The Sensor – Size Does Matter After All
Megapixels, megapixels, megapixels . . . the more the better. Right?
Wrong. I will explain why, but first a brief explanation of what megapixels are and how they relate to your camera.
A camera sensor is an integrated circuit (IC) chip that has the property of reacting to light by responding with an output voltage which is interpreted by the camera’s internal image processor as a digital one or zero. Each photo-reactive site on the chip is called a pixel. The more pixels, the more information about the image the sensor is able to record. This is similar to the pixels used on LCD screens and to a lesser extent the ink dots printed on paper by modern ink-jet printers. In general the more dots, or pixels and the closer they are together, the greater resolution (quality and accuracy) of the resulting image we perceive.
In an ideal universe the only thing we would need to consider is the number or megapixels. But, as we all know, the universe is not ideal. In addition to number, we also need to consider the pixel spacing or density and sensitivity.
As this is intended to be a primer on cameras, I will leave the topic of sensor sensitivity for another time (if ever).
Pixel density, or how many photo-sites are crowded into how small of a space, is a major factor in sensor performance. The problem is one of noise.
Think in terms of moving from the suburbs into a cheap apartment. Inside the isolation of your house your neighbors might as well be on the moon for all the impact they normally would have upon you. But, one of the joys of apartment dwelling is that you get to hear everything that happens next door and above and below you. The noise or cross-talk is communicated from apartment to apartment due to the close quarters and poor insulation (sound deadening).
The same thing happens in camera sensors. Squeezing a large number of pixels into a small sensor chip means that every pixel hears and is influenced by what is going on at the neighbor’s. Modern cameras utilize sophisticated computer algorithms and circuitry to combat this noise problem, but it remains a problem, a problem that becomes more and more prevalent as exposure conditions become less ideal i.e. in low light scenes.
For this reason, with all else being equal, when it comes to camera sensors, the bigger the better.
The picture below compares some common sensor sizes. Full frame (35mm) refers to the most common size of photographic film image. As previously mentioned, the first digital cameras simply replace the film with a similarly sized photo sensor. The small frames shown, 1/1.8”, 1/1.6” etc are sensor sizes that are commonly used in compact digital cameras as opposed to the larger sensors (Full Frame, APS-C and Four Thirds) commonly used in DSLRs.
Megapixel for megapixel a larger sensor will produce a better, lower noise image and allow higher sensitivities than a small sensor.
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