Motion blur

Motion blur

Motion blur is caused when either the camera or the subject moves during the exposure. This causes a distinctive streaky appearance to the moving object or the entire picture (in the case of camera shake).

Motion blur can be used artistically to create the feeling of speed or motion, as with running water. An example of this is the technique of “panning“, where the camera is moved so it follows the subject, which is usually fast moving, such as a car. Done correctly, this will give an image of a clear subject, but the background will have motion blur, giving the feeling of movement. This is one of the more difficult photographic techniques to master, as the movement must be smooth, and at the correct speed. A subject that gets closer or further away from the camera may further cause focusing difficulties.

Light trails is another photographic effect where motion blur is used. Photographs of the lines of light visible in long exposure photos of roads at night are one example of effect.^[3][4] This is caused by the cars moving along the road during the exposure. The same principle is used to create star trail photographs.

Generally, motion blur is something that is to be avoided, and this can be done in several different ways. The simplest way is to limit the shutter time so that there is very little movement of the image during the time the shutter is open. At longer focal lengths, the same movement of the camera body will cause more motion of the image, so a shorter shutter time is needed. A commonly cited rule of thumb is that the shutter speed in seconds should be about the reciprocal of the 35 mm equivalent focal length of the lens in millimeters. For example, a 50 mm lens should be used at a minimum speed of 1/50 sec, and a 300 mm lens at 1/300 of a second. This can cause difficulties when used in low light scenarios, since exposure also decreases with shutter time.

Motion blur due to subject movement can usually be prevented by using a faster shutter speed. The exact shutter speed will depend on the speed at which the subject is moving. For example, a very fast shutter speed will be needed to “freeze” the rotors of a helicopter, whereas a slower shutter speed will be sufficient to freeze a runner.

A tripod may be used to avoid motion blur due to camera shake. This will stabilize the camera during the exposure. A tripod is recommended for exposure times more than about 1/15 seconds. There are additional techniques which, in conjunction with use of a tripod, ensure that the camera remains very still. These may employ use of a remote actuator, such as a cable release or infrared remote switch to activate the shutter, so as to avoid the movement normally caused when the shutter release button is pressed directly. The use of a “self timer” (a timed release mechanism that automatically trips the shutter release after an interval of time) can serve the same purpose. Most modern single-lens reflex camera (SLR) have a mirror lock-up feature that eliminates the small amount of shake produced by the mirror flipping up.

Focus

Focus is the tendency for light rays to reach the same place on the image sensor or film, independent of where they pass through the lens. For clear pictures, the focus is adjusted for distance, because at a different object distance the rays reach different parts of the lens with different angles. In modern photography, focusing is often accomplished automatically.

The autofocus system in modern SLRs use a sensor in the mirrorbox to measure contrast. The sensor’s signal is analyzed by an application-specific integrated circuit (ASIC), and the ASIC tries to maximize the contrast pattern by moving lens elements. The ASICs in modern cameras also have special algorithms for predicting motion, and other advanced features.

Aberration

Aberrations are the blurring and distorting properties of an optical system. A high quality lens will produce a smaller amount of aberrations.

Spherical aberration occurs due to the increased refraction of light rays that occurs when rays strike a lens, or a reflection of light rays that occurs when rays strike a mirror near its edge in comparison with those that strike nearer the center. This is dependent on the focal length of a spherical lens and the distance from its center. It is compensated by designing a multi-lens system or by using an aspheric lens.

Chromatic aberration is caused by a lens having a different refractive index for different wavelengths of light and the dependence of the optical properties on color. Blue light will generally bend more than red light. There are higher order chromatic aberrations, such as the dependence of magnification on color. Chromatic aberration is compensated by using a lens made out of materials carefully designed to cancel out chromatic aberrations.

Curved focal surface is the dependence of the first order focus on the position on the film or CCD. This can be compensated with a multiple lens optical design, but curving the film has also been used.