IMAGE SENSOR

An image sensor is a device that converts an optical image to an electric signal.

Technology / CCD vs CMOS

There are two technologies for an image sensor.

A charge-coupled device (CCD) is an analog device. When light strikes the chip it is held as a small electrical charge in each photo sensor. The charges are converted to voltage one pixel at a time as they are read from the chip. Additional circuitry in the camera converts the voltage into digital information.

A CMOS chip is a type of active pixel sensor (APS) made using the CMOS (complementary metal-oxide-semiconductor) technology. An active-pixel sensor is an image sensor consisting of an integrated circuit containing an array of pixel sensors, each pixel containing a photodetector and an active amplifier. APS is an alternative to CCDs.

Advantages of APS sensors (based on CMOS technology):

1. Consume less power than a CCD 2. APS sensors can combine the image sensor function and image processing functions within the same integrated circuit, so they are faster and less expensive. 3. Windowing readout. 4. Advanced white balance control.

Image sensor format / Aspect ratio

The image sensor format of a digital camera determines the angle of view of a particular lens when used with a particular camera. 35 mm film format is used as a reference to the digital cameras.

Image sensors in digital cameras tend to be smaller than the 24 mm x 36 mm image area of full-frame 35 mm cameras, and therefore lead to a narrower angle of view (aspect ratio 3:2).

The most common aspect ratios are 4:3 and 3:2. Nowadays most digital cameras use sensors around the size of APS-C film with a crop factor of 1.5-1.6. Crop factor is the ratio of a 35 mm frame's diagonal (43.3 mm) to the diagonal of the image sensor. Advanced Photo System type-C (APS-C) is an image sensor format approximately equivalent in size to the Advanced Photo System "classic" size negatives. These negatives were 16.7 × 25.1 mm and had an aspect ratio 3:2. For Nikon cameras: the alternative name for APS-C format is DX technology.

So a modern digital camera is more likely to have either full-frame format or APS-C format (DX Nikon).

Resolution / Pixel count

The number of pixels n for a given maximum resolution (w horizontal pixels by h vertical pixels) is the product n = w × h. Basically, higher resolution means more image detail, but resolution in pixels is not the only measure of image quality. Average resolution of the modern digital cameras varies from 10 to 20 megapixels, but can be up to 60. Effective pixels are the number of pixels that contribute to the actual image. This is always less than the total number of pixels (some are covered with a black due to establish a type black level and others are cropped from the edges). Sometimes there are engineering difficulties that also limit the number of effective pixels.

Conclusion

Now we can read the specifications, for example SONY DSLR-A580Y SMOC sensor APS-C size 23.5 x 15.5 mm, total pixels 16.7 megapixels, effective pixels 16.2 megapixels.