This is a very in-depth article about the optical and optoelectronical functional principles of Digital Single Lens Reflex (DSLR) cameras. Understanding each component inside a DSLR and their underlying technology may help you to achieve better photographic results with a digital camera.
As per Dec. 2022, the chapters on viewfinder optics and phase detection autofocus include some of the most detailed illustrations currently available on the internet. The creation of these diagrams and illustrations involved several months of research including the review of scientific publications, patent registrations, the translation of japanese websites on phase detection autofocus, contacting professionals around the globe, and the disassembly of various DSLR cameras.
The illustration below shows a cross-section through a Canon EOS 1D Mark IV, a very high-end professional DSLR camera that was introduced in 2009. At that time, the recommended retail price of the camera's body was USD 4,999.
Probably the most unique component of a DSLR camera is the reflex mirror. The main purpose of the reflex mirror is to reflect the incident light towards the viewfinder optics in the upper part of the camera. In there, light hits the camera's focusing screen (ground glass) where it forms an image. Since the focusing screen is translucent, the image formed on the screen emits light from its top surface towards the pentaprism. Being deflected and corrected in rotation by the pentaprism (or pentamirror), light will be redirected to the eyepiece so that the photographer gets a picture of the scene from the perspective of the camera lens itself. This is a very helpful feature because actually getting the perspective of the main lens avoids the picture to change from parallax error and also takes filters into account, if applied to the lens. You can find a very detailed description about this complex unit in the viewfinder chapter.
By way of qualification, it should be noted that mirrorless cameras almost always provide a live-view-function that does not suffer from parallax either. However, mirrorless cameras are not covered by this article.
One other striking feature of DSLR cameras is their extremely fast and reliable autofocus. This is achieved by a dedicated autofocus sensor unit, a complex arrangement of optical elements designed to tell the camera whether an image is in focus or not. This unit is typically located in the lower part of a DSLR camera, underneath the mirror. The primary mirror is not fully reflective but has a semi-transparent area. Therefore, light can reach a secondary mirror, placed behind the primary reflex mirror. This secondary mirror reflects light beams to the autofocus sensor unit. The autofocus sensor unit will be described in the autofocus chapter.
In addition, you can find various other illustrations and explanations about DSLR electronics, such as their image sensor and how it can convert light into signals, its readout circuitry, and a DSLR's imaging processor.
The image above summarizes the different paths of light while the mirror is in it’s lower position.
Once the shutter-release button is pressed, both the main reflex mirror and the secondary mirror flip up, exposing the rear area to light. The camera sensor is covered by the shutter that will quickly open after the mirror has reached the upper position. With the shutter opened, the sensor is finally exposed to the incident light, recording the scene. Some interesting facts on sensor physics and processing electronics can be found in the sensor chapter. After the exposure, the shutter will close automatically, followed by the primary and secondary mirror folding down again. The image above shows the exposure of the sensor to light while the mirror is flipped up.