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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

This Article Features Photo Zoom

This begs the question of why the digital cameras available today seem to fall so far short of the potential when it comes to resolution. The situation is rather complex, and a wide variety of factors influence what’s possible. One of these, of course, is cost. What’s possible and what’s affordable to a broad audience tend to be at odds with each other when we’re talking about technology. More advanced sensor designs require more complex manufacturing with less tolerance for error.

To some degree, the size of the image sensor itself dictates the potential resolution. The size of the photodiodes is determined in large part by the technology employed to ensure an acceptable level of image quality. Obviously, for a given photodiode size, a larger physical sensor size allows more photodiodes and, thus, greater resolution. You certainly could opt for a larger sensor size to increase resolution, but that ignores the desire to maintain compatibility with the large number of lenses already available, as well as the desire among many photographers to work with a camera that’s relatively small and lightweight.

Noise-Reduction Technology
Without question, the single biggest factor affecting the rate of development for image sensors is noise. Image sensors are at their core signal recorders; the particular signal they’re recording is the intensity of light. The opposite of signal is noise, which in many ways can be thought of as ambiguity in the signal. The result is random variations in the luminosity and color of individual pixels.

Noise comes from a variety of sources and, thus, there are a variety of methods for combating noise. The first defense against noise is to avoid it in the first place. That calls for utilizing the largest photodiodes possible, because the larger the photodiode, the greater signal that can be captured. More signal equates to less noise. Of course, this directly conflicts with the desire to have increased resolution, which only serves to reinforce the significant role played by noise in reducing image-sensor resolution.

To address the issue of CMOS image sensors being unable to use the full available photosite as a light-recording medium (since other circuitry is included along with each photodiode), microlenses are placed in front of each photodiode on many image sensors. These microlenses focus the light from a larger area (potentially the full photosite) to the smaller area covered by each photodiode. The result is a greater signal and, thus, less noise. The problem is that recent camera models have seen these microlenses brought closer and closer together, to the extent that, in some of the latest sensors, those microlenses are all in contact with each other. That leaves no margin for further reducing noise in this way, obviously.

How A CMOS Works
Each pixel has a converter to convert the charge to voltage.

Advantages: Low power consumption, faster speed is easy, on-chip peripheral circuits possible
Disadvantages: Irregular pixels, random noise


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