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Sensor Pixel Size as aDeterminant of DigitalCamera Image Quality

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Version 0.21Copyright © 2007-2010 by Zack Smith.All rights reserved.

The purpose of this web page is to explain the conceptof sensor element size and to provide some examplesfrom actual digital cameras, so that you can get an ideaof which digital cameras are more likely to offer thebest image quality.

Introduction

A digital camera sensor is what senses an image's lightand converts it into an electronic image. It consists of anarray of millions of pixel sensors. Typically each pixelsensor is made up of sensors for component colors,normally red, green, and blue. These sensors are usuallyadjacent to each other, but in some cameras theystacked on top of one another.

What really matters when determining digital cameraimage quality ("IQ") is actually not how manymegapixels it has, but rather what the size of each ofthese microscopic pixels is. In this article, I measure

pixel size square micrometers (µm2).

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

To illustrate, suppose you have two cameras that bothhave a sensor that is 1/1.8" in size (this represents thearea as a single number since the numerator is always1), but different numbers of pixels.

The one with more pixels crams more pixels intothe same space so they must be smaller than inthe other camera.If the same number of photons are reaching eachsensor per second then fewer photos will bereaching individual pixels in the higher-megapixelcamera.

Therefore it initially appears that the camera with fewermegapixels and therefore large pixels will have betterimage quality with less noise.

This is a good reason to be skeptical about

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the marketing hype and salespeople whoclaim or imply that it is better to purchasehigher megapixel cameras.

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

Sensor quality differs between sensor chipmanufacturers.

1.

Sensor quality varies within the range of productsfrom any one sensor manufacturer. Every productline has a high-end line and a low-end.

2.

Furthermore, any one maker of cameras e.g.Canon may use sensor chips from differentmanufacturers between their camera models. Sothey might put a cheap sensor from onemanufacturer in a cheap camera, but a priceysensor from another manufacturer in theirhigh-end camera.

3.

Newer sensor technology is surely better thanolder technology, because chip manufacturers aregetting better every year, improving the opticaland electrical characteristics of sensors.

So while the Canon A80's 4MPsensor's pixels have an area of 10

µm2, which is big, its quality could beinferior to something made 3 yearslater that uses half the area per pixel.

Thus the technology's generation matters.

4.

Optics can play a big role at high megapixels. Notall lenses are equally finely polished. Somehigh-MP cameras are being sold with lenses thatare just barely sufficient to support the number ofmegapixels. Some cameras with over 12megapixels are being paired with lenses that arenot ground fine enough to support that manypixels.

5.

Lastly, whether sub-pixels (red, green and blue)are adjacent versus stacked is a big factor inimage sharpness. Stacked subpixels are used inFoveon(tm) sensors. Adjacent are used in Bayersensors.

6.

Sensor geometry

Pixels

Now let's talk about the geometry of the sensorsthemselves. A sensor's overall width and height typicallyhave a 4:3 ratio. Each pixel in a sensor has 4 color-

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specific sensor elements -- two green elements, one blueand one red, placed adjacently (except in the Foveonsensor, where they are stacked). Pixels and color-speificsensors are square. Like this:

1/X designation

The information about size of the sensor is alwaysexpressed as 1 over X", where X is a number that variesby sensor. They may also give you the width and heightin millimeters, but sometimes you have to search atechnical manual for that.

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

Determining pixeldimensions from sensorwidth & height

If you are told the actual dimensions of the sensor,determining pixel area is simple:

Area of entire sensor (in mm2) = width inmm * 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 pixeldimensions from 1/X size

While often it is possible to learn the width and heightof a sensor from a company's camera manual orspecification sheet, sometimes all you can get quickly isthe 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...

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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 / 5XHeight = 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 ininchesHeight in micrometers = 25,400 * height ininches

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 numbersfor the total pixels in the camera, e.g. 10MP effectiveand 10.3MP actual. You have to use the actual numberof pixels in the equation above i.e. the higher of the twonumbers usually specified.

