Photo Sensors In Digital Cameras
Creating photographs on digital cameras requires electronic photo sensors, which have replaced film as the image capture media. They were designed to be the electronic equivalent of the 35 mm film format. A 35 mm sensor is called Full Frame, while anything smaller are referred to as Crop Format sensors. Unlike film, the image is processed by digital electronic circuits so developing it is not necessary. Images are stored in devices like an SD card which can then be transferred to a computer for further editing.
The sensor consists of individual pits, called photosites. The photosites represent the pixels that creates a digital image. Each photosite captures the light from photons which will represent the image. The sensitivity of the sensor can be adjusted by the camera’s ISO level, while the aperture size controls the amount of light that hits the sensor. The more pixels the sensor has, the higher the resolution of the image. For example if the camera has a pixel resolution of 3266 x 2450 pixels, it has a total resolution of 8 million pixels or MP (Megapixels). Compare that to 4928 x 3264 which has double the total resolution at 16 MP. The latter has a higher resolution.
Most sensors also use color filters. This is because the photosites themselves don’t capture color, only the light. The most common is the Bayer CFA (Color Filter Array). It is an RGB color array that is placed on top of the sensor that provides color information for the image. When the light hits the filter, it corresponds to the wavelength of the color or a gradient of that color based on the color array. The filter consists of 50% Green, 25% Red and 25% Blue. This type of pattern is also called RGGB (sometimes also arranged as BGGR, RGBG or GRGB). The Green color represents the luminance-sensitive elements of an image and the Red and Blue represent the chrominance-sensitive elements. The Bayer Filter was developed to mimic the way the human eye interprets color. This is because according to study the luminance perception of the human eye is most sensitive to the color green. The colors of an image on digital cameras are based on this concept.
The image quality has plenty to do with the sensor’s size along with the type of lens used. The bigger the sensor, the better the quality because it collects more light to resolve the image. This is the advantage of large sensor size on DSLR, mirrorless and medium format cameras compared to smartphone cameras. That means even if a smartphone camera has the equivalent number of pixels, the camera with the larger sensor size can still resolve a better quality image. With regards to the lens, the build is a very important factor because the lens converges the light with the focal plane to capture the sharpest image. This is why major camera makers develop high-end lenses used in professional photography.
Types Of Sensors
There are 2 common types of sensors used in digital photography. The basic operation of the sensor is to expose it to light to capture the image and convert it into a digital signal.
CCD (Charged Coupled Device)
CCD was the original technology used for digital image capture. A CCD is made from semiconductors, common in electronic chips. Through the use of MOS (Metal Oxide Semiconductors), capacitors are built on the chip to store an electric charge. When image acquisition begins, a bias is applied to the circuit to shift the signals (either vertically or horizontally) captured in each photosite one at a time i.e. “bucket brigade”. This converts the photon to an electrical signal with a certain voltage level. Each photosite represents a pixel that must then be amplified and then it undergoes an analog to digital conversion. The CCD has what is called a passive amplifier.
They were used in the first generation of digital cameras as well as in various types of scanners (includes flatbed and handheld scanners) and photo-copiers. Fax machines, camcorders and some medical imaging devices (dental x-rays or mammographs) also use CCD sensors. The advantage of CCD is it has great dynamic range and noise control. Great dynamic range allows the image to be pushed several stops without increasing the noise. High image quality is the reason CCD are used in certain scientific and commercial applications.
CMOS (Complementary Metal Oxide Semiconductor)
CMOS are the most common sensors used in digital cameras. CMOS are electronic semiconductor IC chips that process photons into digital electronic signals. It works similar to the CCD, but instead of shifting, the CMOS uses an Active Pixel Sensor (APS) which does the signal amplification. This is much faster and more efficient than a CCD. The charge to voltage and amplification circuit are integrated within the CMOS sensor chip. Each one of the photosites in the CMOS has its own amplifier consisting of a transistor, thus they can do their own local processing.
CMOS sensors are more energy efficient and ideal for smaller form factors used in DSLR, mirrorless and smartphone cameras. CMOS requires less power, thus making them ideal for use with chargeable batteries. They are also faster, so they have the advantage of allowing cameras to shoot in bursts to capture the action. CMOS are also much cheaper to produce than their CCD counterparts. The only problem with an active pixel sensor is it has higher noise and rolling shutter issue (e.g. jello effect). Despite the issues, developers have worked around them to include ways to correct the problems (read more about that in this link). This has made the CMOS more commercially viable for consumer electronic products like digital cameras.
Sensor Sizes
Cameras don’t all have the same sensor size, though they may have the same sensor type. The sensor size can determine the resolution of the image. It also determines the amount of light used to create the image. Larger sensors can capture more light resulting in more details and the best images. It would mean better dynamic range, less noise and improved low-light imaging performance.
