An SLR digital camera is one that uses a single-lens reflex and does not use film but stores the pictures as digital or image files. In order for the single lens reflex to work , mirrors are used within the camera to direct light from the lens to the back of the camera and the viewfinder. Digital cameras offer the photographer a wider choice of options than did traditional film cameras. Many photographers that began using film cameras still use film cameras. Many other photographers made the switch to SL digital cameras and the ease that it afforded them in taking photographs.The first digital camera was made by Kodak in 1975 , with the first SLR model created by Sony in 1981. The basics include a mirror that reflects light in a 90 degree angle from the lens to the view finder. This light is then reflected three times so the person can see the image properly. When the picture button is pressed the mirror swings in an upward motion which causes the aperture to become smaller. The shutter opens and then light touches the image sensor. A 2nd mirror will swing over the image sensor which will end the light exposure and the mirror swings back into place. In this period there is no image through the viewfinder , known as viewfinder blackout. This entire motion happens very quickly with SLR cameras , as this motion can occur as much as 10 times in a second.The reason digital SLR cameras are preferred by professionals are that the photographer has a very accurate preview of the actual picture that is being taken. The depth of field refers to the distance between the farthest and nearest object in the picture. These cameras also allow for accurate viewing of depth of field and the lenses can be interchanged. The mirrors used in the camera can be either pentaprism or penta-mirror. Entry level versions typically use a penta-mirror , which is a prism that has 5 sides. Instead of being made of glass the penta-mirror is made plastic which is less expensive and much lighter than pentaprism. The pentaprism is in the same shape as the penta-mirror but it is made of glass. When light passes into the prism it bounces off the back wall , changing light direction by 90 degrees.The ability to change lens will depend on the camera maker and the type of lens. The lenses must have the same shape so that it fits into the lens mount on the camera. Many manufacturers use lenses that can fit in many cameras while some manufactures make lenses that are proprietary in which case will only fit in that manufactures camera. The proprietary nature of some SLR digital camera manufacturers would be one of the main reasons that once a photographer makes a choice in a particular camera he stays with that brand as their entire lens will only work with that brand.Keeping with the same brand of camera helped many photographers make the switch from film cameras to a slr digital camera. In most cases the entire photographer needed to do was change the “body” of the camera as the lens would be interchangeable between the film camera and the digital camera. This would only work on the newer film cameras that had more automatic features than the older film cameras that were manual in operation.