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Comparing Digital Camera Sensors, Part 1

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Digital cameras that use interchangeable lenses (digital single-lens reflex or DSLR cameras) continue to improve in quality and come down in price. Excellent cameras are available for under $1,000. But now, starting with the Canon 5D, there's a (reasonably) affordable model that contains something that was once reserved for only the most expensive DSLRs: a full-frame sensor, measuring the same size as a 35mm negative. In part one of this three-part series, Peter Bauer takes a look at some practical aspects of different-sized camera sensors in digital SLRs. Which camera should you buy? You may want to read this first.
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Digital cameras that use interchangeable lenses (digital single-lens reflex or DSLR cameras) continue to improve in quality and come down in price. Excellent cameras are available for under $1,000. But now, starting with the Canon 5D, there’s a (reasonably) affordable model that contains something that was once reserved for only the most expensive DSLRs: a full-frame sensor, measuring the same size as a 35mm negative. Let’s take a look at some practical aspects of different size camera sensors.

Exploring Digital Camera Sensors

The part of the digital camera that actually captures the photo is the camera’s sensor. The sensor plays the same role as film in a camera, making a permanent record of the light that enters through the camera’s lens. Sensor sizes range from large "scanning backs" (which actually work more like your flatbed scanner than like your digital camera) to sensors smaller than a fingernail (see Figure 1).

Figure 1

Figure 1 The actual sensor is enlarged in the insert.

As you can see, that inexpensive camera uses a sensor that measures about 0.19 X 0.14 inches. It captures a resolution of 640 X 480 pixels—a miniscule image by today’s standards.

Most DSLR cameras today use a sensor that measures approximately 0.9 X 0.6 inches (22.5 X 15 mm), comparable to the APS-C film size. Full-frame sensors, on the other hand, measure 1.4 X 0.95 inches (36 X 24 mm) and compare to a 35 mm negative. Let’s compare the smaller sensor, as found in the Canon 20D and 30D (street price about $1,000, 30D body only) with the full-frame sensor found in the Canon 5D (street price about $2,200, body only). To avoid barely comprehensible strings of decimals, I use millimeters rather than inches in Table 1.

Table 1: Specifications of Two Digital Camera Sensors

 

20D/30D

5D

Sensor dimensions

225 X 1.5 mm

36 X 24 mm

Sensor area

337.5 sq mm

 

Pixel dimensions

3504 X 2336

864 sq mm

Pixel count

8,185,344

 

Pixels per square millimeter

24,253

14,725

Total sensor photo detectors

8.5 million

13.3 million

So, by the numbers, what do the differences between the sensors mean? What advantages does the 5D’s sensor have over the sensor used by the Canon 20D and 30D?

  • 256% larger (square millimeters)
  • 55% more pixels
  • 40% fewer pixels per sq mm (larger pixels)
  • 56% more sensor photo detectors
  • 60% fewer sensor photo detectors per sq mm (over 50% larger)

More pixels and sensor photo detectors. Larger pixels and sensor photo detectors. Large chip. Better performance? Yup. So that you can actually see the difference in the physical sizes of the two sensors, I took a shot (using yet another camera) of the 20D and 5D in "sensor clean" mode. In Figure 2, the 20D (smaller sensor) is on the left.

Figure 2

Figure 2 The difference in physical size between the sensors (the green computer chips) is evident.

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