SLR cameras allow the user to work with the main optical system. This means that the image viewed in through the viewfinder is identical to the image captured on the film. Therefore, SLR users do not have to deal with parallax issues (especially advantageous in close range photography). SLR cameras achieve using a single optical path in one of two ways:

1. use of a reflex mirror (more common)
2. use of a fixed beamsplitting pellicle (special purpose high speed cameras)

typical internal of a SLR camera

shutter mechanisms