Introduction
Camera Lens
A camera lens (also known as photographic lens or photographic objective) is an optical lens or assembly of lenses used in conjunction with a camera body and mechanism to make images of objects either on photographic film or on other media capable of storing an image chemically or electronically. The two fundamental parameters of an optical lens are the focal length and the maximum aperture.
Focal Length
The lens’ focal length determines the magnification of the image projected onto the image plane. For a given photographic system the focal length determines the angle of view, short focal lengths giving a wider field of view than longer focal length lenses.Focal lengths are usually specified in millimetres (mm).
Here is example how focal length affects photograph composition: adjusting the camera’s distance from the main subject while changing focal length, the main subject can remain the same size, while the other at a different distance changes size.
There are two kinds of camera lens according to their focal length:
- Zoom lenses
- Prime lenses
The term prime has come to be used as the opposite of zoom; that is, a prime lens is a fixed-focal-length, or unifocal lens, while a zoom lens has a variable focal length.
Aperture
The aperture indicates the light intensity of that image.A wider aperture, identified by a smaller f-number, allows using a faster shutter speed for the same exposure.[9] We use F value to describe the aperture.
F = Focal length/diameter of aperture.
The lower the f-number, the higher light intensity at the focal plane. Larger aperture (smaller f-numbers) provide a much shallower depth of fiedl than smaller apertures, other conditions being equal.
Image Sensor
An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras, camera modules and other imaging devices. most currently used are digital charge-coupled device (CCD) or complementary metal–oxide–semiconductor (CMOS) active pixel sensors.
Neither technology has a clear advantage in image quality. On one hand, CCD sensors are more susceptible to vertical smear from bright light sources when the sensor is overloaded; High-end frame transfer CCDs in turn do not suffer from this problem. On the other hand, CMOS sensors are susceptible to undesired effects that come as a result of rolling shutter.
###Color separation###
There are several main types of color image sensors, differing by the type of color-separation mechanism:
Bayer filter sensor, low-cost and most common, using a color filter array that passes red, green, or blue light to selected pixel sensors, forming interlaced grids sensitive to red, green, and blue – the missing color samples are interpolated using a demosaicing algorithm. In order to avoid interpolated color information, techniques like color co-site sampling use a piezo mechanism to shift the color sensor in pixel steps. The Bayer filter sensors also include back-illuminated sensors, where the light enters the sensitive silicon from the opposite side of where the transistors and metal wires are, such that the metal connections on the devices side are not an obstacle for the light, and the efficiency is higher.[7][8]
shutter
Shuttering is the process of exposing an imaging sensor to light at a rate equal to or faster than the frame rate. The purpose of the shuttering is to reduce the motion blur within an image frame.
Rolling Shutter
The rolling shutter is where a line or a group of lines is read out while all other lines on the sensor continue to be exposed. When a group of lines are read out at one time, this is called a block readout. Typical line times, depending on the frame rate and sensor architecture, can be several hundred microseconds.
The “Rolling Shutter” can be either mechanical or electronic. The advantage of this method is that the image sensor can continue to gather photons during the acquisition process, thus effectively increasing sensitivity. It is found on many digital still and video cameras using CMOS sensors. The effect is most noticeable when imaging extreme conditions of motion or the fast flashing of light. While some CMOS sensors use a global shutter,[3] the majority found in the consumer market utilize a rolling shutter.
Global Shutter
- A global shutter, unlike the rolling shutter, exposes all pixels at the same time.
- There is no time discontinuity in the image.
- The global shutter has a storage element that allows the pixel to dump the storage charge into a shielded area. This shielded area than can be read out while the next image frame is being exposed. Therefore, there are no discontinuities or image artifacts associated with the electronic exposure.
Why is Global Shuttering Better###
- Image is integrated without discontinuities in time
- Image Quality is superior with less motion blur Measurements are more accurate
- No mechanical moving parts
- Strobes are not required to stop the motion blur
- Special Recording modes such as Slip Sync or Burst ROC are only possible with Electronic Shuttering (Global).
Black Level
black level is defined as the level of brightness at the darkest (black) part of a visual image or the level of brightness at which no light is emitted from a screen, resulting in a pure black screen.
Video displays generally need to be calibrated so that the displayed black is true to the black information in the video signal. If the black level is not correctly adjusted, visual information in a video signal could be displayed as black, or black information could be displayed as above black information (gray).
Auto Exposure(AE)
In photography, exposure is the amount of light per unit area. Determined by :
- Aperture: Controls the area over which light can enter your camera.
- Shutter speed: Controls the duration of the exposure
- ISO speed: Controls the sensitivity of your camera’s sensor to give amount of light.
Exposure is like collecting rain in a bucket. We cannot control the rate of the rainfall, and there are 3 factors we can determine: size of the bucket, duration we leave it in the rain, and the quantity of rain we want to collect.
Collect too much is just like “overexposed”, collect too little is just “underexposed”. In photography, the exposure settings of aperture, shutter speed and ISO speed are analogous to the size, time and quantity discussed above.
