can be increased by pressing the appropriate button, then rotating the main dial (on some older cameras ISO is changed through the Menu button).
The scale is such that doubling the ISO number doubles the sensitivity of the sensor. So increasing the ISO setting from 100 to 200 means that, to get the same overall exposure, you can use a shutter speed that is half as long (or twice as fast).
Each doubling of the ISO also increases the sensitivity by a full exposure ‘stop’ – with the typical full-stop ISO scale progressing 100, 200, 400, 800, 1600, and so on. The top ISO setting varies depending on the age and cost of your digital camera – from ISO 3200 to a staggering ISO 102,400 on some models.
Bizarrely, the top ISO settings on many models are ‘hidden’, and must be enabled using a custom option called ‘ISO Expansion’. The reason for this is that each time you increase the ISO setting, you also get a small decrease in image quality.
Boosting the picture signal also amplifies impurities in the signal known as ‘noise’. This noise shows up as grain and colour mottling in the image – and this gets progressively more noticeable the higher the ISO is set.
PAGE 1: What is ISO?
PAGE 2: How ISO works: free cheat sheet
PAGE 3: When to increase ISO
PAGE 4: Types of noise
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on Wednesday, June 20th, 2012 at 4:00 am under Beginner.
Tags: camera tips, DLSR tips, hot, ISO sensitivity, photography cheat sheet