Sensors and Megapixels

Pixels per inch (PPI), dots per inch (DPI), megapixels, sensors, resolution … it’s enough to confuse anybody.

One of the selling features for a new digital camera is the number of megapixels the sensor holds, so let’s discuss that.

On the Bayer sensor that all digital cameras except Sigma come with, each pixel captures one colour: red green or blue. Sigma’s Foveon sensor has the pixels arranged in three layers so each “space” captures all three colours.

We’re told we need more megapixels because the more we have the more detail we can capture. One megapixel equals 1,000,000 pixels.

This is true to a point, but it is not the only factor to consider.

The small point-and-shoot camera claiming to have the same number of pixels as its larger digital single lens reflex (DSLR) cousin may have the same quantity, but they are not equal in capability.

The small size of the point-and-shoot sensor forces the miniaturization of pixels in order to achieve the same number of megapixels as the DSLR with the larger sensor.

The impact of this Lilliputian sensor with the high number of megapixels is degradation of sharpness and limited performance at higher ISO settings.

My first digital camera was a point-and-shoot with three megapixels from which I routinely printed quality 8×10 photographs. If it had had a higher number of pixels on the sensor, the final image would not have been as impressive.

Larger pixels have a higher dynamic range (the ability to capture the range of tones between black and absolute white) than the shrunken versions.

Full-frame sensors have the same measurements as the 35mm film negatives that preceded the digital cameras, 24mm x 36mm. The term “full-frame” is a misnomer, as there are larger sensors out there in other formats, but it is a descriptive term accepted for the DSLR market.

Most DSLRs come with a somewhat smaller APS-size (Advanced Photo System) sensor with minor size variations depending on the manufacturer.

See the chart for a comparison.

With today’s technology, the battle to create more megapixels on a sensor may well have reached the point of degrading the image in order to market the hype.

This doesn’t mean that you need to run out and get a full-frame DSLR.

There are other factors to consider as well, far beyond the number of megapixels.

Are you starting your system from scratch? Do you have existing lenses you’d like to use? Are those lenses high quality?

Certainly, if you are starting with nothing and have the ability to purchase higher-priced products, you may wish to consider going with the full-frame system for its ability to capture sharper, higher dynamic range photographs.

However, unless you are consistently producing large prints of 11×14 and larger the APS-sized sensors are more than adequate and the human eye will not perceive the difference.

The APS-sized sensors create a crop factor for your lenses that is both a blessing and a problem, depending on the type of photography you are doing.

The crop factor is created because the sensor is smaller than full-frame, so the lens can only focus on the area provided. This factor is in the neighbourhood of 1.5 to 1.7 depending on the camera.

On the plus side, if you have a 70-200mm lens, it now works as a 105-300mm (with the 1.5 multiplier) so will allow you to bring subjects much closer; great for photographing wildlife and sports.

Another positive is that lenses tend to be sharper toward the middle of the frame and the crop is from the centre out, thus creating sharper images.

On the negative side, if you are a landscape photographer who prefers wide-angle lenses, this factor works against you. Your 17 to 50mm lens gives a capture angle of 25.5 to 75mm, meaning you have to go for much wider lenses in order to achieve the angle you want.

Hmmm, perhaps a new 8-16 mm is in order for me …

If you have questions email them to [email protected] or post them after the column at

I look forward to hearing from you.

Happy shooting, and remember to leave the environment as you found it.

Norm Hamilton is a freelance writer and photographer in Whitehorse.

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