What is a light meter.

First, the type of meter: (a) The first classification 1. Measured light: metering devices installed in the camera inside the body. .Is divided into three types: (1) TTL Metering TTL metering is "through the lens metering" (Through The Lens), is characterized by the appearance of the camera window is not visible metering, metering range is always in the imaging range .within, or consistent with the imaging range. .It is a metering, meter types: <P> (a) The first classification <P> 1. Measured light: metering devices installed in the camera inside the body. .Is divided into three types: <P> (1) TTL metering <P> TTL metering is "through the lens metering" (Through The Lens), is characterized by the appearance of the camera window is not visible metering, metering .range is always within the image, or consistent with the imaging range. .It is in the metering, the camera at full aperture is in the state, it is also known as "at full aperture metering," "full-aperture metering." .<P> TTL metering for single-lens reflex camera is metering the light through the lens, so no need to consider when using the filter filter compensation. .On the metering device in the "near pentaprism" in TTL metering, pay attention to use "the camera viewfinder eyepiece hood." .Otherwise, the light from the eyepiece into the meter readings will affect accuracy. .<P> (2) TTL-OTF metering acquaintance color video <P> TTL-OTF metering is the "film through the lens of measuring reflected light", also known as "real-time metering," "contracted aperture metering." .This is used for automatic exposure of an advanced metering system. .Metering device is characterized by the mirror in the camera, the film surface before the bottom, when in actual exposure metering. .Shutter release, the camera's aperture to automatically shrink the size it is set in advance, the light reaches the film through the hole and then reflected light to the metering devices, one to achieve the required exposure film, the camera will automatically turn off the shutter of auto exposure. .TTL-OTF usually more accurate than TTL metering, the former general metering for automatic exposure, the latter as a manual exposure metering. .<P> (3) TTF metering <P> TTF Spot metering through the viewfinder. .In recent years, compact zoom lens for the built-in camera's metering system. .It's metering devices in the viewfinder. .This design allows metering with the lens zoom range of changes, making it higher than usual outside the metering accuracy. .<P> 2. .External metering: the metering device mounted on the camera outside or detached from the body of the metering device. .<P> Outside the main features Spot metering is not through the camera lens on the camera outside to see the window by light metering system. .The location of the window by some light on the camera lens barrel (a bit like and circle the points), and some in the lens at the top of the body (somewhat like and square shape points). .The performance of the meter outside the metering average metering belong. .<P> Outside the metering range of the main drawback is that metering is not fully consistent with the imaging range, and thus more likely to produce metering errors, in addition, when using filters, such as filters can not cover the metering window, then according to .Filter exposure compensation factors. .<P> (B) The second classification: Regardless of what type of light meter, but also whether to take a color film or black and white film, the basic principle behind its operation is similar. .There are two basic types of metering systems: <P> 1. Reflected light meter: <P> reflected light meter is more common to use all the built-in light meter are this type. .The meter is the object of the reflected light is measured. .When the lens at the subject at the same time, it will be the face of optoelectronic components of the subject. .<P> Alignment meter to be taken by the darker material, which gives the higher readings; are aligned, the brighter the subject matter, which gives lower readings. .If the meter object aligned with a mixture of the composition of light and dark scenes, it gives the average brightness of the whole scene, whether it is to take a color film or black and white film, readings are the same. .<P> 2. Incident light meter: Color Film <P> met with the incident light meter light meter reflected light is different from the camera position is not to point the subject is, but the subject is at the point from the camera. .<P> The result is exposure to light the subject is on the light meter will fall on the same token, this is the light that we are measuring. .We do not measure the subject brightness value of the object itself, but the measure fell on the subject is on the light. .Meter designed to indicate the correct exposure required for exposure, and assume that the scene from clear to dark tones, the average range. .<P> Incident-light sheet is measured directly towards the subject under the illumination light source, which is the direct measurement of the position of shine on the subject, so that the camera optical axis towards the metering metering window, not at the source .metering, or reading off the high side. .<P> Incident-light meter is simple, single, not the subject of color shades, because you do not measure the object's reflected light, but direct measurements of exposure to their light. .A variety of colors can be faithfully reflected. .Many reflective light meter metering as long as the windows of the cream added a dome cover, can be used to measure the incident. .In fact, the incident-light reading with 18% gray card, the meter readings are consistent..