Although analog cameras had several film formats including those for medium and large formats, the most popular film size was the 35 mm film. So even today’s digital cameras sensors are often compared to 35 mm film. In fact, sensors that are the same size as 35 mm film are called “full frame” sensors. As you can imagine, in terms of processor chips, these sensors are quite large. So the image sensors in most digital cameras tend to be smaller than the 24 mm x 36 mm image area of a full-frame 35 mm cameras. In fact, some tend to be much, much smaller.

To complicate matters for the end-user, there is no unified measuring system for sensors. Some manufactures measure in inches, some in millimeters. Some measure horizontally and vertically while others measure diagonally. For the most part sensors fit into different types. You will hear the terms like 2/3” type, 1/1.8” type and 1 /2.7” type. These are typical sizes that are generally used in CCDs in smaller point-and-shoot cameras. SLR cameras with interchangeable lenses generally use larger sensors. Two of the most common SLR sizes are 4/3” type and 1.8” APS-C type.

The fractions given in naming sensor types are actually larger than the actual dimensions. We are told that this naming convention is a throw-back to the way that TV camera tubes were named in the 50’s. Obviously, these naming conventions should have been discarded long ago, but like a bad egg, their trail continues.

The table below gives a breakdown of these various sizes and their actual vertical, horizontal, and diagonal measurements. Aspect Ratio is the ratio of width to height.

The APS (Active-Pixel Sensor) naming convention seems to be a move away from the “fraction” type of names. However, it is no less confusing because different manufacturers use different APS sizes. The APS-C type mentioned above is a popular size of APS chip, but some Nikon APS-C chips are slightly larger and some Canon APS-C chips are slightly smaller. Canon also has a larger APS-H sensor.

To get a feel for the difference in sensor sizes, see Figure 1 which gives a visual image of the largest compared to the smallest.

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Figure 1: Relative size comparison of full sensor (25mm x 24mm) to 1/1/8" sensor type (7.18 mm x 5.32 mm).

The reason that the size of the sensor is so important can be easily recognized when you envision the millions of megapixels on the sensor. As you may know, or may have read in our Everything You Need to Know About Megapixel tutorial , manufacturers are still trying to entice photographers to buy their cameras by constantly increasing the number of megapixels in the camera. Since the pixels count in the millions (megapixels) you can imagine that to add more photodiodes to a certain sized sensor to create more megapixels, the pixels themselves have to be smaller. The smaller the pixels, the less light they can handle. When the pixels don’t get enough light, the result is what is called “noise”. Simply put, noise is the presence of specks of color that don’t belong in the photograph.

So, all other things being equal, millions of photodiodes crammed onto a small sensor will result in poorer photos than the same number of photodiodes on a larger sensor. If the sensor is larger or the megapixel count is smaller, each photodiode can be larger. So, given the same pixel count, a larger sensor will capture more light per photodiodes than a smaller sensor and produce a better picture.

A small sensor is not entirely bad. It allows the camera to be small and very portable. That is why digital cameras can be so much smaller than film cameras. Smaller sensors are also less expensive, which can help keep the cost of the cameras more affordable. So there is room for both large and small sensors in the photo marketplace, but the consumer should understand that there are tradeoffs with smaller sensors.