Each of the 3 factors, which determines exposure, controls exposure differently, and each setting also influences other image properties, just like in the chart below, aperture affects depth of field, shutter speed affects motion blur and ISO speed affects image noise.
Auto White Balance(AWB)##
White Balance(WB) is the process of removing unrealistic color casts, so that objects will preserve natural color under different light source. Influenced by : Color temperature of a light source Color temperature is a way of measuring the quality of a light source. It is based on the ratio of the amount of blue light to the amount of red light, and the green light is ignored. The unit for measuring this ratio is in degree Kelvin (K)
Human brain can quickly adjust to different color temperature. Digital cameras usually have built-in sensor to measure the current color temperature and use an alogrithm to process the image so that the final result may be close to what we see(with our eyes,of course). But the algorithm(s) being used may not be accurate enough to make every situtation correct. Under some difficult situations when the in-camera algorithm is not able to set eh color temperature correctly or when some crative and special effects are needed, we can instruct the camera to use a particular color temperature to fulfill our need. The adjustment that makes sure the white color we view directly will also appeare white in the image is refered to as white balance
Dynamic Range##
Dynamic range in photography describes the ratio between the max and min measurable light intensities(white and black, respectively). Incident and reflected light can contribute to the dynamic range of scene.
In a nutshell, dynamic range enhancement is used to make both light and dart parts of a scene clearly. Under intense light source, light spot the photograph from the camera maybe overexposed to be white, and dark spot maybe underexposed to be black. In the dynamic range enhancement algorthm, we can use some evaluation matrixs to make the lightness on the dark spot and on the light spot more balance.
High-dynamic-range imaging (HDRI or HDR) is a set of techniques used in imaging and photography to reproduce a greater dynamic range of luminosity than is possible with standard digital imaging or photographic techniques. The aim is to present the human eye with a similar range of luminance as that which, through the visual system, is familiar in everyday life. The human eye, through adaptation of the iris (and other methods) adjusts constantly to the broad dynamic changes ubiquitous in our environment. The brain continuously interprets this information so that most of us can see in a wide range of light conditions. Most cameras, on the other hand, cannot.
Tone Mapping is a technique used in image processing and computer graphics to map one set of colors to another to approximate the appearance of high dynamic range images in a medium that has a more limited dynamic range. Print-outs, CRT or LCD monitors, and projectors all have a limited dynamic range that is inadequate to reproduce the full range of light intensities present in natural scenes. Tone mapping addresses the problem of strong contrast reduction from the scene radiance to the displayable range while preserving the image details and color appearance important to appreciate the original scene content.
Flicker Detection
Image Histogram
An image histogram is a type of histogram that acts as a graphical representation of the tonal distribution in a digital image. It plots the number of pixels for each tonal value. By looking at the histogram for a specific image a viewer will be able to judge the entire tonal distribution at a glance. Image histograms are present on many modern digital cameras. Photographers can use them as an aid to show the distribution of tones captured, and whether image detail has been lost to blown-out highlights or blacked-out shadows
Sharpness
Sharpness describes the clarity of detail in a photo, can be a valuable tool to emphasizing texture. Two fundamental factors contribute to the perceived sharpness of an image:
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Resolution Describes the camera’s ability to distinguish between closely spaced elements of detail, such as the two sets of lines shown below: Resolution is limited by your digital sensor.
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Acutance Describes how quickly image information transitions at an edge, so high acutance results in sharp transition and detail with clearly defined borders.Acutance depends on both the quality of your lens and the type of post-processing. It’s the only aspect of sharpness which is still under your control after the shot has been taken.
Image stabilization
Image stabilization (IS) is a family of techniques used to reduce blurring associated with the motion of a camera or other imaging device during exposure. Gusts of wind tearing at a surveillance camera can make it shake or vibrate enough to make images come out as useless blur. A heavy truck, or a train, passing close by can have similar consequences. Image stabilization techniques are used in consumer products such as digital still cameras and video cameras. Today, there are two methods to tackle the problem – optical image stabilization and electronic image stabilization.
###Optical Image Stabilization### An optical image stabilization system usually relies on gyroscopes or accelerometers to detect and measure camera vibrations. The readings, typically limited to pan and tilt, are then relayed to actuators that move a lens in the optical chain to compensate for the camera motion. In some designs, the favored solution is instead to move the image sensor, for example using small linear motors.
Either method is able to compensate the shaking of camera and lens, so that light can strike the image sensor in the same fashion as if the camera was not vibrating. Optical image stabilization is particularly useful when using long focal lengths and works well also in low light conditions.
The main disadvantage of an optical solution is the price.
###Electronic Image Stabilization###
Electronic image stabilization, also known as digital image stabilization, has primarily been developed for video cameras.
Electronic image stabilization relies on different algorithms for modeling camera motion, which then are used to correct the images. Pixels outside the border of the visible image are used as a buffer for motion and the information on these pixels can then be used to shift the electronic image from frame to frame, enough to counterbalance the motion and create a stream of stable video
References
9.Kingslake 1989,[page needed]