Exploring different image sensor sizes

In the world of digital imaging, you constantly hear or read the term “full-frame sensor.” What exactly does this mean and what are the advantages and disadvantages?

A full-frame sensor is designed to capture the full size of the film frame. In the case of Canon, this is 36 X 24 mm, or the full size of the 35mm film format. The chief reason that all dSLR sensors are not full frame has to do with the cost of manufacturing the larger sensors combined with the physical size of the camera body and its ability to accommodate a chip that large. As chip sizes get larger, the yield gets drastically lower and, subsequently, the price increases. The semiconductor industry’s advances in affordability have been driven by the ability to make circuits smaller and smaller, but an imaging chip must remain large, and such large chips get cheaper much more slowly. Owning its own chip-making facility has made Canon a leader in full-frame sensor technology.

Consider several of the more popular sensor sizes and the cameras that use them:

  • APS-C sensor. This is named for the now-all-but-defunct film format introduced and popularized by Kodak. This is the sensor size used in the Canon EOS Digital Rebels, the EOS 30D, and the EOS 40D. This smaller size results in what is known as a crop factor. In the case of Canon, this is 1.6x, meaning a 100mm lens now has the effective focal length of 160mm, a 200mm lens has the effective focal length of 320mm, and so forth. As you have probably already surmised, this is an advantage for those wanting more of a telephoto effect but a big disadvantage to those needing a wide-angle lens.
  • APS-H sensor. This sensor is slightly larger than the APS-C sensor but is not quite full-frame. This sensor has a 1.3x crop factor so the telephoto effect is less pronounced. This sensor is used in the EOS-1D Mark III.
  • Full-frame sensor. As the name implies, this sensor is the full size of 35mm film and consequently there is no crop factor. This lack of a magnification factor is considered an advantage by many: Your lens “is what it is,” meaning a 100mm lens is a 100mm lens. This sensor is used in the EOS 5D and the EOS-1Ds Mark III.

Just how much difference does sensor size make? There’s a great deal of attention paid to pixel count in the world of digital cameras. Cameras are often categorized by the number of pixels they have in their image sensor. However, as mentioned earlier in this chapter, not all pixels are equal, and size does matter. From Canon you get a pretty good explanation of why. A large CMOS sensor offers better image quality than a smaller one because the larger sensor contains bigger-sized pixels. The relationship between image quality and pixel size can be readily understood if you imagine the pixel as a kind of bucket used to collect not water but light. This micron-sized bucket not only gathers light but also has a pho-todiode that produces a voltage when photons (light) strike it. A regular bucket with a larger opening can collect more water in a shorter time than a smaller one. This is similar to the larger CMOS sensor compared to the smaller one: The large one gathers more light in a shorter time and therefore can respond more sensitively.

For your purposes, it helps to know that larger pixels enlarge better, called upsizing. You can increase the size of a photograph with bigger pixels through interpolation and get a much smoother image with noticeably less noise.


NOTE

APS stands for Advanced Photo System, which is a film format size standard developed primarily by Kodak but cooperatively with Nikon, Canon, Fuji, Minolta, and other camera manufacturers. It was introduced in 1996, and used primarily for point-and-shoot cameras. The standard relates to film frame dimensions, and has been carried over to digital image sensors. There are three categories, including APS-C (the classic, with an aspect ratio that relates to a 4 x 6 print), APS-P (the panoramic, with an aspect ratio that relates to a 4 x 12 print), and APS-H (the high-definition, a larger sensor/film size that relates to a 4 x 7 size). Kodak’s APS products are sold under the Advantix brand.

Category: Photography System

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