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The image sensor is the device at the heart of every digital camera.
There are two types in common usage
neither offers a distinct advantage in image quality, but CMOS sensors are lower cost to produce.
Both types perform the task of recording light energy and then converting this into an electrical signal. This signal is then converted to a digital code which can be processed by the camera's computer to form a digital image.
Sensors record the light energy at individual receptor sites called pixels.
Modern sensors have thousands of these pixels across their width and height.
For example a sensor 4500 pixels wide x 3000 pixels tall has a total of 14,500,000 pixels – or 14.5million. This would be described commercially as a 15 mega pixel sensor with and aspect ratio of 3:2
The size of the sensor can be expressed by the number of mega pixels and by its physical dimensions.
The number of mega pixels essentially only relates to the size of print that can be produced at a given print resolution.
Higher mega pixels do NOT necessarily mean higher image QUALITY.
Sensors that are physically larger in dimension tend to have larger pixels – this means the recorded light has a higher signal to noise ratio. This DOES give better image quality especially when higher ISO settings are used.
Smaller sensors require shorter focal lengths to achieve the same angle of view compared with larger sensors. This means that required aperture sizes are also proportionally smaller – with the result that the depth of field for smaller sensors is inherently deep – which limits the creative effects available to the photographer. The smaller apertures also mean that diffusion effects will occur at lower f numbers – so, small compact cameras are often limited to a smallest aperture of around f8.
The ratio of an image sensor size to a so called “full frame” sensor is known as the crop factor or focal length multiplier.
This means that field of view (and therefore degree of magnification) for a given lens is increased by the crop factor when compared to using the lens on a full frame camera.
E.g. a 200mm lens mounted on a camera with image sensor having a crop factor of 2 will give the same angle of view and magnification as a 400mm lens mounted on a full frame camera.
The cost of manufacturing sensors increases almost exponentially with physical size.
So a sensor which is twice as large may cost 4 times as much to produce. This explains (at least partly) the higher price of larger image sensor Dslrs versus smaller image sensor Compact cameras.
Even when subjects are perfectly focused, a blurred image can result.
There are two causes for this
Movement of the subject during the exposure.
Movement of the camera (and therefore image sensor) during exposure.
causes can be resolved by using a high shutter speed.
regard to “freezing” subject movement – the shutter speed
needed will depend on the speed of the moving object. One possible
solution is to “pan” the camera – which means following the
subjects movement by deliberately moving the camera such that
relative to the cameras view the subject does not move.
When shooting static objects with an exposure that doesn’t permit high shutter speeds (with small apertures or in low light situations) the ideal solution is to prevent camera movement by using a tripod (or placing it on a stable surface) and using a remote shutter release (or the countdown timer option)
When the situation requires that the camera be hand held then camera shake may result.
Image stabilisation systems help overcome camera shake by up to four exposure stops.
In terms of shutter speed this means up to 1/8th of the lower limit for a non stabilised system, may still give good non blurred images.
This greatly extends the low light capability for shooting static images when hand holding - particularly in combination with the relatively high ISO settings that may be used with modern Dslrs.
Image stabilisation systems may either be built into lenses or directly into the camera body.
There are pros and cons for each type of system.
Building the system into the lens has scope for a greater degree of stabilisation, especially at longer focal lengths and (with Dslrs) gives the benefit that the image in the optical viewfinder is also stabilised.
The downside is that constructing a lens with a stabiliser adds significantly to the cost and not all lenses are available with stabilized versions.
Stabilisers built into the camera body have the advantage that the image stabilisation will function irrespective of the attached lens.
The downsides are a lesser degree of maximum stabilisation and an optical viewfinder will not have the stabilised view.
The image recorded by on a cameras image sensor depends upon three primary factors
The brightness of the scene being photographed.
How much of this light is able to fall upon the image sensor during exposure – determined by the aperture size and the shutter speed.
By how much the analogue light signal recorded by the image sensor is AMPLIFIED during conversion to the digital data used to create the final image.
The ISO setting deals with the last of these three factors.
ISO settings are normally calibrated such that each time they are doubled the effective brightness of the exposure will change by one stop – i.e. the effect will be the same as halving the shutter speed or doubling the aperture area.
This is best understood by considering an example.
Suppose a photo taken at the following settings is too dark
Aperture – f5.6
Shutter speed – 1/250 sec
to retake the photo with the brightness of the exposure increased by 1 stop (which is the effect from DOUBLING the light intensity) the options are any ONE of the following
INCREASE aperture size by LOWERING the f number to f4
SLOW shutter speed by changing to 1/125 sec
the ISO to 200 – to “boost” the light signal received.
Unfortunately increasing the ISO to achieve desired exposure does have a downside – that of increasing digital NOISE.
The ISO setting at which noise becomes unacceptable depends upon numerous factors, the main one being the signal to noise ratio.
Basically the larger (as in physical dimensions NOT number of mega pixels) the sensor, the higher the incoming light signal to noise ratio – i.e. there is proportionately less noise to amplify for a given exposure – so the image quality is better.
For this reason, Dslrs (with larger sensors) perform better than compact cameras and camera phones (smaller sensors) at higher ISO settings and therefore give greater image quality and exposure flexibility and creative control in low light situations.
In summary, achieving exposure by raising ISO can be considered as a “last resort” method to “boost” exposure best used only when adjusting aperture or shutter speed (or using flash to brighten the subject) is either not viable or undesirable for creative control reasons.
JPEG (Joint photography experts group)
Light falling upon each pixel on the image sensor is converted to electrical signals which are amplified and then converted into digital data.
This digital data can then be processed into a format that computers (or printers) can recreate as an image.
This raw uncompressed data is capable of reproducing the original image with a very high degree of accuracy but has the downside of having a large file size.
The JPEG system processes the image using complex series of algorithms that take account of our eyes' limitations in detecting changes in colour and brightness intensity. This enables image data to be compressed with a reduction of file size but without a perceived loss in image quality.
The degree of compression and resulting reduction in file size can be increased to varying degrees until the reduction in image quality becomes apparent.
The extent to which lower image quality can be perceived depends upon the image size and the distance at which it is viewed.
For example small images displayed on websites may be compressed much more than files to be printed in large format at high resolution.
Most cameras provide the choice of captured images being stored on the memory card either in the original RAW format or being processed as JPEGS “in camera” at a choice of compression levels. (Or being stored in both formats simultaneously)
When the camera processes a JPEG image, in addition to the data compression various other image settings such as white balance, sharpening and colour saturation may also be applied.
If shooting in JPEG it is recommended to use the highest quality setting as additional compression (and therefore reduction in file size) can always be applied latterly in post production.
An aspect of JPEG images that it is important to understand is that each time an image is processed – even at the same compression setting, some loss of data and therefore image quality may occur. For example opening an image, rotating it and then saving it and then repeating this exercise, will cause additional compression each time.
It is therefore preferable to save off any image modifications as a copy file and thus preserve the original highest quality original version.
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