Digital SLR also known as single lens reflex use a mirror to control where the light coming in through the lens goes. The light can path through either to the viewfinder or to the camera sensor. Pocket cameras always have the light fall on the camera sensor. Here are some of what this difference mean.

The first implication of using a single mirror like in the Canon Digital SLR to divert the light is that Digital SLR cameras can not provide live view of the scene on their LCD screen. The LCD screen is thus used for menu options and for post viewing of photos that were already taken. The only way for photographers to compose a photo is by looking through the optical viewfinder. With pocket cameras the camera CCD sensor is always exposed to the lens and thus the LCD screen provides live view and can be used to compose photos. On the other hand with pocket cameras composing photos using the viewfinder is less accurate as the viewfinder uses a separate lens and the photo seen through it is not exactly the same one that will be taken.

There are advantages to using an optical viewfinder in SLR cameras like the Canon Digital SLR. The picture you see though the viewfinder is exactly the one to be taken. There is no time delay between the scene and what you see as with LCD view that is not real time but just close to it. The quality of the photo seen through the optical viewfinder is exactly the quality of the photo to be taken. LCD screens can distort colors and are limited in resolution in performance. For example it is nearly impossible to use the LCD screen to manually set the focus while using the viewfinder a photographer can manually set and find tune the focis with ease.

The LCD live view does have some advantages. The biggest one is the ability to compose photos in situations when the optical viewfinder can not be used. In order to use an optical viewfinder your eye needs to literally touch the viewfinder. This limits the options for where the camera can be positioned and the photo shooting angles. For example if you want to take an overhead photo of a crowd by lifting the camera as high as you can then using the optical viewfinder is not an option. On the other hand with the LCD live view you can lift the camera and still compose a good photo.

LCD screens are also limited in use in very bright scenes for example during a sunny day. Another disadvantage of the LCD live view is that in order to provide it the camera needs to keep the CCD sensor operating all the time. The result of that is more noise as the CCD chip gets warmer with time.

Some new Digital SLR cameras provide the good of both worlds. By splitting the light coming from the lens to both the optical viewfinder and the CCD sensor they allow using either the optical viewfinder or the LCD live view to compose photos.