Anyone looking at the cameras the manufacturers have been offering over the past several years could easily believe that the higher the camera’s megapixel count the better. But, as the old jazz standard says: ‘It ain’t necessarily so!’ 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.

 

Anyone looking at the cameras the manufacturers have been offering over the past several years could easily believe that the higher the camera’s megapixel count the better. But, as the old jazz standard says: ‘It ain’t necessarily so!’ 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.

Sensor Size
One key issue camera manufacturers shy away from disclosing is actual sensor size. We remain bemused by the way camera manufacturers always quote sensor size in totally meaningless fractions. How big is a 1/2.5-inch imager? Is it larger or smaller than a 1/1.8-inch CCD – and by how much?

For the past couple of years Photo Review has translated these irrational ‘measurements’ into dimensions in millimetres so readers know just how small most digicam sensors actually are. (As far as we’ve been able to determine, we’re the only magazine and website that does this.) The table below shows the dimensions of some of the most popular sensor sizes.

Sensor “Type”

Imaging Area dimensions
(w x h x diameter)

1/2.5-inch

5.76

4.29

6.9

1/1.8-inch

7.18

5.32

8.93

1/1.7-inch

7.6

5.7

9.5

2/3-inch

8.8

6.6

11.07

Compare these dimensions with the larger sensors used in DSLR cameras. A typical ‘APS-C’ sized sensor measures 25.1 x 16.7 mm and has a diagonal of 30.1 mm. A 35mm film frame (and ‘full-frame’ DSLR sensor) measures 36 x 24mm with a diagonal of 43.3 mm.

Photosite Area
If you know the size of a camera’s image sensor and the pixel dimensions of the largest image it can produce you can calculate the surface area of its light-collecting photosites. The more light the sensor collects, the less the signal has to be amplified to produce an image. This is probably the most critical factor in determining overall picture quality, because amplifying low-level signals inevitably leads to an increase in the noise that is associated with all electronics.

When you have a 6-megapixel camera with a sensor that measures 5.76 x 4.29mm and use it to produce an image with 2816 x 2112 pixels, the surface of each photosite measures 0.002045 millimetres squared. That’s just over two microns in each direction!

However, if you have a 6-megapixel camera with a sensor measuring 23.7 x 15.6mm and use it to produce an image with 3008 x 2000 pixels (which is roughly the same resolution as the sensor mentioned above but with a different aspect ratio), each photosite measures 0.00788 millimetres squared – or 7.88 microns squared. Each photosite on this sensor has almost 16 times the surface area of each photosite on the smaller sensor.

The number of photons a photosite can capture is proportional to its surface area. The random noise generated by the random emission of photons from most common light sources is relatively constant.

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).

Other Factors
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, which also boosts 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 may produce very noise-affected images.

Output Size
The resolution you need in a DSLR will also be influenced by how big you want to print your pictures – and how much you are likely to crop shots for enlargement. More megapixels means more detail is recorded and, consequently, you can make larger prints or crop shots without worrying about the image’s pixel structure becoming visible when the result is enlarged. However, 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.

Another factor to take into consideration is 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. And 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. When full-frame prints from Raw image files are viewed at that distance it’s very difficult to see any difference between a 6-megapixel and a 10-megapixel camera. (The shots from the 10-megapixel camera have slightly greater potential for enlargement, however.)

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output

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. Note also that it may be difficult to see much difference between prints made from cameras with the ‘ideal’ resolution for the output size and those made from cameras with slightly lower resolution imagers with larger photosites – especiallywith the biggest enlargements.

Film vs Digital

For photographers who are still fretting about film vs digital, PMA International offers the following comparison: A 35mm colour negative has approximately 14 million to 17 million colour dye blobs. This can be thought of as 14MB to 17MB in digital terms. If we think we need a sampling rate of two-to-one for proper sampling to surpass the resolution of 35mm film, a sensor will need to generate a file of approximately 28MB. With 24-bit colour, the threshold for non-compressed images is in excess of 8 megapixels to give approximately the same resolution as 35mm film.

We hope that puts the issue to rest.