Chapter 02
How Many Megapixels Do You REALLY Need?
Don’t be seduced by the megapixel counts touted in advertising materials and on camera packaging. It’s no longer true that the higher a camera’s megapixel count the better. The only thing more megapixels will give you is the ability to enlarge and crop pictures without individual pixels becoming visible. Other factors are much more important in determining overall picture quality.
Output Size
Megapixel resolution plays an important role in how large you can print your pictures. Because the more megapixels you have, the more detail is recorded, high-resolution cameras allow you to make larger prints or crop shots without worrying about the image’s pixel structure becoming visible.
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The diagram above provides a guide to the ideal resolutions for three popular print sizes. Note that output resolution decreases as print size increases because larger prints are viewed from a greater distance.
Paying for pixels you don’t need is a waste of money. It’s better to invest in a camera with a better quality lens, larger sensor and more effective image processor.
You should also consider how well your printer can reproduce fine detail. There’s little point in shooting or scanning pictures at high resolution and creating huge image files unless you have a printer that can reproduce all the detail in the image.
There comes a point beyond which high output resolution on its own is irrelevant. Only the pickiest of viewers will look at an A3+ print close up; most of us prefer a viewing distance of 1-2 metres. At that distance it’s very difficult to see any difference between prints from a 6-megapixel and a 10-megapixel camera with the same sized sensors. (The shots from the 10-megapixel camera have slightly greater potential for enlargement, however.)
Sensor Size and Picture Quality
The size of a camera’s image sensor is the main determinant of picture quality and the larger the sensor area, the higher the potential for producing top-quality digital pictures. Equally important is the size of the actual photosites on the sensor that collect the image-making light. The larger the photosites, the more light they can collect and, consequently, the more image data they make available to the camera’s image processor. (Details of how to calculate the size of sensor photosites are provided below.
For the past couple of years Photo Review has translated the irrational ‘measurements’ quoted in manufacturers’ specifications into dimensions in millimetres so readers know just how small most digicam sensors actually are. The table above shows the dimensions of some of the most popular digicam and DSLR sensor sizes and, to emphasise the differences between the two camera types we’ve used red to highlight the digicam section of the table and green for the DSLR section.
As you can see, the sensors used in digital SLR cameras are substantially larger. The size differences are easiest to appreciate in the diagram below, which compares the areas of the most popular digicam sensor (1/2.5-inch type) with three DSLR sensor sizes sensor and a 35mm film frame.
Calculating Photosite Area
If you know the size of a camera’s image sensor and the pixel dimensions of the largest image it can produce, it’s easy to calculate the surface area of its light-collecting photosites. This is important because the more light each photosite collects, the less the signal has to be amplified to produce an image and the better the image quality will be.
The actual calculation is straightforward. Simply divide the length of one side of the imaging area by the number of image pixels that correspond to that side. For example, a 6-megapixel camera with a 1/2.5-inch imager produces a high resolution image of 2816 x 2112 pixels. Dividing the width of the sensor (5.76 mm) by 2816 pixels gives us 5.76 ø·2816 = 0.0020454 mm (or 2.045 microns).
The table below shows a range of typical photosite sizes for current consumer digicams in red and DSLR cameras in green with typical examples for each resolution category.
(1) Nikon Coolpix L6 (2) Olympus SP550UZ (3) Sony Cyber-shot DSC-T100
(4) Canon PowerShot G7 (5) Nikon D40, (6) Olympus E-410, (7) Pentax K10D, (8) Canon EOS-1D Mark III, (9) Canon EOS5D
Photosite Size and Picture Quality
The surface area of a sensor’s photosites dictates the number of photons (particles of light) it can capture. The more photons collected, the more information the camera can process – and the less the image is affected by the background noise that is generated by all electronic devices, which is relatively constant.
Image noise can affect digital photographs under a variety of situations, including:
* When the number of photons (the fundamental ‘particles’ of light) striking the sensor is very low;
* When the camera’s sensitivity (ISO) is set to a high value;
* When the temperature of the sensor is high,
* When there are errors in the transmission of the signal from the sensor to the processor, and
* When the digital image signal is amplified substantially.
The diagram below illustrates why larger photosites are less affected by noise than smaller photosites.
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The larger photosite on the left can collect many more photons of light than the smaller photosite on the right. But the amount of background noise is the same for both photosites. Consequently, the larger photosite has a much better signal-to-noise ratio. It can also collect more light with the same exposure time and, therefore, respond with higher sensitivity.
Identifying Image Noise
In most cases, noise can only be seen when the image is enlarged substantially – at least 200 times. Noise-affected pictures look ‘grainy’. Sometimes you may see a pattern of tiny white or coloured dots, scattered randomly throughout the image. Noise also reduces the sharpness of edges between bright and dark areas in the picture and can make it look unsharp and a little flat.
Sometimes noise can only be seen in shadowed areas, where exposure levels are low. It is common for this shadow noise to show a pattern of coloured dots. Long exposures are often associated with ‘hot’ or ‘stuck’ pixels, which can be seen as a pattern of bright and coloured dots that is repeated in all shots taken under the same conditions.
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Enlarged sections of two identical shots taken with ISO 200 (left) and ISO 1600 sensitivity, showing typical image noise.
Sensor Size and Tonal Reproduction
Sensor size can also influence the camera’s ability to record a full range of tones from white to black. Our regular camera tests consistently show that cameras with smaller sensors fail to achieve this in bright conditions. It’s common to find blown-out highlights and blocked up shadows in such shots. In contrast, DSLR sensors with larger photosites can usually record the full dynamic range in the subject (although you may need to shoot raw files in order to extract all the highlight and shadow details in brilliant sunshine).
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The illustration above has been cropped from a shot taken with a high-resolution digicam in bright sunlight. Note the lack of detail in the bright areas on the left wall of the house and also under the verandah near the front door. Contrast these areas with a similar shot taken with a DSLR camera, which can capture a full range of tones in both brightly-lit and shadowed areas because of its larger photosites.
Lens Quality
The size and quality of a camera’s lens can influence image quality at least as much as the sensor’s megapixel count. There’s a big difference between a small, cheap glass lens that you might find on an entry-level digicam and the high quality, multicoated lenses you would buy for a DSLR. Five megapixels is probably the limit of resolving power for most point-and-shoot camera lenses. Beyond a certain point, diffraction will begin to reduce the resolving power of the lens-plus-sensor system, as we have discovered from Imatest tests on many 8- and 10-megapixel digicams.
A further consideration is the way the camera’s image processor handles the image data. In many digicams, the image processor automatically sharpens the image by default. This can further degrade picture quality, especially if it’s already been reduced by diffraction. It’s not uncommon to find a 10-megapixel digicam with worse performance than a 6-megapixel camera with a similar-sized sensor.
Subject lighting will also play a role in image quality, especially with small-sensor digicams. In dim conditions, photographers are forced to increase ISO speeds. However, with a small-sensor digicam, this will increase image noise, thereby reducing image quality. As we’ve outlined above, larger photosites produce less image noise and give photographers more flexibility with ISO settings. Shooting at ISO 800 may be feasible on a DSLR while the same setting on a digicam will probably produce very noise-affected images.
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