Most of the modern 35 mm (small format) and medium format cameras are all of this type. A mirror in the camera body is pivoted at an angle of 45 degrees just in front of the focal plain shutter. The light rays passing through them is reflected above the focusing screen. The focused image on this screen is seen again through a prism placed at the top.
The prism is called ‘penta prism’ since it has five sides and it turns the light rays to 90 degrees with the view finder without reversing the image. The light rays take a total internal reflection inside the prism.
The stop value (f) or the aperture is set before the exposure is made. When the shutter is released the deviating mirror is automatically swung up above and the focal plane shutter is mounted directly in front of the film, momentarily exposes the film.
The advantage of an SLR camera is that the photographer is able to see the image exactly what the lens is viewing, (the field of coverage).
But the disadvantage is that (it is not once you get used to it) there is a black out in the view finder at the exact moment of exposure.
The set aperture closes to the set value only just before the exposure is made. Otherwise it is fully opened to allow maximum light through the lens for convenient sharp focusing.
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