Pixel Size on Sensor


Today’s Question: I’m thinking of switching from a full frame camera to the micro four-thirds format. If the sensors have equivalent pixel count, how is image quality affected by the smaller pixel size of the four-thirds sensor when compared to the larger pixels of a full-size sensor.

Tim’s Quick Answer: While a smaller individual pixel size on the sensor improves the ability to resolve fine detail, the bigger concern would be the increase in noise (and reduced dynamic range) that results from the smaller pixel size.

More Detail: Technically, of course, the sensor doesn’t actually have pixels, but rather photo sites that gather an electrical charge based on the amount of light striking each photo site during the exposure.

In order to compare the size of the individual photo sites, you need to consider both the overall resolution of the sensor (generally presented as how many millions of pixels, or megapixels, the sensor captures) as well as the overall dimensions of the sensor. With increased resolution or decreased sensor size, individual photo sites of course need to be smaller.

As you can probably appreciate, smaller photo sites represent greater relative resolution, which translates into a greater ability to resolve fine detail. So you could reasonably expect a camera that employs a sensor with smaller individual photo sites to render greater overall detail in a photo.

However, the problem with smaller photo sites is that less information can be recorded by each individual photo site. In the context of a photographic image, the information being captured is light. So you’re capturing less light with a smaller photo site than you could with a larger photo site.

Capturing less light means the sensor will capture less dynamic range, increasing the risk of clipping for highlight or shadow detail. In addition, less light gathered will require greater amplification, which in turn will result in more noise in the final photo.

There are obviously many other variables involved, so you can’t automatically assume that a smaller sensor will result in greater noise. But as a general rule, you can expect a sensor with a higher density of photo sites to provide decreased dynamic range and increased noise, all other things being equal.

I would, of course, recommend evaluating individual camera models when it comes to overall capture performance. Some sensors (and overall camera systems) are better than others when it comes to maximizing the amount of information that can be gathered in a photographic exposure while minimizing the amount of noise.