What are Effective Pixels?

Understanding Digital Pixels in Photography

If you look at the specifications of any digital camera you will notice two listings for the pixel count: effective and actual (or total).

Why are there two numbers and what do they mean? The answer to that question is complicated and gets pretty technical, so let's take a look at each.

What are Effective Pixels?

Digital camera image sensors consist of a number of pixels, which collect photons (energy pockets of light).

The photodiode then converts the photons into an electrical charge. Each pixel has only one photodiode.

Effective pixels are the pixels that are actually capturing the image data. They are effective and by definition, effective means "successful in producing a desired effect or intended result." These are the pixels that are doing the work of capturing a picture.

A conventional sensor in, for example, a 12MP (megapixel) camera has an almost equal number of effective pixels (11.9MP). Therefore, effective pixels refers to the area of the sensor that the 'working' pixels cover.

On occasions, not all sensor pixels can be used (for instance, if a lens cannot cover the whole sensor range).

What are Actual Pixels?

The actual, or total, pixel count of a camera sensor includes that (approximately) 0.1% of pixels left over after counting the effective pixels. They are used to determine the edges of an image and to provide color information.

These leftover pixels line the edge of an image sensor and are shielded from receiving light but are still used as a reference point which can help in reducing noise. They receive a signal that tells the sensor how much 'dark' current has built up during an exposure and the camera compensates for that by adjusting the value of the effective pixels.

What this means to you is that long exposures, such as those taken at night, should have a reduction in the amount of noise in the deep black areas of the picture. There was more thermal activity while the camera's shutter was open, which caused these edge pixels to activate, telling the camera sensor that there may be more shadow areas to be concerned with.

What are Interpolated Pixels?

Another factor of concern with camera sensors is that some cameras can interpolate the number of sensor pixels.

For instance, a 6MP camera may be able to produce 12MP images. In this case, the camera adds new pixels next to the 6 megapixels it captured to create 12 megapixels of information.

The file size is increased and this actually results in a better image than if you were to interpolate in an image editing software because the interpolation is done before JPG compression.

However, it is always important to remember that interpolation cannot ever create data that was not captured in the first place. The difference in quality when using interpolation in camera is marginal.