In the old film days, you decided on the ISO when you bought your film. With modern digital cameras we can manually adjust ISO for each shot, or allow the camera to adjust it automatically. Why adjust digital camera ISO? Why not just set it to the highest number possible and forget about it?
First, what is ISO? In film, ISO tells you the sensitivity of the film to light. The more sensitive the film, the higher the ISO. With digital cameras, we have a sensor in place of the film, but the principle is the same. If I set the ISO to 200 it is twice as sensitive as setting it to 100. The more sensitive the sensor is set – a higher ISO – the faster we can set the shutter speed for a given aperture. Here is a great article about aperture and setting aperture. I also talked here about aperture when discussing depth of field. If you are shooting your child’s soccer game, you would want a higher shutter speed to stop the ball or freeze their kick. If you are shooting a waterfall, it is common to set a slower shutter speed to give a longer exposure time. This softens the waterfall, giving it a flowing effect.
You may think, “My camera has a maximum ISO of 6400, why not just set it there and not worry about it?” Like everything in life there are tradeoffs. The higher you set the ISO, the grainier the image comes out. What does that mean? Here are two photos of the same object shot at the same aperture setting but different ISO and shutter speed. The first was shot at ISO 200, the second at ISO 6400 with the same aperture (f/4.0).
When you click on the images, you can see an example of how the very high ISO produces a grainy picture. If you are at your child’s school program and you aren’t allowed to use a flash, then use 6400 or the highest ISO available on your camera and use the highest shutter speed you can. The quality of the image is less important than capturing the event clearly. If you are outside at the soccer game on a sunny day, set the ISO to 100 or 200 to prevent an overexposed image – where all you see is a white picture.
There is another alternative. If you are unsure about what ISO to use, most cameras have an Auto ISO option. There is no reason to not use this setting if you are unsure about what ISO to use. Many people will use Auto for standard shoots because it is one less thing to worry about. However, if you are trying to get that photo of your baby sleeping in some funny pose and don’t want to wake them up – boost the ISO.
Camera Info: These images were shot with a Nikon D600 using an 85mm f/1.8 lens