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ISO stands for International Standards Organization, which doesn't really tell us much about what it is. Except, of course that it is an international standard, so where ever you see this, it means exactly the same thing.

ISO is a measure of light sensitivity. How much light to you need to expose a photograph correctly. It applies equally to film and anything else that is sensitive to light, it tells you just how sensitive to light something is compared to anything else with an ISO number.

The image sensor in a digital camera directly replaces the light sensitive film so the ISO number is now something which is built in to every camera. Even if neither your camera nor its manual make any reference to an ISO number it still has one. It's just that you can't change it, so it doesn't really matter what it is.

With most cameras, however, you can change the ISO number and you will have a menu item for doing just that. On selecting this, you will be presented with a list of numbers and an "auto" option. This is likely to be the default setting for your camera and, if you are a beginner, this is probably the best place to leave it. Your camera will automatically adjust its ISO depending on how much light there is.


You might wonder why anyone would ever want to use any other setting, but there are very good reasons. Not least of which is that part of learning photography is learning to be in control of the photographic process and setting the correct ISO number is an important part of that.


First things first. The higher the number you set, then the more sensitive your camera will be. The scale is linear. That means, if you double the number, then your camera will be twice as sensitive to light. So in simple terms, setting the ISO number is quite straightforward. If the conditions are bright, set a low number and when it get's darker, or you go indoors, change to a higher number. This is what an auto setting will do, but there are times when this simple solution is not the right one.



Using a high ISO number will not only make your camera more sensitive to light, it will also make the images it produces more "noisy". Just what this noise looks like and how much of it there is varies tremendously between cameras. Not only that, but whether the noise is a bad thing or not is a matter of personal taste. Some people feel that noise can actually enhance certain types of photograph. In the heyday of film, some photographers would deliberately shoot on high ISO material for its "grainy" appearance. Grain and noise whilst being created in entirely different ways are visually quite similar.



It is also possible to remove noise from an image, either with a good photo editor or a dedicated noise reduction program. However, good as these solutions are, if the highest quality is what you are after, nothing beats using a low ISO number in the first place.


The two most obvious occasions when you would want to use the lowest ISO number, no matter how dark it is, are landscape and night photography. In practice this means using a tripod. Instead of making your camera more sensitive to light, you expose the image for longer, hence the need of a tripod. This, of course, will work with any static subject and your pictures will be noise free. You could always add noise to your pictures in editing, but this way, you have the choice.


Not only are there times when you would want to use a low number when it's dark, there are also times when you might want to use a high ISO even if it's a bright sunny day. One reason is that it will allow you to use a very fast shutter speed. This is important when you zoom right in or use a long lens on your Dslr camera and are holding the camera by hand.


Long lenses greatly magnify the effects of camera shake and the only cure is to use a faster shutter speed. In terms of overall image quality, it is probably better to sacrifice some, in the form of increased noise, for the benefit of having a rock steady shot. Also, if you want to make sure that everything in your picture from front to back is in focus, then you need to use a very small aperture. You may find that increasing the ISO number is the only way to achieve this.


One last point. If you want to start experimenting with different ISO numbers, the first thing you should do is take a few test shots at each of your available settings. Take a good close look at the noise each generated by your camera at each different ISO level and make your own decision about how bad, or good, it is to you. Once you know that, you will be able to make an informed decision about the best setting to use, rather than let your camera decide for you. Who's taking this picture anyway?

ISO number