Image Sensors: The Biggest Thing in Digital Photography Since Megapixels

By Ben Davis
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When digital cameras first appeared on the market, most rated in the appalling 2- or 3-megapixel range. When the first 5-megapixel digital camera hit the market, people could see the difference. These cameras still could not compete with 35mm film, but the digital revolution was catching up quickly. Today, digital cameras have knocked analog cameras off retail shelves, and the variety of cameras will fit anyone's amateur or professional needs. From professional DSLR to point and click cameras, digital cameras have earned their place at the top. However, despite the variety of uses and designs, most people still believe that more megapixels make a better picture. This isn't always the case.

Like many technologies, megapixels have become a marketing benchmark, used to advertise the product without proving the product's quality. Truly savvy photographers are more interested in the image sensor than in how many pixels will pick up light. If we compare a camera to a car, an image sensor is like an engine, and megapixels are more like the horsepower. A car may be measured in horsepower, but the engine makes the car move.

Of course, megapixels matter. There is a clear difference between a 5-megapixel camera and a 16-megapixel camera, regardless of the image sensor in each camera. But when you're comparing two 16-megapixel cameras, it is important to understand the different technologies and make your purchasing decision based on your needs. At a basic level, all image sensors consist of two parts: a photo-sensing plate and at least one output node. The photo-sensing plate absorbs light, much like film. The output node then converts the light into a digital signal. What makes image sensors different is how they combine these two elements.

The first image sensor to make digital cameras a worthy contender to replace 35mm film is the Charge-Coupled Device (CCD). In this setup, when the iris opens, the image sensor plate dedicates every pixel to absorbing light. All of this is filtered through a small number of output nodes to create a high-quality picture. For this kind of image sensor, the more pixels on the plate, the more accurate an image will be. Usually, these images take up more storage space, and if you're going to post your pictures online, a photo taken with a CCD image sensor might be overkill. Most websites that allow you to post your own images online will downgrade the image quality to save space and optimize for the site itself. On the other hand, if you like to frame your pictures or put them in scrapbooks, high-resolution CCD cameras offer a professional look in most lighting situations. In addition to creating larger files, these cameras also use more battery power to quickly turn light into a digital signal. With a CCD sensor, the cost of high-quality pictures is a quickly filling memory card and a quickly draining battery.

To combat the storage space and battery consumption of CCD image sensors, Complimentary Metal-Oxide-Semiconductor (CMOS) sensors came to the market about five years after the first consumer CCD-based digital cameras. In the original design, the output nodes were placed on top of the photo-sensing plate. Because the output nodes were on top of the light plate and blocking the light, the camera picked up only half of the light, so a 16MP CMOS camera would only deliver 8MP in performance. However each picture took up significantly less space, so these cameras were great for webcams, security cameras or toys, where pixilated or grainy images are not as much of a problem.

A spin-off technology was developed to correct the blocked light issue of the CMOS sensors by keeping the photo sensor in front of the output nodes. These MOS image sensors make the image size as large as a CCD camera, but unlike a CCD camera, the communication between the photo plate and the output nodes is highly efficient and uses less battery power than a CCD camera. The MOS image sensors are much more power efficient and use less battery power, but every image takes up as much file space as a CCD picture. As an alternative to CCD image sensors, MOS image sensors deliver equal quality, with every pixel absorbing light, while using more output nodes to share the burden of translating light to a digital signal.

The story would end here, if not for one more major advancement in CMOS technology. People were still not satisfied with the storage-sucking images from CCD and MOS image sensors, but the pixilated images from old CMOS cameras made them unusable by serious photographers. Photographers want to share their pictures, either on a social network or by printing and framing the pictures. The low-resolution CMOS pictures do neither. New, backlit CMOS image sensors corrected the fatal flaw of the first generation: Instead of placing the output nodes above the photo plate and blocking half of the light, the setup is reversed. Light hits the photo-sensing plates first, and the output nodes behind the plate convert the data to a digital signal. With the original CMOS sensors, a 16MP camera delivered 8MP of quality. Because the entire pixel plate is used in the updated CMOS technology, the resolution advertised accurately represents the camera's performance. This technology radically improves the design and image quality without raising the price of a camera.

Today, image sensors are best judged by your particular needs. If you plan on taking a lot of pictures in low-light settings and cannot use your flash, you'll want a CCD image sensor because it will pick up more light and deliver the cleanest image; however, you'll pay for it in higher storage space needs and a shorter battery life. If you will take pictures in multiple settings and plan on printing the images and framing them, MOS image sensors offer better variety and will work well in multiple lighting situations. If you are a casual photographer and mostly post your pictures online, backlit CMOS sensors allow for high quality digital images without eating up your storage space.

The average amateur photographer will not be able to spot the difference between the three main classes of image sensor, but with experience comes knowledge. Rather than using the wrong camera for years, we suggest determining how you will use the camera now and building your knowledge base with the right camera.

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