Image Sensors: Bigger is Better!
It’s time to get techie about digital camera pixel and sensor size. I offered some opinions on pixel counts in the last post. There is still the issue of pixel structure, but it is a gnarly technical one that is rife with minutiae, so I’ll save it for another day. This time I’ll concentrate on sensor size, which relates to pixel quality, but will limit the discussion to sensors for mere mortals (smartphone to full-frame), and ignore the ginormous sensors used in expensive digital medium-format cameras.
First, a simple concept: given pixels of the same size, more will fit on a larger sensor. Makes sense, doesn’t it. In reality, all pixels are not the same size. Manufacturers tend to make them smaller so that more will fit on a smaller sensor, and even larger sensors are getting packed with increasing numbers of shrinking pixels. But all else being equal, pixels on a larger sensor will be larger than pixels on a smaller sensor.
What the heck is a pixel?
Each pixel (or “photosite”) on a camera’s sensor is basically a little light detector. Light in, data out. Individual pixels only detect light amplitude. They have no intrinsic color or image detection capability. If you place a filter that only passes one color in front of a light detector, obviously it will then detect variations in the level of light of that color. Most current camera sensors place an array of red, green, and blue filters in front of the pixel array, one color per pixel. This is the “Bayer filter” you have probably heard mentioned. The camera’s image processing engine then synthesizes the full photographic color spectrum from the red, green, and blue level data derived from the sensor. The fact that you have a large number of pixels in an array is what makes it possible to capture images.
Pixel size is a major factor in the performance differences between small and large sensors. Pixel counts affect resolution, but the quality of the data produced by each pixel can have a much more noticeable effect on final image quality, particularly in terms of noise. Noise dilutes the data we actually want, muting color and showing up as a graininess in our shadows and, as the noise increases, more visibly in our midtones. Yech. If we can keep noise under control we can boost sensitivity (ISO), making it possible to capture clean, noise-free images in lower light. By now you’ve probably guessed where we’re going … larger pixels produce less noise. More accurately stated, larger pixels have a higher signal-to-noise ratio. Because the noise generated by a pixel remains more or less constant, the more light it receives the higher its signal-to-noise ratio will be. Quite simply, bigger pixels receive more light. More of the good stuff and less of the bad.
Manufacturers continue to find ways to design sensors with higher pixel counts, and therefore smaller pixels, that can produce low-noise output. BSI (Back Side Illumination) is one example, allowing more light to reach each pixel by moving the sensor chip’s wiring out of the way.
Now that we’ve got some of the techie stuff out of the way, and understand that bigger is basically better when it comes to pixels and sensors, let’s compare some common image sensor sizes.
|Type||Dimensions (approx.)||Area (approx.)|
|Smartphone (1/1.3")||4.8 x 3.6 millimeters||17.3 square millimeters|
|Compact Camera (1/1.7")||7.44 x 5.58 millimeters||41.51 square millimeters|
|Micro Four Thirds||17.3 x 13 millimeters||225 square millimeters|
|APS-C||23.7 x 15.6 millimeters||370 square millimeters|
|Full Frame||35 x 24 millimeters||840 square millimeters|
There are a few more variations that fit in between the sizes listed above, but what we have is enough for our discussion. In fact, we only really need to look closely at the largest and smallest. Once you grasp that difference the rest are easy.
At 35 mm x 24 mm even the largest full-frame sensor in our list isn’t exactly what you’d call a “big thing,” but compared to the teeny 4.8 mm x 3.6 mm smartphone camera sensor it is huge. Take a look at the sensor surface area, which is what will determine the number and size of pixels that will fit. 17.3 square millimeters for the smartphone sensor compared to 840 millimeters for a full-frame sensor: the full-frame sensor is almost 50 times larger! That’s the kind of difference we’re talking about, and it should be pretty clear that there will be corresponding variations in image quality. Differences between adjacent sizes in our chart are naturally a lot smaller: 2.4x from smartphone to compact camera, or 2.3x from APS-C to full frame don’t seem like a huge step up, yet there are subtle but noticeable differences. The relatively small smartphone to compact camera gap also reveals why the compact camera market is drying up. You already have your phone with you, so why carry an extra piece of equipment that only offers a small increase in image quality?
Ultimately it comes down to: how much image quality (at current technological standards) do you need? To a large extent that will depend on why you take photos in the first place. If you just want to show friends and family where you’ve been and what you’ve done (including what you had for lunch … a trend that I simply don’t get), then a smartphone camera is fine. Save your money. But if you want images that you can truly be proud of, images that have integral beauty and depth, images that express rather than simply show, then you need to move up to something bigger.
At the next level you’re looking at micro four thirds, APS-C, or full frame, or one of their close relatives. If you can sacrifice a small amount of quality and control, then go with micro four thirds or APS-C. As small and lightweight as full-frame cameras have become, micro four thirds and APS-C cameras and lenses are still a lot smaller, making them more convenient for travel or just carrying with you at all times. But if only ultimate image quality will do, and you’re prepared to deal with the extra size and weight of a full-frame system, then that large full-frame sensor will give you the nice big pixels you need.
Another parameter that changes with sensor size is angle-of-view, but that is a separate subject for yet another post.