Have you ever wondered how is it that, even though the viewing lens of the camera is not in the same plane as your view finder, you are still able to focus and get the exact image on your film? For this you need to understand the working of your SLR camera. A single lens reflex camera gets its name from the single reflex lens is use: it has only a single lens to view, as well as capture, the frame.

1 - Front-mount lens (4-element Tessar design)
2 - Reflex mirror at 45-degree angle
3 - Focal plane shutter
4 - 35 mm film or sensor
5 - Focusing screen
6 - Condenser lens
7 - Optical glass pentaprism (or pentamirror)
8 - Eyepiece (can have diopter correction ability)

If you look through the cross section of the SLR camera, you will notice there is a lot more to a camera than a film and a lens. Light enters the camera through the mounting lens and the image is obtained on a reflex mirror that is inclined on an angle of 45 degrees. The image obtained on the mirror is laterally and vertically inverted. This image is projected on to a penta prism above it. The penta prism in the camera rectifies the lateral and vertical inversion through a series of refractions and reflections. This image is then visible through the viewfinder, exactly the way as it is in real life. After focusing and adjusting through the viewfinder, when you press the click button to capture the image, the reflex mirror goes up in the direction of the arrow. The focal plane shutter opens and the film is exposed to light, thus capturing the image. All SLR cameras work in this manner, except digital SLR cameras like the Olympus E-410 use sensors instead of film. You would have often observed that while you press the click button, the view from the view finder is blocked. This happens because of the upward movement of the reflex mirror to clear the path for light to enter the film plane.