Camera basics

Both cameras could earn our Editor's Choice award – and we only give out about 15-20 of these each year across all hardware/software products. That means that both cameras represent an exceptional value and reflect a product that is notably better than the competition. The fact that the digital SLR is so much more expensive doesn't mean it's less of a value. You're paying for a different set of features. Sensor size is one such feature.

In the original "A Tale of Two Cameras," I explained that the critical first decision to make when buying a digital camera is not your budget, or the megapixel rating, but deciding whether or not you want a camera with a large imaging sensor (i.e. a digital SLR) or a standard point-and-shoot digital camera with a small imaging sensor. Sensor size plays a big role in the price difference between these two camera types.

Both of the cameras we will be using are 8 megapixel cameras, but the EOS-20D has a sensor that has about 9 times as much surface area. To use the diamond ring analogy, both cameras are like rings with 8 diamond stones. The more expensive EOS-20D, however, has diamonds that are around 9 times as large, and are several grades better in clarity and color. Like diamonds, price does to increase proportionally to size. A 4.5 carat diamond is not 9x as expensive as a half-carat one. For photography, a larger sensor translates into improved dynamic range and tone, and reduction of noise. Often overlooked are the additional consequences on lens design and depth-of-field.

Dynamic Range, Tone, and Noise

Larger sensors sizes have better dynamic range and tone. Digital SLRs will be able to capture more nuanced shadows, texture, and the fine gradations of tone and color better than a regular digital cameras can. In practice, a large sensor digital camera should be able to better capture the soft details hidden in shadows or the texture that makes clouds look fluffy and have mass rather than white blobs.

Noise is the digital equivalent of film grain. For low-light photography, you will need to increase the ISO sensitivity (like using higher speed film) in order to keep your shutter speed fast enough to prevent blurred pictures. As you increase the sensitivity of the sensor, you also increase the amount of grain. This is important for low-light and indoor shots. Small sensors usually only reach ISO 200 before noise becomes a problem. Larger sensors, particularly those in Canon digital SLRs have substantially less noise, allowing you to reach ISO 3200 with sufficient quality for making standard 8x10 prints. The difference between ISO 200 and ISO 3200 is "4 f-stops." This means that an image requiring a shutter speed of 1 second at IS0 200 only needs a shutter speed of 1/32 seconds at ISO 3200.

Sensor Size and Lens Design

The lens of a digital camera is just as important as the sensor when it comes to final image quality. A smaller sensor allows you to use a smaller lens. This affects you in several ways. First of all, it's cheaper to produce a smaller lens. Not only is there less raw material to deal with, it's also technically easier to design the optics. Therefore, digital SLRs are substantially more expensive because not only do you have to pay for a larger sensor, but you need to pay more for a lens with the same level of quality. This also means that lenses for digital SLRs are considerably larger and heavier, which can be problematic when traveling. More importantly, differences in lens design and sensor size have consequences on the type of pictures you can take due to differences in depth of field.

Depth-of-field refers to the difference between the objects that are in focus and those that are out-of-focus. If you'd like the foreground and background to be "perfectly sharp," you are looking to have a very high depth of field. On the other hand, if you'd like the background to blurry for a more artistic look (think about a movie where the focus changes from a character in the foreground to the background), you are looking for a shallow depth field. Larger sensors sizes allow you achieve a more shallow depth of field than small sensor sizes. Therefore, it's often better to have a small-sensor camera when your desire is to have both the foreground and background in focus. The larger sensor lets you be more creative with your photographs, but you'll more frequently have the problem with out of focus images.

The differences in depth of field have to do with the consequences of sensor size and not simply aperture.

That statement can be surprising for amateur photographers, particularly those who have not shot both 35mm and medium/large format film. As a photographer, depth of field is controlled by changing the aperture of the lens. By closing down the iris that allows light into the camera, you increase your depth of field; it's similar to a person squinting. Since photographers only deal with aperture, a common mistake is to assume that you can achieve a shallow depth of field with a small-sensor digital camera as long as you have manual aperture control. This isn't true because depth of field is also affected by focal length and focus distance as well as the "circle of confusion" (the definition of what's blurry and what's sharp). All those variables work out in such a way that smaller sensors have a net increase in depth of field. The easiest way to think about it is that depth-of-field is based upon the true focal length of a lens as opposed to the "effective" 35mm focal length. Don't worry if this doesn't make sense – the real-world examples will clear things up for you.