Saturday, August 13, 2011

ISO and Shutter Speed....

In my last post we talked about aperture, this time we are going to talk about the other two sides of the exposure triangle...ISO and shutter speed.
Just like aperture, ISO and shutter speed each can be measured in stops of light.
Starting with ISO, the settings are:  50, 100, 200, 400, 800, 1600, 3200, 6400 and so on. Each one will be one more stop of light. So if we go from 100 to 800 ISO that would be three stops of light MORE.  Adjusting ISO makes the sensor more sensitive to light, which enables us to shoot in dark locations and keep a fast shutter speed. The trade off with ISO is the higher the ISO that is used, the more noise you will introduce into the image. Keep in mind that noise on screen looks worst than what it will in print. BUT a noisy image is preferred over a missed, under exposed or blurred image.
Shutter speed is the next component of the exposure triangle. And just like aperture and ISO it is measured in stops of light. Most Cameras can go as long as a 30 second exposure all the way to 1/8000 sec exposure. If you are hand holding your camera, most likely you wont ever use a shutter speed slower than 1/30 and if you use flash you probably are limited to a synch speed of around 1/250. Using those assumptions the shutter speeds at one stop of light intervals would be 1/30, 1/60, 1/125 and 1/250. If we were to keep going it would be 1/500, 1/1000, 1/2000, 1/4000 and 1/8000.  Each number represents one stop of light, so if we went from 1/250 to 1/2000 that would be three stops of light LESS. The things to think about with shutter speed is the slower your shutter the more you introduce motion blur from the subject (and possibly motion blur from you holding the camera) and the more light you let in. The faster the shutter speed is the less light you allow in and you will "freeze" the subject. 
As a general rule your shutter speed should be equal to your focal length. So if you are shooting with a 85mm lens you would want to be shooting at 1/100, if you are using a 35mm lens you "should" be able to use a shutter speed of around 1/30. This is going to be very dependent on how steady you are.
In my next post we will discuss how to use the exposure triangle to be creative with your work.

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