• lens (1)
• movable mirror (2)
• focusing screen (5)
• viewfinder (6-8)
• digital sensor (CCD or CMOS) (4)
• digital memory
• microprocessor
• high-speed digital port (Firewire or USB)
• battery

When the mirror is down, the image is projected onto the focusing screen.

To explain an SLR, I need to show you how the image travels through the camera to your eye or to the sensor.

The lens (1) would focus the image on the sensor (4). but a movable mirror (2) reflects it up to the focusing screen (5), usually a thin piece of frosted glass. It forms an image that can be seen from above. Let me tell you why that is done.

When I was a little kid, my mother had a camera without the optics above the focusing screen. She had to look down at the focusing screen and twist a bezel until the image was clear. The need to look down at an inverted mirror image led to the development of the SLR.

Mom’s camera was expensive and awkward. Because it had two lenses, one for the viewfinder, the other for the film,  changing the camera lens required changing the viewfinder lens as well.  Putting a movable mirror in front of the film eliminated one lens and saved time … an important step in making cameras cheaper, simpler and better. But still not as easy to use as the SLR.

Looking down at a mirror image is awkward; better to be looking straight at the scene when you raise your camera. Asahi Optical developed a design that lets you do just that. They put optics above the focusing screen so you can look at the scene, raise the camera and see the same scene through the viewfinder without needing to take your eye off your object or move your head.

The reflected picture on the focusing screen is inverted and reversed. That’s a problem. In an SLR camera, the optical distance from the eyepiece (8) to the focusing screen is too close for an adult with normal vision to see clearly. That’s not good either.

Asahi solved both problems at once. First they moved the image to infinity with a condenser lens (6).  They solved the second problem with a very impressive innovation, the pentaprism (7). It flipped and reversed the image perfectly.

The marketing department needed to generate a catchy name for this new technology. Recognizing that reflects and reflex sound almost the same and having only one lens, they called the design ‘single lens reflex.’ They formed an acronym from pentaprism and reflex: Pentax.

That camera was the ‘Asahi Pentax.’ In its time, it was considered the most advanced 35mm camera in the world. So far no one has improved on the basic design concept. No wonder it made the leap into digital cameras.

The important point here is the optical distance – the distance that the light travels – is exactly the same distance that the light will travel to reach the sensor as it must travel to reach the focusing screen bouncing off the mirror. The result is that when the image is in focus on the screen, it will be in focus on the sensor.

We have already seen that this design lends itself to interchangeable lenses. Even though a SLR camera does not need the interchangeable lens feature, in my opinion, it would be crazy to go to all the expense of putting the mirror, pentaprism, etc. in the design and not allow the different lenses to be attached to the body.

Since the camera uses digital electronics rather than film and the machinery needed to handle it, SLRs are more reliable and mechanically much simpler. Instead of a shutter, the sensor captures light for a predetermined period, converts it digital data and stores it in its own computer memory.

When the memory gets full, the photographer downloads the data to a computer through some high-speed data connection.

So the short answer to the question of “how does a digital SLR camera work?” is “Very, very well, thank you.”