Frontwithlens1Inside EOS

Have you ever wondered what goes on inside your EOS camera? You press the shutter button and, hey presto, the image appears on the LCD monitor.

At EOS magazine we believe that knowing how your camera works helps you improve your photography – by eliminating user error. Here's a short guide to some of the parts that make the magic happen.

The digital sensor inside your EOS camera is a marvel of miniaturisation. The 22.2 x 14.8mm sensor of the EOS 1000D (right), for example, contains millions of tiny photodiodes – 10.1 million, to be precise – lined up in 2598 rows with 3888 photodiodes in each row. The photodiodes are often called pixels – an abbreviation of ‘picture elements’.

The sensor unit is positioned behind low-pass filters. These are necessary because they help to prevent moiré patterns – the wavy lines that can appear across an image when newsprint or woven cloth is being photographed. The patterns appear when the distance between lines in the subject match the pixel pitch of the sensor. The low-pass filter reduces the risk of moiré patterns by spreading the light before it reaches the sensor. This creates the soft appearance of a digital image. Separate low-pass filters are needed for spreading the light horizontally and vertically. An infrared filter suppresses red fringing.

Nowadays sensors in EOS cameras are self-cleaning. A piezoelectric element creates a high-frequency vibration when the camera is switched on or off. This shakes off any dust which might have settled on the front filter of the unit. The dust is caught by a sticky band around the edges of the sensor.

DIGIC processor
The odd thing about digital sensors is that they are not digital. Each pixel is a photosensitive cell which responds to light by generating a small electrical current – the brighter the light, the stronger the current. This is an analogue process. The camera contains an image processor (above) made up of millions of tiny transistors etched into silicon board. These convert the analogue signals into digital data. Canon’s name for this processor is DIGIC. The processor has evolved over the years and the latest EOS models launched since 2017 use DIGIC 7.

AF unit
The autofocus sensors are in the base of the camera. The main mirror, which reflects light up to the focusing screen, is actually translucent. Some of the light passes through to a secondary mirror where it is reflected down to the autofocus (AF) unit. This unit contains prisms which split the light and direct it to different parts of an AF sensor. The two images are analysed and the phase difference tells the camera how much the image is out-of-focus and the direction the lens needs to move to make the image sharp. If Live View is used, both mirrors swing up to lie flat under the focusing screen. This allows uninterrupted light to reach the sensor, but means that no light reaches the AF unit. Focus is achieved by contrast detection of the image on the sensor. This is usually slower than phase detection.

Focal plane shutter
EOS cameras feature a focal plane shutter. The sensor is set at the focal plane, marked on the exterior of some camera bodies with the symbol
O . This is where rays of light from the lens come to a focus. The focal plane shutter is positioned immediately in front of the sensor. The shutter has two opaque metal blinds set across the width of an opening. Before an exposure, the first blind covers the opening, stopping any light from reaching the sensor. The second blind is above the opening. (Each blind is actually a series of metal blades which can overlap to occupy less space than the extended blind.) The two blinds are controlled by a unit which is part-mechanical, part-electronic. When the shutter button is pressed the first curtain opens, exposing the sensor. The second curtain follows to terminate the exposure. The shutter speed is determined by the time between the movement of the first and second curtains. The shutter-cocking system then resets the curtains to their original positions.

Mirror system
Single-lens reflex cameras have a mirror which reflects rays of light from the lens up to the focusing screen. This mirror is hinged so that it can swing out of the way during an exposure. The mirror drive mechanism (right) is quite complex. The mirror swings up to cover the focusing screen, which gives the temporary viewfinder blackout. The mirror must not be allowed to bounce at the end of its movement as this can cause vibrations in the camera during the exposure.

The above parts are small but significant parts of your EOS camera, but by no means all that it takes to make up your EOS. Think too of the eight-sided pentaprism (or pentamirror in entry level cameras), the metering sensor which sits between the pentaprism and the eyepiece lens, then the focusing screen which sits beneath the pentaprism. So too the exploded diagrams shown above illustrate just how many parts there are to your camera.

Take a thought then the next time you press the shutter button of the marvel of innvoation and technology that's contained in that little black box you're holding.

Learn how to get the most out of your Canon EOS camera with EOS magazine. We'll show you how your camera works, how to utilise the powerful feature set and how to get your images right in-camera.

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cmos02eurABOVE Digital sensor

BELOW Sensor unit, plus exploded view



DIGIC processor unit


ABOVE LEFT AF unit, with main mirror (top left) and secondary mirror (top right), and AF unit underneath.

ABOVE RIGHT Focal plane shutter unit

BELOW Mirror drive mechanism



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About this article

This article excerpt has been taken from the January-March 2011 issue of EOS magazine called 'Inside EOS'.