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Image Compression

And their effect on image quality

Good versus Bad pixels

The image sensor.
A camera's lens projects an image on the surface of an image sensor. This surface is pretty small and a collection of millions of separate sensors. Each sensor is like a solar cell, and produces an analog electrical signal that is then converted into a digital value. All these values together make the photo.

The more pixels, the more details a picture can have. We want sharp pictures, so we need lots of pixels; Megapixels. One MegaPixel = 1,000,000 pixels. But how many MP is enough? For high quality snap shots, about 8 MP is more than enough. Professional photographers may need much more depending on what they are working on. For many jobs 8 MP is fine too, but when the result might be a huge sharp print, much more pixel are needed.
To double the resolution of a 1MP sensor, the amount of pixels both length and width must be times two. So: (1*2) * (1*2) = 4 MegaPixels
To double the resolution, the amount of MegaPixels must be times 4
To double the resolution of a 4 MegaPixels sensor, you'll need 4*4 = 16 MP
To double the resolution of a 16 MegaPixels sensor, you'll need 16*4 = 64 MP
To double the resolution of a 64 MegaPixels sensor, you'll need 64*4 = 256 MP
So, when having an 8 MP camera, buying a 10 MP camera will not give you much sharper pictures, you'll probably not notice the difference.

The picture above shows well how much the image sensor size differ in digital photo cameras. All compact cameras have tiny sensors, and aren't fit to make quality photos (so little light divided by so many pixels). Full frame sensors are great, but so are the size and price of the camera. The APS-H and APS-C formats reduce the size and price a bit, but the new Micro Four Thirds cameras promise to be a real in-between format.

"Full frame" sounds like the biggest possible sensor, but there are bigger ones. The name is because it's size is equal to the 35 mm film format: 24 x 36 mm (it's called 35 mm because that's how wide the film rol was cut).

Type Size example MP MP/cm2
Full frame 36 x 24 mm Canon EOS 5D Mark II 21 2.4
APS-C (Nikon DX) 23.1 x 15.4 mm Nikon D7000 16.2 4.6
Micro Four Thirds 18 x 13.5 mm Panasonic Lumix DMC-G2 12.1 5
1/1.6" (1/1.63") ±8 x 6 mm Panasonic Lumix DMC-LX5 10.1 24

Noise, and other problems.
The light sensitive cells on an image sensor are very tiny, and the exposure is short. If too little light can be collected by the cells, the readout is an inaccurate measurement, visible as noise, especially in the dark areas. Noise is not only polluting the image, reducing the image quality, it's also almost impossible to compress, causing an unwanted increase of file size. Another problem with very small cells, is the lens's quality not being able to deliver that high a resolution, and dust being relatively huge.
What is more important than the amount of MegaPixels is the quality of the pixels. I once had a digital compact camera of 6 MP, but the quality of the pixels was such crap; I had to reduce the length and width of the images by 50% to get decent pixel quality, which really left me with a 6/4 = 1.5 MP camera!

How to get the best image quality in digital photography.
- Get yourself some big pixels! The larger each pixel on the image sensor is, the more light it receives, the better the image data becomes. Less noise, better colors, smaller files, and sharper images because the lens's resolution is a little less critical.
Of course; the image sensor size of a 12MP camera with bigger pixels will be bigger and more expensive than a 12MP camera with tiny pixels.
How big are your camera's pixels? Search for the size of the image sensor, and divide it's MegaPixels by that surface area.
A 14 MP camera with a sensor of 0.8 * 0.6 cm = 0.48 cm2,
has a pixel density of 14/0.48 = 29.2 MP/cm2
A 14 MP camera with a sensor of 3.6 * 2.39 cm = 8.6 cm2,
has a pixel density of 14/0.12 = 1.63 MP/cm2
Today (2010), quality starts at about 5 MP/cm2, if you'd ask me.
The lower the MP/cm2, the bigger the pixels, the better the quality.
And one more thing: there is some space between the pixels, for the communication "wires" and such The smaller the pixels the more light is lost in between the pixels.
Can pixels be too big? No. If you make beautiful pictures, you're probably not using an in camera flash, and often no direct sunlight, so you'll often need all the light you can catch. The size weight and prize of the camera can become too much though. If your camera has to be a compact, be sure to get a big sensor (1/1.6" or 1/1.7") and not too much megapixels.

- Get a new camera. Each year, the image sensors get a little better. A camera from 6 years ago with the same MP/cm2, won't be as good as a new one.

- Set your ISO low. The lower the sensitivity of your image sensor is set, the more light information is gathered.

- Always shoot in RAW. Most digital cameras have sensor cells for Red Green and Blue. A JPG saves at most 256 tones per channel, and in a lossy way. A 12 bit RAW saves up to 4096 tones per channel, and lossless. Because is saves so much more image information, you can afterwards choose exactly the tones you want. Forgot about JPG, use RAW if you care for quality. (jpg with a high quality setting is great for the final result, but certainly not for shooting). Only some compact cameras have RAW.

- Hold your camera still! If you cause motion blur, all these mega pixels won't do you any good.

A 6 MegaPixel camera with good quality pixels, can outperform a 24 MegaPixel camera with bad quality pixels. Choose a good camera, and care for your pixels.


There is much more to tell about image quality, but if you're not a geek, you may skip this last part:

Because a 10 MP camera produces 10,000,000 RGB pixels,
one would expect there to be:
10,000,000 Red sensors
10,000,000 Green sensors
10,000,000 Blue sensors
All together 30,000,000 sensors

But in reality, there are only:
2,500,000 Red sensors
5,000,000 Green sensors
2,500,000 Blue sensors
That's all together only 10,000,000 sensors!
There are 20,000,000 sensors missing!!

The missing 20,000,000 measurements are simply predicted, thus guessed really! 2/3 of a typical photo is made up...
Why? Most digital cameras use a "Bayer filter arrangement" on the image sensor, build like this (the color shows to what color the area is sensitive):

The Bayer arrangement of color filters on the pixel array of an image sensor.
Each cell is sensitive for only 1 color. Red or Green or Blue.

This explains why the green channel has often the highest quality.
To get higher quality pixels, each MegaPixel sold to us should only produce 0.25 RGB MegaPixels, so that for every RGB pixel, there is at least 1 sensor for each color.

Now go out and shoot some cool photos!

Giesbert Nijhuis

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