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Tuesday, October 5, 2010

Megapixels And ISO: Have We Reached The Limit?

Although there’s no question that the pace of increase has slowed, noise-reduction advancements are poised to fuel continued resolution boosts

You’ve heard the assertions before: The “megapixel wars” are over. We’re operating in the land of diminishing returns. It’s all evolution and no revolution. We’ve reached a plateau. Truth be told, I began preparing to write this article on the assumption I’d be explaining exactly why image sensors are indeed reaching a limit and why we’ll need to start looking elsewhere for improvements in digital cameras. But along the way, I was more than a little surprised at what I discovered, and now I’m convinced there’s still a lot of room for improvement with the image sensors that are the cornerstone of digital photography.

A Little Background
To be sure, things do seem to have slowed down on the resolution front. It took only two years to double from 3 megapixels to 6 megapixels among early digital SLRs. But six years ago, the top digital SLRs were in the 16-megapixel range, and as of this writing, we certainly haven’t doubled that. That would suggest development has slowed down, that we’ve reached some sort of limit, and that we really can’t expect much in the way of increased pixel count. Of course, the increases in image-sensor resolution have come in fits and starts, as a new high-resolution image sensor naturally isn’t replaced overnight. When you take a wider view, you realize that while there certainly was a big jump early on, we’ve been making steady progress ever since.

Some of this is clouded by the fact that image sensors tend to be developed for and targeted to a particular line of digital cameras. This makes an accurate comparison difficult, since you need to consider trends across multiple camera lineups for which there are varying goals. Early on, there generally was only one line of digital SLR from each major manufacturer. There are now several lines from each camera maker, and some of those lines focus on an audience that’s more interested in a small, lightweight camera at a relatively low price point than having a camera with the maximum possible resolution.

Anatomy Of An Imaging Sensor
Before delving more deeply into the issue of whether we’ve reached a limit in what image sensor technology is able to provide moving forward, it can be helpful to have an understanding of that technology.

Today’s digital cameras use two primary types of image sensors: the charge-coupled device (CCD) and the complementary metal-oxide semiconductor (CMOS). These two technologies actually have a lot in common. Both were created in the late 1960s (quite old by technology standards), and both have very similar physical structures. Individual photodiodes collect an electrical charge when exposed to light, and that electrical charge is converted into a voltage value that’s recorded as data. It’s what happens in between that represents the biggest difference between these two types of image sensors.

In a CCD, the electrical charge accumulated by each photodiode must be transferred to the edge of the sensor, where the charge can be converted to a voltage value and recorded as data. This process is, in some ways, inefficient. It requires greater power to transfer the charge than it would to simply transfer the voltage value, as well as more time to transfer the charge row by row to the edge of the sensor than if all values were recorded in parallel.


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