Understanding ISO

In traditional film photography the ISO was a measure of the sensitivity of camera film to light. If you were shooting in low light conditions you needed to use a type of film that was more sensitive to light (had a higher ISO number) in order to get a decent shot. The big drawback for photographers was that once the film was loaded into their cameras they were unable to change the ISO setting until they had finished shooting the roll of film and could load another with a different ISO speed.
Now ISO is used to describe and quantify the sensitivity of the cameras digital sensor to light. The higher the ISO number the the more sensitive the cameras sensor setting, allowing you you take pictures in low lighting conditions. The great benefit of ISO in the digital world is that you can change the cameras ISO setting for each and every photograph you take. You no longer have to shoot ‘the whole film’ at the same ISO.
ISO forms a part of the famous exposure triangle along with aperture and shutter speed and as a result is directly related to these two elements. As mentioned above increasing the ISO setting of your camera will enable you to take a picture with a quicker shutter speed in order to get the same exposure. Assume you are shooting at an aperture of f8 and a shutter speed of 1/60 of a second at ISO 100. By doubling the ISO to 200 (every time you double the ISO you increase it by 1 stop) you would now be able to take the the same exposure at f8 but with a shutter speed of 1/125 of a second.
Problems with High ISO
While the above sounds great you should be aware that the potentials for quicker shutter speeds with increased ISO come at a cost. One of the side effects of increasing your cameras sensor sensitivity is that the resulting images will show increased noise or grain. To see the effect of the noise created by ISO try taking two identical shots, one at ISO 100 and one at ISO 1600. You’ll more than likely soon see these grainy effects in the one shot at higher ISO.
As a result in most circumstances it is preferable in low light conditions to select a large aperture (lower f-number) if you can in order get a faster shutter speed. Increasing the ISO should be your last resort when you are shooting wide open (with your largest aperture) and you are still not getting fast enough shutter speeds in order to get a sharp image.
Despite the above limitations and effects of high ISO settings digital slr cameras built in the last 2-3 years are becoming increasingly better at removing the noise when using high ISO settings. Most DSLR cameras will let you shoot up to ISO 400 without you noticing any immediate noise or grain creeping into your images. In the next year or two it is likely that this good performance will increase to 1600 and maybe beyond as manufacturers are constantly working to increase high ISO performance.
Summary
ISO is an incredibly useful tool that can help you get a shot, especially when light levels are low. While increased grain in images is an obvious side effect of high ISOs do not always fear it. Grain in your images can be lend a very artistic effect to a photograph, in particular in black and white images.

In traditional film photography the ISO was a measure of the sensitivity of camera film to light. If you were shooting in low light conditions you needed to use a type of film that was more sensitive to light (had a higher ISO number) in order to get a decent shot.

The big drawback for photographers was that once the film was loaded into their cameras they were unable to change the ISO setting until they had finished shooting the roll of film and could load another with a different ISO speed.

Now ISO is used to describe and quantify the sensitivity of the cameras digital sensor to light. The higher the ISO number the the more sensitive the cameras sensor setting, allowing you you take pictures in low lighting conditions.

The great benefit of ISO in the digital world is that you can change the cameras ISO setting for each and every photograph you take. You no longer have to shoot ‘the whole film’ at the same ISO.

ISO forms a part of the famous exposure triangle along with aperture and shutter speed and as a result is directly related to these two elements. As mentioned above increasing the ISO setting of your camera will enable you to take a picture with a quicker shutter speed in order to get the same exposure. Assume you are shooting at an aperture of f8 and a shutter speed of 1/60 of a second at ISO 100. By doubling the ISO to 200 (every time you double the ISO you increase it by 1 stop) you would now be able to take the the same exposure at f8 but with a shutter speed of 1/125 of a second.

Problems with High ISO

While the above sounds great you should be aware that the potentials for quicker shutter speeds with increased ISO come at a cost. One of the side effects of increasing your cameras sensor sensitivity is that the resulting images will show increased noise or grain.

To see the effect of the noise created by ISO try taking two identical shots, one at ISO 100 and one at ISO 1600. You’ll more than likely soon see these grainy effects in the one shot at higher ISO.

Take a look at the  image below (shot at 1600 ISO) and you can probably work out some grain. However this is a much better shot that could have been achieved by using the on-board flash.

High ISO Can Cause Grainy Images

High ISO Can Cause Grainy Images

As a result in most circumstances it is preferable in low light conditions to select a large aperture (lower f-number) if you can in order get a faster shutter speed. Increasing the ISO should be your last resort when you are shooting wide open (with your largest aperture) and you are still not getting fast enough shutter speeds in order to get a sharp image.

Despite the above limitations and effects of high ISO settings digital slr cameras built in the last 2-3 years are becoming increasingly better at removing the noise when using high ISO settings. Most DSLR cameras will let you shoot up to ISO 400 without you noticing any immediate noise or grain creeping into your images. In the next year or two it is likely that this good performance will increase to 1600 and maybe beyond as manufacturers are constantly working to increase high ISO performance.

ISO Summary

ISO is an incredibly useful tool that can help you get a shot, especially when light levels are low. While increased grain in images is an obvious side effect of high ISOs do not always fear it. Grain in your images can be lend a very artistic effect to a photograph, in particular with black and white images.

Enjoy this article? If so please subscribe to Digital SLR Guru to receive free updates!

Tagged as: exposure, Tips & Tutorals

Leave a Comment

Previous post:

Next post: