When purchasing a digital SLR camera, the issue of sensor size comes into play very often. The sensor is the light-sensitive surface inside the camera body that captures light in order to make a photograph. The size of this sensor has a variety of effects on your digital photos.
Introduction to Digital Camera Sensors
A DSLR camera’s sensor sits inside the camera body, on the other end of the camera’s lens. When the shutter button is pressed, the mirror moves out of the way and allows the light passing through the lens to fall on the sensor. The sensor records the light, and the rest of the electronics inside of the camera convert that light information into a photograph. Where older cameras had film, a digital camera has a sensor.
Most photographers describe the size of a camera’s sensor by comparing it to the size of a traditional 35mm negative. The APS-C sensor used in Canon’s Digital Rebel cameras (XT, XTi, XS, XSi) and mid-range models (20D, 30D, 40D) is slightly smaller than a 35mm negative. The other camera manufacturers, e.g. Nikon, Olympus, Sony, also use smaller-than-35mm negative sensors in many of their digital SLR cameras.
The following photo does a great job of illustrating the size differences between different sensors.
Photo by occecid.
The “full-frame” sensor on the left is the same size as a 35mm negative — this is the sensor sized used in Canon’s 5D and 1Ds cameras. The “APS-C” sensor on the right is what is present in most of Canon’s entry-level, amateur, and “prosumer” models. The “APS-H” sensor on the top is a third size that appears in some of Canon’s professional cameras.
Nikon also uses a full frame sensor in some of their professional camera bodies — they refer to this as their “FX” format. FX sensors are present in the Nikon D3 and D700 cameras as of this writing.
Effects of Sensor Size on Photographs
The size of the sensor inside a camera determines the field of view that will be expressed in each photograph. Most of the lenses used on DSLR camera bodies cast a light circle intended to cover a full frame sensor/35mm negative. Since a cropped sensor is smaller than a 35mm negative, it captures a smaller portion of the area covered by a full-frame sensor. Here is a rough illustration:
By looking at this illustration, it becomes apparent why photographers refer to a sensor that is smaller than full-frame as a “cropped” sensor. By capturing a smaller area of light, the field of view that is recorded in the picture has the appearance of having been cropped from the center of a 35mm photograph.
Cropped sensors also have an effect on depth of field — they increase depth. The difference is not overly dramatic when comparing a full-frame sensor to a size such as APS-C or Nikon’s DX format (which appears in cameras such as the D70 and D300). However if you were to compare the depth of field produced by a DSLR to that of a point & shoot camera, the difference would be quite pronounced. While taking a portrait at f/2.8 with a DSLR would give you a nice blurry, out-of-focus background, f/2.8 on a compact digital camera would put much more of the scene in-focus. The sensors in pocket-sized compact digital cameras are but a fraction of the size of those found in DSLRs, therefore differences in depth of field are very noticeable.
Focal Length Multiplication Factors
Since a cropped sensor only captures a portion of the scene recorded by a full-frame sensor at any given focal length, we must use a multiplication factor in order to describe the fields of view produced by a cropped sensor. Canon’s APS-C sensor has a multiplication factor of 1.6x, and Nikon’s DX format has a factor of 1.5x.
What do we do with these multiplication factors? Well, we multiply them by the given focal length to determine the field of view. Put a 50mm lens on a Canon Digital Rebel XTi. The XTi’s APS-C sensor is cropped, so we multiply 50mm by 1.6. 50 * 1.6 = 80, which means that looking through a 50mm lens on a 1.6x cropped sensor camera yields a field of view that is equivalent to 80mm on a full-frame body or 35mm film. Therefore in order to achieve the “normal” field of view traditionally seen through a 50mm lens, you must us a 30mm lens on a camera with a 1.6 crop factor. 30 * 1.6 = 48mm… close enough.
It is important to remember that sensor sizes do not change the actual focal length of a lens — 50mm will always be 50mm. A cropped sensor simply changes the field of view in a photograph taken at a given focal length.
Effects of Sensor Size on Your Wallet
Larger sensors have more surface area, requiring more material, which costs more money. Therefore full-frame digital SLR cameras have higher price tags. Prices on full-frame cameras have dropped over time as consumer demand has increased and material costs have fallen, but you are still going to pay a little more for a full-frame camera.
As I am writing this article, Canon’s 1 year-old EOS 40D (cropped) costs about $1,000.00. Meanwhile, their nearly 3 year-old EOS 5D (full frame) will run you $2,300.00. Nikon’s D300 (cropped) is listed at about $1,600.00, while the D700 (full frame) is $3,000.00.
