The Rebel T1i Sensor

In Chapter 2 we took a quick look at the anatomy of your Rebel T1i, and you
learned something about the architecture of an SLR camera. In that discussion
I mentioned the image sensor in your camera. As you already know, in a digital
camera, the image sensor takes the place of a piece of film and is the mechanism
by which the camera can “see” an image.
The image sensor is a chip that is mounted parallel to the back of the camera so
that the light focused by the lens hits it head-on. In the canon eos 500d, the image sensor
is the same size as a piece of APS film.

The Rebel T1i image sensor is based on a technology called complementary metal-
oxide semiconductor, or CMOS. All Canon SLRs use CMOS image sensors, while
many other SLRs and most point-and-shoots use Charge Coupled Device, or
CCD, image sensors. You will often see people praise the merits of one sensor over
another, and although these can be interesting engineering discussions, as a photographer
the only thing you need to worry about is final image quality.

If yourimage sensor delivers excellent images in a range of lighting situations—which
the Rebel T1i does—then the relative merits of sensor technology are fairly moot.
Whether CMOS or CCD, most image sensors employ the same basic design. A
rectangular area on the sensor is sensitive to light. This area is divided into a grid,
with one cell for each pixel that the sensor can capture. So, the canon eos 500d's 15-
megapixel sensor is divided into a grid with 15 million cells (for those of you who
are sticklers for accuracy, it’s actually closer to 15.1 million cells), and on each cell
is a tiny electronic circuit called a photo diode.
When you turn your camera on, the image sensor is given a charge, so that each
photo diode has a certain voltage stored in it. When you press the shutter button,
the sensor is exposed to light. When a photo diode is struck by light, it releases
some of its voltage, and the more light that it receives, the more voltage it releases.
After the shutter closes, the camera measures the voltage at each cell, to
determine how much signal struck each cell.

Since the camera knows how bright each pixel should be, and since it has so many
pixels on its sensor, the image sensor can build up a very detailed image. Obviously,
all of this happens in a fraction of a second, and in practice you don’t
have to worry about what the image sensor is doing at the individual pixel level.
However, you do have to think about the amount of light that you expose the
sensor to, because if there’s too much or too little, your shot will be ruined.
What’s more, you can change the method by which the sensor is exposed and
achieve very different results.


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