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Monday, December 16, 2013

Introduction to Photography Exposure Terms – Part 3: ISO

By Marc B

Last week we introduced beginners to aperture, and the week before we wrote about shutter speed. This week (in Part 3 of the Introduction to Photography Exposure Terms), we’re introducing ISO or sensitivity.  Using these settings effectively, will help you achieve the right exposure and therefore the right look and feel for the photo you want to take. Remember to play around with these settings and experiment. You will learn with practice, so start shooting!

ISO or Sensitivity
The ISO setting on a DSLR is used to increase or decrease the sensor’s sensitivity to light.  Current ISO sensitivity ratings are based on the original sensitivities of film i.e 100, 200, 400, 800 etc.  The ability to change the ISO quickly is very useful, as it allows you to adjust your photo according to the available light in the area you are shooting. 
If the shutter speed is too slow and you have no alternative then you can increase the ISO which will allow you to have a faster shutter speed with the same amount of light.  Be aware that as you increase your ISO you also increase the appearance of ‘digital noise’ in your image. ‘Digital noise’ shows up as a messy multi-coloured haze across the image and is particularly noticeable in block areas of colour or shadow.  The ‘digital noise’ gets worse as you increase the ISO.
Typically the ISO has a range from 100 ISO up to 25600 (or more, depending on your camera). When talking about ‘general photography’, most photos are shot in the range of 100 to 1600 ISO.  The best ISO to choose depends on a number of variables. One of these variables is when the content of the image is the absolute first priority (eg a time critical photo for a news story where the technical result is not as important as ‘just getting the shot’). In this case, a high ISO is fine. It will ensure the photo has a better chance at being sharp.
 
Examples
The photo on the left shows a cropped area of an image shot with an ISO of 100.  It is very clean with good contrast and next to no digital noise.
The photo on the right was shot at almost exactly the same time but with the ISO set to 6400.  The difference is less contrast and a lot of ‘digital noise’.
 
The ISO setting can be a powerful tool when deciding on your exposure settings.  Adjusting the setting will allow you to move to a faster or slower shutter speed while keeping your aperture consistent. Keep in mind that as you increase the ISO you also increase the appearance of digital noise in your image with a corresponding decrease in image quality.
Experiment and have fun!
Next week, Part 4 of our ‘Introduction to Photography Exposure Terms’ will be on Exposure (bringing it all together).

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