The Best ISO Setting
With the advent of digital photography came huge flexibility in the ability to change the ISO setting of the camera. Previously you needed to wait until you were ready to change your film in order to move to a different ISO. Now you can change the ISO any time and as a result it’s one of the most misused controls. Few photographers really understand the effect of changing the ISO setting and a question that's often asked is "what's the best ISO setting".
What is an ISO Setting?
The ISO setting is a common standard using the photography industry to measure of how sensitive the cameras sensor is to light. The higher the ISO number, the more sensitive the sensor is. For example a setting of ISO200 is more sensitive than ISO100.
When we use a more sensitive ISO setting it can make it easier for the camera to achieve the exposure of the image. For example if we need a shutter speed of 1/60” (second) when using ISO100, if we increase the ISO to ISO200 we can use a faster shutter speed of 1/120”. The benefit of the faster shutter speed is that we are less likely to experience camera shake when taking the picture.
High ISO Drawbacks
If higher ISO settings allow us to use faster shutter speeds and this is an advantage, why would we not use the maximum ISO setting. The answer is simple noise. You see the camera sensor doesn’t really become more sensitive, it’s actually the signal to the sensor that’s boosted. And with this boost to signal strength noise is also introduced and boosted.
Noise is the enemy of the photographer and can seriously degrade the quality of your photography. You can see a simple comparison below.
Avoiding Camera Shake
Avoiding camera shake is the best and probably only reason that you will want to use a higher ISO. Camera shake can also damage the quality of your photography and can make image appear soft. Look at the following two images.
This first image was captured using ISO80 which is lowest ISO this particular camera can be set to. It should produce the lowest noise levels and be the highest quality.
Now compare this to the next image.
This image was captured at ISO400 and therefore contains higher levels of noise than the first image. Despite this the image is noticeably sharper.
The Best ISO Setting to Use
The best ISO setting to use is therefore the lowest one that allows you to avoid camera shake. You might not realise it but longer focal length lenses require faster shutter speeds to avoid camera shake. As a rough rule of thumb, the shutter speed should be at least as fast as the inverse of the lens focal length. This means if you are using a 20mm lens then your shutter speed should be faster than 1/20”. If your lens is a 100mm focal length then aim for shutter speeds faster than 1/100”.
The one thing you must avoid is placing your camera in Auto ISO. Manufacturers are usually overly conservative and ISO setting tend to rise too quickly in the auto mode. Get into the habit of manually selecting your ISO, watching the shutter speed and only increasing the ISO when you notice the shutter speed getting too slow.
But the ISO setting in your camera is just one of three settings that allow you to take control of the expoosure. The other two are shutter speed and aperture. Learn more by reading my article Creative Camera Control. And if you haven't already done so, I recommend reading my article what is an f-stop.
If you would like to know more about this subject why not consider my book “Beginning Photography the Right Way”.