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Pixel Size as a Determinant of Digital Camera Image Quality

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Copyright © 2007-2009 by Zack Smith.
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The purpose of this web page is to explain the concept of sensor element size and to provide some examples from actual digital cameras, so that you can get an idea of which digital cameras are more likely to offer decent image quality.

Introduction

A digital camera sensor is what senses light and converts it into an electronic image. It consists of an array of millions of pixel sensors. Typically each pixel sensor is made up of sensors for specific colors, for instance red, green, and blue. These sensors are either adjacent to each other or, in some cameras, stacked on top of one another.

What really matters when determining digital camera image quality ("IQ") is actually not really how many megapixels it has, but rather what the size of these microscopic pixels. In this document I state pixel size in terms of area in square micrometers (µm2).

The size of a pixel directly impacts how much noise an image will have in low light, and in some cases even in daylight. The bigger the pixel is, the lower the noise because more photons can reach a bigger pixel sensor.

To illustrate, suppose you have two cameras that both have a sensor that is 1/1.8" in size (this represents the area as a single number since the numerator is always 1), but different numbers of pixels.

  • The one with more pixels crams more pixels into the same space so they must be smaller than in the other camera.
  • If the same number of photons are reaching each sensor per second then fewer photos will be reaching individual pixels in the higher-megapixel camera.

Therefore it initially appears that the camera with fewer megapixels and therefore large pixels will have better image quality with less noise.

This is a good reason to be skeptical about the marketing hype and salespeople who claim or imply that it is better to purchase higher megapixel cameras.

Of course, pixel size is not the sole determinant of image quality. There are others:

  1. Sensor quality differs between sensor chip manufacturers.

  2. Sensor quality varies within the range of products from any one sensor manufacturer. Every product has a high-end line and a low-end. That's how business works.

  3. Furthermore, any one maker of cameras e.g. Canon may use sensor chips from different manufacturers between their camera models. So they might put a cheap sensor from one manufacturer in a cheap camera, but a pricey sensor from another manufacturer in their high-end camera.

  4. Newer sensor technology is surely better than older technology, because chip manufacturers are getting better every year, improving the optical and electrical characteristics of sensors. So while the Canon A80's 4MP sensor's pixels have an area of 10 µm2, which is huge, its quality could be inferior to something made 3 years later that uses half the area per pixel. Therefore the technology generation matters.

  5. Optics can play a big role at high megapixels. Not all lenses are equally finely polished. Some high-MP cameras are being sold whose lenses are just barely sufficient to support the number of megapixels.

Sensor geometry

Pixels

Now let's talk about the geometry of the sensors themselves. A sensor's overall width and height typically have a 4:3 ratio. Each pixel in a sensor has 4 color-specific sensor elements -- two green elements, one blue and one red, placed adjacently (except in the Foveon sensor, where they are stacked). Pixels and color-speific sensors are square. Like this:
   
   

1/X designation

The information about size of the sensor is always expressed as 1 over X", where X is a number that varies by sensor. They may also give you the width and height in millimeters, but sometimes you have to search a technical manual for that.

This 1/X" format is an old way of describing sensor sizes devised for Vidicon television cameras. It means that the diagonal of the 4:3 sensor is 1 over X inches, times two thirds. The reasons for this arcane standard is a story for the history books, not here.

Determining pixel dimensions from sensor width & height

If you are told the actual dimensions of the sensor, determining pixel area is simple: Area of entire sensor (in mm2) = width in mm * height in mm
Area of entire sensor (in µm2) = 1,000,000 * area in mm2
Area of one pixel = area of sensor in µm2 / # pixels

Determining pixel dimensions from 1/X size

While often it is possible to learn the width and height of a sensor from a company's camera manual or specification sheet, sometimes all you can get quickly is the 1/X" value.

To determine 4:3 sensor width and height from 1/X", let's solve this equation:

(1 / X") * 0.667= sqrt ( (4a)2 + (3a)2 ) Or simply... 0.444 / X2 = 16a2 + 9a2 And from this we get... 0.444 / X2 = 25a2 And then this... sqrt (0.444 / 25X2) = a And like so... 0.667 / 5X = a And finally... Width = 4a = 4 * 0.667 / 5X
Height = 3a = 3 * 0.667 / 5X ...but we need the total area, too: Area = width * height = 0.21333 / X2 Plus we need to convert to metric! Width in micrometers = 25,400 * width in inches
Height in micrometers = 25,400 * height in inches
Area in µm2 = 645,160,000 * area in inches So! The final equation is: Area of the entire sensor in µm2 = 137,630,000 / X2 The area occupied by one pixel is: Pixel area = area of sensor in µm2 / Y
...where Y = total pixels

Camera manufacturers typically provide two numbers for the total pixels in the camera, e.g. 10MP effective and 10.3MP actual. You have to use the actual number of pixels in the equation above i.e. the higher of the two numbers usually specified.

Point and shoot pixel area values

Camera Sensor size Megapixels Pixel area (µm2)
Canon Powershot A630 1/1.8" 8.2MP 5.2
Casio Exilim Z1080 1/1.75" 10.3MP 4.36
Canon Powershot G7 1/1.8" 10.3MP 4.1
Canon Powershot A640 1/1.8" 10.3MP 4.1
Canon Powershot A650 1/1.7" 12.4MP 3.8
Casio Exilim Z1200 1/1.7" 12.39MP 3.8
Canon Powershot G9 1/1.7" 12.4MP 3.8
Nikon Coolpix 5100 1/1.72" 12.43MP 3.74
Canon Powershot A570IS 1/2.5" 7.1MP 3.1
Canon Powershot A720IS 1/2.5" 8.3MP 2.6 (!!)
Canon Powershot A80 1/1.8" 4.1MP 10.

Special case of a 16:9 Panasonic camera

The Panasonic Lumix DMC-LX2 is the same camera as the Leica D-LUX 3. Both manufacturers claim the sensor, which has the "widescreen" format, can be thought of as a 1/1.65" even though that standard was invented for old-time television cameras. I was unable to find the manual online to get the real dimensions. The Panasonic version of this camera generally does not review well, perhaps because the odd-dimensioned sensor is a new and unusual creation that has not been refined yet.

Camera Sensor size Megapixels Pixel area (µm2)
Panasonic Lumix DMC-LX2 1/1.65" 10.2MP 5.0

Digital SLR pixel area values

Camera Sensor size Megapixels Pixel area (µm2)
Nikon D700 36 x 24 mm CMOS sensor 12.9MP 67.0
Nikon D3 36 x 24 mm CMOS sensor 12.9MP 67.0
Nikon D70 23.7 x 15.6 mm CCD sensor 6.3MP 58.7
Nikon D40 23.7 x 15.5 mm CCD sensor 6.3MP 58.7
Canon EOS 350D / Digital Rebel XT 22.2 x 14.8 mm CMOS sensor 8.2MP 40.1
Nikon D60 23.6 x 15.8 mm CCD sensor 10.8 MP 34.6
Nikon D80 23.6 x 15.6 mm CCD sensor 10.8MP 34.5
Nikon D40X 23.6 x 15.6 mm CCD sensor 10.8MP 34.5
Canon EOS 400D / Digital Rebel XTi 22.2 x 14.8 mm CMOS sensor 10.5MP 31.3
Canon EOS 40D 22.2 x 14.8 mm 10.5MP 31.3
Nikon D90 23.6 x 15.8 mm CMOS sensor 12.9MP 28.9
Canon EOS 500D / Digital Rebel Ti1 22.3 x 14.9 mm CMOS sensor 15.1 MP 22.0

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