What is the image sensor?

Put simply, the image sensor = film!

In the old days of film cameras, you pointed the camera at your subject, the light from the subject entered the camera via the lens and hit the film at the back of the camera. Well essentially, the image sensor takes the place of film.  Thus, the image sensor is that part of the digital SLR camera that captures the image before sending it to the storage device.

So why do you need to know any more about image sensors? The reasons are both in terms of how they relate to that so often used term, “megapixels” and secondly, how different sizes of image sensors effect your choice of lens. This second issue is referred to as the “crop factor”.  But let me start by talking about megapixels.

The Image Sensor & Megapixels

As mentioned, the image sensor sits at the back of the camera and essentially captures the light hitting it and in turn capturing the image before sending it to the camera’s storage device.  How does it do this?  The image sensor is filled with millions of “photosites”, more commonly called “pixels”.  Each pixel can be thought of as a “bucket” that fills up with light.  If these buckets or pixels are not permitted to fill up with enough light (if, for instance the shutter doesn’t stay open long enough), then the image will be under-exposed.  If these buckets or pixels are permitted to fill up with too much light (if, for instance, the shutter stays open too long), then the image will be over-exposed.

These pixels are set out in an array on the image sensor. The size of the image sensor and therefore the array will be different for each camera.  For example, they may be set out in an array of 5,616 columns and 3,744 rows, giving the image sensor a total of 21,026,304 pixles (i.e. 5,616 x 3,744) or approximately 21 megapixels. Therefore, this is where DSLR cameras get their most important classification in terms of their “megapixels” – from the number of pixels on the image sensor.

How important are megapixels? Read “The Truth About Megapixels” to find out.

The Image Sensor and How It Effects What You See

So now, what is this “crop factor” issue all about?   This is an issue that will affect your lens choices to a certain degree.  Before discussing this, it is important to get a handle on “focal length”.  Every lens has a focal length. The focal length is generally the distance, measured in millimeters (mm), between the lens and the image sensor. More importantly, the longer the focal length the closer the subject will appear when looking through the camera’s viewfinder. Conversely, the shorter the focal length the further away the subject will appear. Another way to look at it is that while a longer focal length tends to bring the subject closer to the camera, a shorter focal length gives a wider view of the entire scene.  Thus a lens with a focal length of 25mm will have a much wider angle view than will a lens with a 300mm focal length when viewing a scene from the same spot.

How does all this relate to crop factor? The image sensors found in today’s digital SLR cameras are smaller that the 35mm film frame found in traditional film cameras. For example, the Canon EOS 50D has a sensor that measures 22.3mm x 14.9mm, while a traditional film camera had a film frame that measured 35mm by 24mm.  Thus in this case the film frame is approximately 1.6 times larger than the digital image sensor. So? Well, today’s lenses continue to be made as if they are for the 35mm film format (with exceptions). Think of the lenses as taking the light that enters it and projecting it towards the back of the camera as a beam of light that measures as a 35mm by 24mm rectangle. BUT, instead of projecting it onto a 35mm by 24mm film frame as it had done in the past, it is now projecting it onto an image sensor that is 22.3mm x 14.9 mm (based on the above example).  Thus the image sensor is “cropping” the beam of light being projected onto it. This has the same effect as if the lens was zooming in on the subject.

Assuming this photo was taken with a film camera and the same lens was transferred to a digital SLR camera then . . .

Therefore, because the image sensor is cropping the image and effectively zooming in on the subject, it in turn is changing the focal length of the lens. For example, in the case of the Canon EOS 50D, because the image sensor is smaller than the traditional film frame by a factor of 1.6, then this

. . . the image is dramatically cropped, in this case by a 1.6 crop factor due to the increased focal length.

number is used as the “multiplication factor” for all lenses which are then attached to this camera. So, if you attach a standard 50mm lens, then the actual focal length is no longer 50mm but it becomes an 80mm lens (50mm x 1.6), effectively zooming in more on the subject then the 50mm lens would have if it were attached to a film camera.

To get the true focal length of any lens attached to this camera, you would then have to multiply the focal length by 1.6 in this case, all because of the smaller image sensor. The important point here is that given that each digital SLR manufacturer has a different sized image sensor, each will therefore have a different crop or multiplication factor for their camera.

There is one exception to the crop factor: “full-frame” digital SLR cameras.  As the term implies, full-frame cameras are digital SLRs that have an image sensor that is 35mm by 24mm and therefore does not crop the image and therefore no multiplication factor needs to be applied to any lens being attached to it. Therefore, a 50mm lens attached to a full-frame digital SLR remains a 50mm lens.  Needless to say, these are relatively expensive cameras!

You will find more about Focal Length and the crop factor in the Choosing a DSLR Lens section.

The next article in this series is “Understanding JPEG & RAW File Formats”

If are looking to start exploring digital SLR cameras, jump to the “Choosing a DSLR Camera” section.

Otherwise go back to Getting Started in Digital SLR Photography.

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