What is ISO?
Your camera’s ISO setting is its' sensitivity to light. The higher the ISO, the more sensitive it is. This is measured according to international standards, so ISO100 on one camera will be exactly the same as ISO100 on another.
Each ISO setting is double the one before: if you increase the ISO from 100 to 200, you double the camera’s sensitivity; and if you increase it from 200 to 400, you double it again. This carries on through the ISO scale.
This is deliberate. The ISO settings are designed to double (or halve) the exposure in the same way that the lens aperture settings & shutter speed settings are, and this is why the lens aperture, shutter speed and ISO are often described as the ‘exposure triangle’.
For example, if you wanted to use a faster shutter speed without changing your aperture, you can increase the ISO instead.
The relationship between lens aperture, shutter speed and ISO can quickly get complicated, but if you take time to understand and practice you will soon get to grips with it.
There are drawbacks to changing the ISO which mean that in practice you tend to change the ISO only when you have to. Drawbacks to upping your ISO depend on your camera, but the higher the ISO number the grainier your photograph may start to appear, this is something that will differ from camera to camera depending on the newness and quality of your camera.
An example of when you may use a high ISO could be if you are somewhere dark and either can't use a flash or if you don't have a tripod and need to let extra light in to the camera and don't want camera shake - you can up your ISO to maintain a faster shutter speed.
In a church for example or a cave maybe. Some historical buildings wont allow use of flash so if you have no tripod with you - having a higher ISO will give you the extra light on the sensor that you need.
However is you are trying to capture the best Landscape image possible - remember to take your tripod and use the lowest ISO that the camera naturally offers without being enhanced - most cameras offer ISO 100 as their lowest ISO - this will give you the best quality image from the camera as it will have the least amount of noise in the image. To understand this fully, next time you go out with your camera, take the same shot using all of the different ISO settings and when you get home look at them all next to each other on your computer and see the difference.
I used High ISO's inside the cathedral, as it let more light in when I didn't have my tripod and so wasn't able to use a slower shutter speed without getting camera shake.
On the image below I used an ISO of 100 - it is a bright sunny day so there was plenty of available light.
Watch out for the next top tip.
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