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How to use the ISO setting on a digital camera

Digital cameras, like film cameras, often have an ISO setting, but what does this means and how do you use it? All is explained here...

|  Digital Camera Operation
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Words & Pictures Peter Bargh of ePHOTOzine

When you buy a film for a traditional camera you'll notice it has an ISO setting. ISO is short for International Organization for Standardization and in film terms refers to the sensitivity of the film. The camera usually has an automatic setting that recognises which film has been loaded and adjusts to the correct ISO setting - this range is usually between ISO25 (slow) and ISO5000 (fast) which covers most film options. The photographer has the option to load the film with the necessary sensitivity to suit the shooting conditions, using a slow speed film in bright conditions and a fast film in low light. With some film it is possible to change the ISO setting and compensate with the processing.

So why not use fast film all the time?
The faster you go the more grain you'll see in the picture this reduces image sharpness and detail. Contrast is also reduced making the vibrant colours of an image become more muted. Fast film should only be used to get you out of a tricky situation where slower film wouldn't deliver a picture at all, and if used carefully you can get decent photos.

Digital cameras don't use film so why do they have a ISO setting?

Digital cameras record the image using a CCD that converts light to analogue signals, which are then processed in the camera to create a digital image. You can adjust the sensitivity of the CCD so that it increases in low light allowing it to record the image. Unlike a film camera the ISO can be adjusted automatically so you don't have to decide which speed to set just point and shoot. In low light the camera will increase the ISO to ensure the photo is recorded.

Like increasing film speed you do have drawback to upping the ISO of a digital camera. The higher the sensitivity, the more likely pictures are subject to noise, which appears as randomly-spaced, brightly coloured pixels. Most digital cameras have an automatic sensitivity range of between ISO100 and ISO400 and some have a manual override. Some go up to ISO1600. The beauty with adjusting the ISO of a digital camera is that you can check quality on the LCD preview screen and reshoot if necessary. This is easier if the LCD has a magnifying option as it's hard to spot noise on a small non-magnified version.

When to use faster ISO settings
In most situations it's best to fix the camera on a lower ISO setting by turning the ISO auto option off in the camera's menu. Then you are assured highest quality, but you may need to use a tripod in lower light conditions.

The benefit of increasing the ISO is to improve the flash range changing the ISO from ISO100 to ISO400, for example, doubles the distance the flash covers which is useful if you're shooting your child on a school stage or a concert at a local venue.

Here's a simple table to help you work out the range of your camera's flash
The instruction book will normally state the flash range based on the camera set at ISO100 which is listed below as a factor of x1. So let's say the flash can reach a subject 3m away. If you increase the film speed to ISO800 multiply 3 x the conversion factor of x2.8 to show the new extended flash range of 8.4m.

CCD sensitivity (ISO setting) Conversion Factor
ISO 25 x0.5
ISO 50 x0.71
ISO 100 x1
ISO 200 x1.4
ISO 400 x2
ISO 800 x2.8
ISO 1600 x4
It's also useful if you are photographing a subject such as close up of a flower in the shade. You may have set the exposure manually using a small aperture to ensure maximum depth of field but the resulting shutter speed is too slow to prevent the wind movement blurring the subject. A faster ISO will help increase the shutter speed to freeze the movement.

These two close ups are cropped from the main photo to show the effect of adjusting the ISO. The version above left is set to ISO100 and the one above right is at ISO400. Notice the image looks broken up with irregular dots on the ISO400 version. This is noise. There may be times when it's better to have noise and a sharp photo than no noise and a blurred subject because of camera shake or movement.

When using a faster ISO setting it's worth looking to see if your digital camera has an auto sharpen mode and turn this off as the sharpened noise can be really exaggerated.

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Very useful info in this section for the beginner, thanks Peter.
Yes, thanks. It's cleared my mind of a few questions.
Useful and explained well Thanks Peter, here's to more tips.
Great info, a really valuable piece of advice! many thanks
Thanks, very useful and very helpful.
A well laid out article, and very useful.
Thanks, I understand now.
as a beginner i found the info very suporting and helpful .as i am just starting out with photography and you have cleared a couple of problems i was having . thanks peter .
I also found this to be extremely informative and useful. Many thnks Peter, advice I wish I had found before.
Thanks - I hadn't known or thought about the use of the auto-sharpen function in conjunction with ISO. Also the examples provided show very clearly the impact of ISO
Good advice and food for thought very clear and helpful advice. Many thanks Peter

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