EOSM3EFM22STMEVFDC1angledBSLMirror, mirror

Have you ever wondered what goes on inside your EOS mirrorless camera? Unlike single-lens reflex (SLR) cameras, there's no mirror, and this means that they operate in a different way.

Why is this important? Well, it affects size – no mirror box means smaller and lighter bodies. However, performance is also altered. Read on for more.

SLR vs. mirrorless

Until the EOS M arrived in 2012, every previous EOS model has been a single-lens reflex (SLR) camera (top right). Light from the lens hits a reflex mirror (1) and is reflected up to a focusing screen (2). From here it passes into a pentaprism (3) and out to the viewfinder eyepiece (4). The pentaprism ensures that the image in the viewfinder is the right way up and the right way round. At the same time, some of the light passes through a translucent area at the centre of the reflex mirror and hits a secondary mirror (5). This reflects the light down to an autofocus sensor in the base of the camera (6).

When you press the shutter release button to take a photograph (right, centre), the mirror flips up to let the light through to the sensor at the back of the camera (7). The SLR viewfinder is optical, using mirrors and lenses to create the image you see.

The EOS M-series models are compact system cameras (CSC). They have no reflex mirror, focusing screen, pentaprism or eyepiece (bottom right). The autofocus sensors are embedded in the digital sensor. Light from the lens goes straight to the sensor (1). This is a much simpler system than the single-lens reflex, giving advantages and disadvantages. The image you see on the LCD monitor or in an electronic viewfinder is digital, taken directly from the camera sensor.

DSLR advantages

• Optical viewfinder and eyepiece provide a clear image even in bright sunlight.
• Holding the camera to the face to look through the viewfinder reduces camera shake.
• Dedicated autofocus sensor offers fast, accurate focusing.
• Many EOS SLRs offer faster continuous shooting speeds than the EOS M-series models.
• Battery life tends to be better because there is less use of the power-hungry Live View screen.

DSLR disadvantages

• SLR cameras are larger and heavier than CSC cameras.
• Mirror movement creates a distinctive noise and has the potential to increase camera shake.

Mirrorless advantages

• Small camera body with image quality of an EOS digital single-lens reflex.
• Dedicated lenses can be lighter and smaller.
• The lack of a reflex mirror can provide quieter shooting, meaning you can be more discreet and there's less interference when shooting video.

Mirrorless disadvantages

• Hybrid focusing can be slower than dedicated AF sensor of an SLR, especially in low light.
• If no viewfinder, the LCD screen can be difficult to view in bright daylight and the camera must be held away from the face to view the Live View screen, increasing the risk of camera shake.
• EVF (electronic viewfinder) can be less comfortable viewing.
• EVF and Live View screen are both power hungry, so batteries tend to be consumed more quickly.


There is no clear winner in the DSLR v. mirrorless debate. There are advantages and disadvantages to each system. Both have their proponents, some of them vehement: “DSLR photography is dead” and “The future is mirrorless” are views frequently expressed. They are, in our opinion, premature. Canon has a major investment in professional and enthusiast DSLR cameras and lenses and it is not going to phase these out any time soon. Mirrorless cameras will improve and increase in popularity, but the two systems will run in tandem for the foreseeable future.

Download the full article from April-June 2017

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