Posted by: admin 3月 8th, 2012

The more a photographer learns, the more he realizes that photography is not an exact science like mathematics or computing where a concrete answer or solution to a problem can be found after all the important data or information have been systematically entered and worked on.

In photography, something can always go wrong, even when every effort has been made to ensure a reliable result.

A photographer is always concerned about the correct measurement of light and its possible effect on the scene or, subject before taking a picture. This involves the question of aesthetics or the way a photographer interprets his subject. The photographer must choose the correct combination of shutter speed and f-stop which will allow him to record the subject accurately, making the correct exposure so that the film emulsion receives sufficient light from the scene to record tones and colours accurately.
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Camera manufacturers are well aware of the kind of problems which photographers face, and they have provided ingenious solutions to most of them. Using silicon chips that act like tiny computers, they have incorporated automatic exposure modes which cover most situations.

But there are still many tricky lighting problems which require an understanding of exposure to be handled properly.

To simplify the matter, just consider all the different lighting conditions which one may encounter while taking a leisurely stroll around the local neighbourhood.
First of all, there is the weather and the intensity of sunlight. It can be anything from dry and sunny to hazy cloud cover with sunny spells,
or overcast with bright to grey skies and flat daylight.

The light can be seen as very bright and hard or soft and more diffused. There will also be light reflected from the walls of buildings and reflective surfaces. All these combine to give a variety of lighting effects for a scene or subject and each will require careful consideration with regard to exposure settings.
 
Then there are the subjects that one may encounter while strolling. Perhaps some lively children, sparkling with energy, could be playing their games on the pavement, with the sun behind them producing a lovely back-lighting effect. This would require an exposure to be made for the shadow side to capture the faces and not the sunny side, otherwise the faces would be underexposed; and the backlight effect lost.

It is always wise to watch out for the strong contrast between sunny areas and shadows because the brightest parts can be more than 200 times brighter than the darkest shadows. Although this is not a problem for normal vision, photographic film (or digital cameras)is incapable of recording such large differences between light and dark areas.
When a photographer is confronted with such a situation, he must make a decision on which are the important areas for his picture and make an exposure accordingly. This is relatively simple if it is the face or skin tones of a person which are important in the picture, but there are many exceptions.
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These exceptions or tricky lighting problems can often fool the built-in light meters found in most modern cameras and result in photographs that are either over or underexposed.
There are several different types of light meters available to photographers but because the majority of SLR (single lens reflex) and compact cameras are equipped with built-in reflected light meters, we will concentrate on the pros and cons of using such a metering system.

Reflected light meters measure the amount of light which is reflected off a subject or scene and enters the camera lens. The light or image, which is seen through the viewfinder by the photographer, is measured by a light meter housed inside the camera body, and the light reading is processed by a micro-processor and then presented to the photographer as information which he will need to obtain a correct exposure for his picture.
This system of measuring light through the lens is commonly known as TTL (Through The Lens) metering and is divided into three categories.

Averaging meters examine the entire scene found in the viewfinder and give all parts of the picture equal importance to obtain an average light reading for the picture.

Spot-reading meters measure only a small area of the picture, usually the central portion of the viewfinder which is marked by a small circle, and base the exposure on this reading.

Centre-weighted meters, which are most commonly used by camera manufacturers, especially for SLR cameras, are a compromise between averaging and spot-reading meters and take a reading from the central portion of the picture as well as the averaged out reading from the edges of the picture to arrive at fairly accurate exposures for most common situations.

Automatic digital cameras are largely equipped with these centre-weighted metering systems.

The better examples of digital cameras offer good results producing a high proportion of correctly exposed and sharp pictures but if a photographer wants more control over his pictures then it is probably better for him to use a manual or semi-automatic camera, as these cameras allow the photographer to choose any combination of aperture and shutter speed to give the desired interpretation of any subject.

After all, many scenes or subjects contain areas of intense or very dim light, for which an averaging light meter will only produce an average measurement - that is how it is programmed.

A photographer who is aware of this fact can bracket his exposures (in other words, make several more exposures above and below the meter's recommended reading) to ensureachance of getting the correct exposure.
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