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Camera Basics - Sensors Pt. 2
Category: Photography Basics
Updated 6 months ago by renan

Although there are various sensor sizes available for different camera models, it is equally important to understand how these sensors are being used within the camera. The manufacturer's decision on how many pixels and what kind of imaging technology is used, offers different degree of image quality or "flexibility" to the photographers.


PHOTODECTECTORS. A digital image output from a camera is composed of an array of much smaller elements called a pixel (short for picture element). Each of these pixels are produced by the sensor using multiple photo sensitive components.

closeup cmos sensor

Magnified view of a typical CMOS Sensor

As you can see in the image, the sensor is composed of very small elements called the photodetectors. These photodetectors capture the component colors of the light that falls into each area using filters, and sends them out as electrical signals, some photodetectors capture Red, some Green and others Blue. The size and position of the photodetectors vary depending on the manufacturer. The color of a pixel is determined by combining the values from these elements in a process called "demosaicing" which results in varying degree of accuracy in color reproduction in the captured image. This is why you will notice that in some cameras the output is a slightly cooler hue compared to another which might have slightly warmer colors. In other cameras, some colors may appear too strong and others may appear too pale.

This quality of the sensor is something you cannot see in the specifications, but can only be judged based on actual outputs.


SENSOR RESOLUTION. Depending on how many times the sensor is subdivided vertically and horizontally determines the smallest detail a sensor can capture. A sensor that has 3,000 columns of pixels and each column is a row of 2,000 pixels will have more details than a sensor that only has 1,500 columns and 1,000 rows. These measurements are called the sensor's resolution. It is commonly expressed by the horizontal and vertical pixel count or the total number of pixels, so, for the first sensor the resolution is 3,000 x 2,000 pixels or 6,000,000 pixels or 6 megapixels (MP). The first expression is more informative as it also shows the actual vertical to horizontal ratio, but the second one is more generic and easier to memorize.

Due to the advancement in production technology of image sensors, it is very common to see large sensor resolutions such as 8-12 MP even on basic smartphones, 12-18 MP on point-and-shoot cameras, 16-24 MP for most four-thirds and APS-C systems, and 24-50 MP can easily be found on Full Frame cameras.

However, large resolution doesn't automatically equate to better image quality. The higher the resolution, the size of the photodetectors becomes proportionally smaller until a point when the sensitivity and reliability of the photodetectors becomes questionable.


SENSITIVITY (ISO) - The sensitivity of a sensor is measured using the ISO system which was introduced by the International Organization for Standardization for film cameras. These values has been adopted to represent a similar standard in digital sensor sensitivity. The sample below is from a typical output from a Canon APS-C sensor without any post processing.

iso sensitivity comparison

The higher ISO values represent higher sensitivity. In digital cameras, this is achieved by amplifying the output signal of the sensor proportionately according to the ISO value selected. This is very useful for capturing a relatively dark scenes as it will make the scene look brighter. However, as these signals were produced artificially by amplification, there is a tendency for the output to become inaccurate, especially if the photosensors are very small. This inaccuracy is evident in images captured by small sensors at high ISO where specks of random variation of colors start to appear in dark areas -- these are called "noise".

To sum it up, the compromises you have to make when selecting a proper sensor size for your needs are:

1. Size of the sensor - large sensors have really good image quality but is expensive and also requires a large body to accommodate it.

2. Type of sensor - CMOS is adequate even for professional use, cheaper and is more efficient. CCDs and Foveon may have a bit of an image quality advantage, but, are more expensive and also requires expensive components to make them work.

3. Resolution - Higher image resolution may give more details, but also reduces the accuracy of color reproduction, specially on small sensors.

4. Sensitivity - The capability of the camera to artificially raise the sensitivity of the sensor is useful but may also introduce unwanted levels of noise in your output.

Other factors you may also need to look into when checking a sensor and image production capabilities are Dynamic Range, Noise Reduction, Sharpening, and other post production options. These vary among manufacturers and deals greatly with correcting the final image output.


Related Links:
Camera Basics - Sensors Pt. 1


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