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The Camera
A camera is a device for capturing images which can then be reproduced and viewed at a later date.
A camera consists of an enclosed hollow with an opening (or aperture) at one end for light to enter and a recording surface for capturing the light. Okay, in today’s world it is an expensive box with buttons, dials, menus and lenses (expensive pieces of glass) but the principle is exactly the same as it was when photography was invented.
Very early cameras without any lens where called pinhole cameras, as the hole had to be very tiny to create a sharp image. This minimises the amount of light getting in, thereby creating a very faint image. Therefore all modern cameras have a lens to focus the incoming light which allows for much larger apertures to be used and therefore brighter images to be created.
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The word camera comes from the Latin camera obscura which means 'dark chamber', an early method in which an entire room formed part of an imaging system. A small hole in one wall created an image on the opposite wall. An artist would often attach paper to the wall and create a drawing over the projected image.
A modern interpretation of this principle is the pinhole camera and is sometimes taught to children in science. |
The image capture is achieved by either photographic film or an electronic sensor. Modern photographic film consists of a plastic-coated strip with a light-sensitive emulsion of silver halide salts. When the emulsion is exposed to light, it forms a latent (invisible) image. Developing the film uses chemicals to change this into a visible image and then to 'fix' the film so that it is no longer sensitive to light.
As the film is sensitive to light it must be kept in complete darkness inside the camera until the point the photographer wants to take the picture. It is then it is exposed for a very short time to the light entering the lens. This is the job of the shutter which is usually mounted just in front of the film and opens for the required amount of time before closing again.
The button used to take the picture on a camera is often (and wrongly) called the shutter. The correct name is the shutter release button.
The Image Sensor
 A digital camera has a lens, an aperture, and a shutter just like the film camera. The fundamental difference is that the film is replaced with an electronic sensor which consists of millions of individual light-sensitive cells (called pixels) that can record the amount of light falling on it. However to measure the colour of the light and not just the intensity, a coloured filter needs to be put over each pixel. The most commonly used Bayer Filter has 25% blue pixels, 25% red pixels and 50% green pixels (as the eye is more sensitive to green) although others exist. Each pixel is only capable of recording the amount of that pixel's primary colour and not the full colour.
To convert this grid of individual primary coloured pixels into an image that can be stored on the memory in the camera, a process called demosaicing must be carried out by the imaging processor inside the camera. This converts the single-coloured pixel into a full colour pixel by looking at the surrounding pixels. For example, every blue pixel has four green pixels on each side and four red pixels on each diagonal. From this it can compute the RGB (red, green, blue) value for that pixel.
 The end result of this process is an image with the same number of pixels as the sensor but each one a full colour pixel. When we look at an image on a computer - this is exactly what we are seeing: a grid of pixels, the colour being defined by the amount of red, green and blue. If there are a large number of pixels in an image, you will not see the individual dots as they will be very small compared with the size of the image. The number of pixels in an image is called the 'pixel resolution'. The higher this is, the better the image will generally look.
Pixel Resolution and Spatial Resolution
It is a common mistake to refer to a camera image size in pixels (eg. 3456 x 2304) or its total number of pixels (eight megapixels) just as the resolution of the camera. This should be referred to as the pixel resolution.
In contrast, a camera's ability to resolve detail in the images that it captures is called spatial resolution and is a property of the whole camera system and not just the pixel resolution. A poor quality lens on a camera will give that camera a lower spatial resolution than when fitted with a high quality lens.
Spatial resolution can never exceed the pixel resolution. |
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| The above image has 60,000 pixels in it. |
This version of the image (which has been enlarged to the same size as the one on the left) has only 1,600 pixels. The individual pixels are clearly visible. |
CCD, Super-CCD, CMOS and Foveon
You will sometimes see the abbreviation CCD used in place of the word 'sensor'. A CCD is just one type of image sensor albeit the most common one.
A CCD (charge-coupled device) is the most common type of sensor. It consists of a rectangular grid of pixels as described above. However Fuji has developed its own variant called Super-CCD which uses octagonal photo sites as shown in the picture below. Super-CCD offers greater horizontal and vertical resolution at the expense of diagonal resolution. A recent variation is called Super-CCD SR which uses two different types of photo sites: high and low sensitivity. Fuji claims that this increases the dynamic range of the sensor by two stops (+2 EV) compared with a normal CCD.
