What is depth of field preview?
When you look through the viewfinder of a Canon EOS camera – or just about any modern SLR – you re looking through the lens when the aperture is set to its widest setting. This means as much light as possible is coming through, which makes it easier to see the scene and focus and so on. However, depth of field is at its shallowest when the lens is wide open. So if you want to get a sense of the depth of field that will result from some other aperture setting you need to close the lens down physically to that setting. The depth of field preview button available on most Canon EOS cameras does just this. Unfortunately the differences in depth of field between various settings can be hard to make out in the viewfinder itself. So all the DOF preview button usually does is to make things look darker. And if you re shooting in low light conditions, making the viewfinder darker may mean you can hardly see anything at all. So DOF preview buttons, while they have their uses, are of somewhat limi