Point and shoot pixel areavalues

Camera Sensorsize

Megapixels Pixel

area(µm2)

Pixeldensity

(MP/cm2)

CanonPowershotA630

1/1.8" 8.2MP 5.2 19.3

CasioExilimZ1080

1/1.75" 10.3MP 4.36


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CanonPowershotG7

1/1.8" 10.3MP 4.1

CanonPowershotA640

1/1.8" 10.3MP 4.1

CanonPowershotA650

1/1.7" 12.4MP 3.8

CasioExilimZ1200

1/1.7" 12.39MP 3.8

CanonPowershotG9

1/1.7" 12.4MP 3.8

NikonCoolpix5100

1/1.72" 12.43MP 3.74

CanonPowershotA570IS

1/2.5" 7.1MP 3.1

CanonPowershotA720IS

1/2.5" 8.3MP 2.6 (!!)

CanonPowershotA80

1/1.8" 4.1MP 10.

Special case of a 16:9 Panasoniccamera

The Panasonic Lumix DMC-LX2 is the same camera asthe Leica D-LUX 3. Both manufacturers claim thesensor, which has the "widescreen" format, can bethought of as a 1/1.65" even though that standard wasinvented for old-time television cameras. I was unableto find the manual online to get the real dimensions. ThePanasonic version of this camera generally does notreview well, perhaps because the odd-dimensionedsensor is a new and unusual creation that has not beenrefined yet.

Camera Sensorsize

Megapixels Pixel area

(µm2)

Panasonic LumixDMC-LX2

1/1.65" 10.2MP 5.0

Bayer digital SLR pixel areavalues


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Camera Sensorsize

Megapixels Pixelarea

(µm2)

Pixeldensity

(MP/cm2)

NikonD700

36 x 24mmCMOSsensor

12.9MP 67.0

1.5

Nikon D3

36 x 24mmCMOSsensor

12.9MP 67.0

1.5

Nikon D70

23.7 x15.6 mmCCDsensor

6.3MP 58.7

1.7

Nikon D40


6.3MP 58.7

1.7

CanonEOS 350D/ DigitalRebel XT

22.2 x14.8 mmCMOSsensor

8.2MP 40.1

2.5

Nikon D60


10.8 MP 34.6

2.9

Nikon D80


10.8MP 34.5

2.9

NikonD40X


10.8MP 34.5

2.9

CanonEOS 400D/ DigitalRebel XTi


10.5MP 31.3

3.2

CanonEOS 40D

22.2 x14.8 mm

10.5MP 31.33.2

Nikon D90


12.9MP 28.9

3.5

CanonEOS 500D/ DigitalRebel Ti1


15.1 MP 22.0

4.5

CanonEOS 7D

22.3 x14.9 mmCMOS

18 MP 18.5 5.4


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sensor

Foveon digital SLR pixelarea values

Foveon sensors have some natural advantages over theBayer format. Whereas the Bayer's pixel area is like amosaic, with pixel area divided into two green sensors(½ the area), a red sensor and a blue sensor (¼ the areaeach), the Foveon's entire pixel area is available to eachcolor because the color sensors are stacked.

Therefore Foveon color sensors receive more light thanBayer elements do given the same area. Or to put thatdifferently, a Bayer sensor would have to be larger thana Foveon for its pixel sensors to receive the sameamount of light.

In addition, because all sensor colors are at onephotosite, Foveon sensors naturally produce sharperimages than Bayer sensors, even those with 2 times asmany pixels.

There is a wrinkle in the stacking approach, however.The sensor is made of silicon, which is only translucent.In order words is passes some light but absorbs it aswell. As you go deeper into the silicon, it consequentlyabsorbs fewer and fewer photons. Furthermore differentwavelengths penetrate to different depths. Bluepenetrates silicon the shallowest, so the blue detector isthe top and thinnest layer. Green penetrates deeper butnot as deeply as red, so green is the next layer belowblue, and then red is the deepest and thickest layer.

My experiments in infrared photography using theSigma DP1 showed that the Foveon sensor is highlysensitive to these long wavelengths despite fewerphotons reaching the red layer.

As far as I know, the DP1 and DP2 use the same sensorhowever there are varying reports as to how manypixels each has. So here are two calculations:

Camera Sensorsize

Megapixels Pixelarea

(µm2)

Pixeldensity

(MP/cm2)

SigmaDP2/DP2s

20.7 x13.8mm

4.7 MP 60.8 1.6

SigmaDP1

20.7 x13.8mm

4.8 MP 59.5 1.6

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