A CMOS sensor is similar to a CCD in that it utilizes a traditional grid of pixels. The difference is that the individual pixels are randomly accessible and, in theory, the data from the sensor can be accessed much more quickly. Early CMOS sensors suffered from significant noise problems, however Canon has developed some excellent low noise sensors used in all their DSLRs.
Foveon is the only sensor used in a mainstream camera (the DSLRs made by Sigma) where the individual pixels are not limited to a single primary colour. Every pixel records a value for the red, green and blue component, thereby eliminating the need for a Bayer filter and the demosaicing process. Although the sensor is able to record much more of the light entering the camera (as the colour filters over each photosite on a traditional sensor block two-thirds of the light from ever being detected), a matrixing process to convert the colours into a standard colour space can increase the noise in low-light situations.
In a comparison between a 14MP Foveon sensor (4.7MP of each primary colour) and a 12.7MP traditional sensor, the latter has better mono detail, but the Foveon has greater colour resolution.
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Left to right: Normal CCD layout, Fuji Super-CCD HR, Super-CCD SR, Super-CCD SR II. |
The Numbers Game (Megapixels)
After reading the previous section you could easily believe that the number of megapixels a camera has is the most important indication of quality. Many of the leading manufacturers would also like you to believe that. If they can convince you that the five megapixel camera you bought 18 months ago belongs on Antiques Roadshow and that their latest eight megapixel will improve both your images and your appeal to the opposite sex, you'd probably rush out to the shops and buy one now.
Let's look at the facts (here comes yet another bullet list).
- An eight megapixel camera has 60% more pixels than a five megapixel camera - however this is an increase in the area of the sensor. The image's horizontal and vertical resolution increase significantly less than this (in fact it's about 26%). Therefore your prints can only be made 26% larger without a loss in quality. To double the print size from a five megapixel camera would require a 20 megapixel sensor.
- The five megapixel camera will produce perfectly good 10x8 / A4 prints. So will the eight megapixel camera. Under close scrutiny an A3 or 12x18" print may benefit slightly from the higher megapixel rating, but how often do we print that size anyway?
- The physical size of the eight megapixel sensor may not be any larger than the five megapixel sensor. To see why this matters read the next section.
- Each picture file size will be roughly 60% greater, meaning more space taken up on the hard disk, more time spent doing backups (which of course you do on a regular basis) and larger, faster and more expensive memory cards needed for the camera.
That upgrade isn't looking too sensible at the moment is it? But we like to give a balanced argument, so …
- A higher megapixel camera will allow you to crop the image more at the editing stage.
- If you want to print at 12 x 18" or A3 size, then the eight megapixel may produce better images (see next section).
- Ignoring the number of megapixels, a modern camera will have improved in many other ways over an older model. So although the pixel count in itself might not be justification to upgrade, there may be other features (such as a better zoom lens, improved low light performance, etc) that may justify being extra nice to your partner in the weeks before Christmas.
The message we want to get across here is not to fall into the trap of thinking that a camera's megapixels figure is the most important measure of image quality - it is actually very low down on the list. Nikon make a fabulous entry level DSLR (the D40) which does not come cheap and is 'only' six megapixels. But take a look on www.flickr.com, search for "Nikon D40" and look at the results for yourself. You won't find any eight or 10MP compact to match it (even ones made by Nikon).
Can You Have Too Many Pixels?
This may sound a strange question - why would anyone choose a lower resolution camera over a high resolution one? In theory, increasing the resolution will give you a more detailed image. However the physical size of the sensor, or more specifically the physical size of the individual pixels, also has a major impact on quality. The smaller the size of a pixel, the less amount of light it will be able to gather, and therefore to capture images in low light will require greater boosting of the signal from the sensor, the consequence being much more noise being introduced into the image.
If you have two sensors of the same physical size (one with seven megapixels and the other with 12 megapixels), the size of each pixel will be smaller on the higher resolution sensor and therefore the potential for noise increases. High resolution sensors must be made larger to maintain the pixel size and keep noise under control.
Sensor size is one of the major differences between a compact camera and a digital SLR. Typically a compact's sensor will be between 5-8mm wide whereas a DSLR sensor will be 18-36mm wide. The most common size (called APS-C) is around 24mm wide. See later section on how sensor size and noise are related.
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Photographs
This is a site about photography so I'm sure you are expecting to see plenty of pictures.
For now, why not take a peek at the flickr galleries belonging to the two authors of this site.
Colin's Flickr Page
Phil's Flickr Page
"A camera is the 'Save' button for the mind's eye." - Roger Kingston
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