Digital SLR cameras explained: 10 things every new photographer must know

Digital SLR cameras are the preferred tool of most professional photographers and they’ve been the first choice of camera for enthusiast photographers for decades, but what is a digital SLR, how do they work and why are they so popular? Our head of testing, Angela Nicholson explains all you need to know.

Digital SLR cameras explained: 1. Why Single Lens Reflex?

The name single lens reflex camera seems rather odd today, but there was a time when twin lens reflex (TLR) cameras were very popular – there are still one or two models on sale today.
The two lenses of an TLR have the same focal length and their focusing mechanisms are linked, but they are used for two different tasks.
The ‘viewing lens’ is used for focusing while the photographer looks in the waist-level viewfinder, while the ‘taking lens’ sits in front of the film, ready for exposure in a separate chamber.
The word ‘reflex’ in the name stems from the fact that TLR and SLR cameras have a reflex mirror, essentially a mirror at 45 degrees, that reflects light from the lens into the viewfinder.
In a TLR the mirror is fixed and the scene is visible in the viewfinder throughout the exposure.
In a digital SLR camera, however, the mirror flips up during exposure to allow the light to reach the film or sensor, this blanks out the viewfinder for the duration of the exposure.

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Digital SLR cameras explained: 2. The viewfinder is optical

Digital SLR cameras have an optical viewfinder that receives light from the same lens as is used to capture the image.
When the mirror is down, the light exiting the lens is reflected up into a pentaprism (or pentamirror) which bounces the light around to produce an image on the viewfinder screen that is the right way round.
This enables SLRs to be smaller than TLRs and solved the problem of parallax error encountered with rangefinder cameras – they don’t see the scene through the lens.
Modern digital SLR viewfinders are generally bright and pleasant to use, but unlike the electronic viewfinders found in many compact system cameras, they can’t show the impact of camera settings.
The reflex mirror lifts just before an exposure to allow light to reach the sensor, this causes the viewfinder to go black during exposure.

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Digital SLR cameras explained: 3. Exposure is controlled by shutter speed, aperture and sensitivity

There are three means of controlling exposure with a digital SLR camera: shutter speed (exposure time) aperture and sensitivity, often called The Exposure Triangle.
While shutter speed and aperture size control the amount of light that reaches the sensor, the camera actually has a base sensitivity that remains constant.
Other sensitivity settings are created by applying gain (amplification) to the image signal (more on this later) to replicate the effect of a more sensitive medium.

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Digital SLR cameras explained: 4. Most offer a collection of exposure modes

Most modern SLRs offer a wide range of exposure modes from fully manual to fully automatic with semi automatic options such as aperture priority and shutter priority mode in between.
There is also usually a collection of scene modes in which the camera selects exposure and processing settings that suit that particular type of scene.
These scene modes may be selected by the photographer, but many cameras now offer a mode in which the camera detects the type of scene before it and then sets appropriate exposure and processing settings.
In aperture priority mode the photographer sets the aperture to control depth of field and the camera sets a shutter speed that will produce a good exposure.

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Conversely, in shutter priority mode the photographer sets the shutter speed to freeze or blur movement as they wish, and the camera sets an appropriate aperture. In manual mode the photographer has complete control over shutter speed and aperture.
Many digital SLR cameras now have the option to set sensitivity automatically. If this mode is selected when shooting in manual exposure mode, the photographer still sets the shutter speed and aperture, but the camera determines the overall exposure by varying sensitivity as it calculates is necessary from shot to shot.

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Digital SLR cameras explained: 5. The sensor produces an electrical signal

When the reflex mirror is lifted and the shutter opened, the sensor is exposed to light enabling an image to be recorded.
The sensor is covered with photo receptors, often referred to as pixels, each of which has a micro lens over it to direct the light downwards.

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In most cases the sensor has a Bayer pattern filter array over it, this enables the camera to interpret colours even though each receptor actually only detects luminance (brightness).
This red, green, green, blue (RGGB) filter has two green filters for every red or blue filter to reflect the dominance that green has in our vision.
Light falling on the sensor generates an electronic signal. The brighter the light, the stronger the signal. This electronic signal is then converted into a digital signal which can be processed into a digital image.
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