The largest sensors are used in Large Format and Medium Format cameras. The Large Format sensor measures greater or equal to 4 x 5 in and greater than the Medium Format sensor size. The Medium Format is larger than the Full Frame format (24 x 36 mm) but smaller than the Large Format size. An example of the Large Format camera is that found in the Wista Field-45DX 4 x 5 in sensor. The Hasselblad X1D-50C uses a Medium Format 43.8 x 32.9 mm sensor.
For consumer digital cameras, the Full Frame and Crop Format is more commonly used. As mentioned earlier, the Full Frame format is based on the 35 mm film camera in terms of its size (36 x 24 mm). It has no crop factor so what the photographer sees through the viewfinder is how the image will look like. Crop Format is much smaller than the Full Frame format. An example of crop sensors include the APS-H 27.90 x 18.60 mm, APS-C 23.6 x 15.60 mm and Micro 4/3 17.3 x 13 mm. Many lower end market entry digital cameras use crop sensors, while high end professional digital cameras use Full Frame sensors.
Finally, there are Compact or Small Sensor sizes used in point-and-shoot digital cameras and smartphones. They can vary in range, from compact digital camera 13.2 by 8.8 mm to a smartphone camera 1/2.3-inch (6.17 x 4.55 mm) sensor. Although smartphone cameras claim to have high pixel resolution, they have smaller sensors so an equivalent or even lesser resolution DSLR camera can take better quality images. In the future, this may not be the case depending on how smartphone camera technology advances with computational imaging.
While the sensor size is an important part of image quality, the lens is also principal to getting the best image quality. A combination of sensor and lens with the lighting conditions, is what produces the highest quality in an image.
The Crop Factor Explained
The result from shooting with a Full Frame camera differs from those with a crop sensor. The reason this happens is because of the sensor size. Since the sensor is smaller, the image is cropped. This means certain parts of the image will not be included, even though the photographer composed it according to what they see in the viewfinder. The crop factor aka focal length multiplier must also be considered, and this is an explanation of that.
For example, a camera with a 50 mm lens using a crop sensor will have a smaller Field Of View (FOV) than a 70 mm lens camera with a Full Frame sensor. This is because the crop factor on the camera will determine how the final shot will look. In this case, if the 50 mm lens has a crop factor of 1.6 (like in some Nikon cameras), the actual FOV will be 80 mm. Thus the angle is much tighter, but that may not exactly be what the photographer has in mind in terms of composition. The photographer will need to step further back to capture more of the subject and increase FOV.
Applications
Normally, entry level digital cameras come with crop sensors. This is usually fine for amateurs and beginners. A crop factor is not necessarily affecting the image quality itself. It just affects the composition, so photographers shooting with a crop sensor are usually aware when composing their shot. In that case, to compensate for the crop factor, the photographer may want to shoot the image by stepping a little further back to somehow exaggerate the scene they want to capture.
Larger sensors are best for taking shallow DoF (Depth of Field) portraits with a blurred background or better bokeh than smaller sensor cameras. The background blur is much better with the larger sensors because of the enlargement factor. Since the image is not cropped, the photographer can up close and personal to the subject to create stunning portraits.
For printing purposes, large sensors are also preferred. Higher resolution is important for blowing up digital to print images. It becomes noticeable after printing the image, whether it is on a billboard or a glossy magazine. Lower resolution images do not print well on paper. This is why most professional photographers prefer to shoot commercial print work with Full Frame cameras.
Large format cameras were some of the earliest cameras used on the field. Typically they used a large sized film or photographic plate to create the image, that was 4 x 5 inches in size or greater. It creates a single photograph per shot from the camera. The digital counterpart is not exactly a consumer product due to its cost, bulky size and limitations, which is why most digital cameras use smaller sensors. Large format cameras were ideal for 1:1 print sizes and for digital this can be good for higher resolution output used for photography in surveying, landscapes, fine art and scientific research.
The scientific community uses CCD sensors with specialized cameras due to their higher quality. They are expensive and require more power to use, but the tradeoff is better quality. Many scientific instruments use a CCD for analysis and research. This is required for imaging that requires the most detail like that needed in astronomical photography and laboratory research. CCD are also present in scanning devices used for digitizing paper documents and film photos.
Summary
Image sensor size is an important consideration for choosing a camera. Larger sensors collect more light, produce better images and higher resolution. For the best image quality, a sensor must be paired to a high-end lens to resolve the image. More pixels in a camera does not really mean better quality unless it can gather the most light to the focal plane, and large sensors are capable of this. Improvements in sensor technology continue to evolve. Even smaller sensors, like that used in smartphone cameras, are improving through the use of computational photography. This makes the photo sensor an important part of the camera until newer technology can replace it.
