For starters, the number of megapixels has very little to do with a camera's sensor size. It only refers to the number of pixels on the sensor, or the camera's resolution. It determines how large you can print your image, but not how sensitive the sensor is. And it certainly is no indication of image quality. (If you're shopping for the best digital camera for your needs, find out what features you need to really focus on by reading our free "How to choose: Digital cameras" advice and watching our Digital Camera Buying Guide video.)

In fact, the Sony sensor is about 5 to 6 times smaller than Nikon's. So, each pixel on the Nikon D700 (Click on the image at right for a closer look) is larger than pixels in the Sony subcompact. And it's what allows each pixel to be much more sensitive to light, in turn, making the whole sensor more sensitive.

This is especially noticeable in very brightly lighted situations as well as low light. So, for example, when shooting a portrait on a sunny day, the Sony may not be able to capture the details in someone's hair if the sun is shining too brightly. With the D700, you're much more likely to see all the details in this highlighted area.

Cameras with small sensors also fall short in low-light situations. Details in a shadow area will often be swallowed up by dark tones. They'll also introduce something we refer to as visual noise, or unwanted "grain" or spots of color, that will further degrade image quality.

For example, if you use the Sony to take a group portrait at an evening event, in low light, faces may be mottled with specks of reddish or purplish spots. With the D700, that's far less likely to happen. In fact, with the D700, you can crank up the ISO setting (up to ISO 6400) to capture the shot, letting you increase the SLR's sensitivity to capture an image in low light.

If you crank up the ISO to its maximum setting of ISO 3200 on the Sony DSC-W300, you'll start to have problems. The image would most likely be tainted with specks of false color. It's why we suggest that you raise it no higher than 200 ISO, according to the "Max. ISO with best quality" column in our Ratings of digital cameras. This score recommends the highest ISO setting at which a camera can produce a good image. Better models should excel in low light.

For the D700, you'll likely be able set the ISO much higher-probably 800 or 1600 ISO, if not higher. (At the moment, we haven't included the D700 or any other full-frame SLRs models in our Ratings of digital SLRs, which is why we don't have an exact number for our "Max. ISO with best quality" test.)

SLRs with full-frame sensors have other benefits as well. Since they are the same size as a 35mm frame of film, they generally eliminate what we refer to as a lens magnification factor. A basic SLR like the Nikon D60 has a magnification factor of 1.5x, which means if you attach a 50mm lens, it will have a focal length of 75mm. This factor is important because it boosts a zoom lens's telephoto capabilities, while reducing its wide-angle. On full-frame SLRs, a 50mm lens remains a 50mm lens. For more on lens magnification factors, check out the "Sensor Size and Lens Magnification Factor" section of my first impressions of the Canon EOS 40D Digital SLR.

Shortly, our labs will be checking out the performance of this full-frame sensor technology and comparing it with conventional SLRs.

For more on the SLRs mentioned in this post, check out the Nikon D700, Canon EOS 5D Mark II or Sony Alpha A900 (Click on the last image at right for a closer look).

—Terry Sullivan