The key feature of Single Lens Reflex (SLR), and Digital SLR, cameras is their optical viewfinder showing the view through the camera's lens.

The large bright viewfinder makes DSLR cameras more practical when using wide angle or telephoto lenses. The photographer can see how the lens represents the scene, which is particularly important for long focal length telephoto lenses where there is a high degree of image magnification.

This usability with a wide range of focal lengths has resulted in a flexible and adaptable camera system. Pro photographers love the fact that, whatever the photographic task, there is a lens that makes the DSLR a suitable tool for the job.

Three main parts of the camera allow photographers to look through the lens using the viewfinder. A series of mirrors and reflecting devices divert the light from the lens to the viewfinder, presenting it the right way up and around in the process.

Movable Mirror.

The key breakthrough in SLR design was the development of a practical automatic mechanical system of moving a mirror out of its position in the light path between the camera’s lens and shutter. When the photographer presses the button to take the picture, this first starts the mechanical system moving the mirror out of the way so the light can pass unimpeded through the shutter when it opens and onto the image recording device, film or digital sensor. The sound of the moving mirror arriving in its up position gives SLRs their characteristic sound, not the shutter movement.

The mirror in its up position blocks any light from coming back down through the viewfinder and degrading the image. The mirror movement can cause vibration problems so many DSLRs have the facility to lock the mirror into the up position before taking the picture.

Viewing Screen

This sits underneath the pentaprism and the light from the mirror is projected onto this plastic screen. This screen has all the focusing and other aids the photographer sees in the viewfinder. This system gives a good view of the image for accurate manual focusing. This is still important in the world of autofocus, as in some low light conditions autofocus may not be reliable.

Pentaprism

This is the device in the top part of the camera and it reflects the light internally from the mirror, and then presents it to the viewfinder the right way up and the right way round. This compensates for the reversal effect of the mirror and the inverted image produced by the camera’s lens. The better DSLR’s use a glass prism; cheaper models use a mirror box instead of a prism.

TTL metering.

This refers to the camera’s internal light meter measuring the light coming Through The Lens, hence the acronym TTL. There is no need for exposure compensation when a polarizing filter reduces the amount of light entering the camera’s lens. The inbuilt meter measures the light after any filters, so it is automatically compensated for.

Stop Down Metering:

This is old technique where the lens aperture had to be set to its final value for accurate TTL meter readings. When this resulted in a small aperture, it became hard to focus and compose, as the image in the viewfinder was dim. Now it is almost a thing of history as most lens and camera combinations leave the aperture at maximum size, only reducing it to the required size during the taking of the photograph. The camera’s exposure calculating system takes into account any differences between the desired lens aperture setting and the actual amount of light coming through the lens.