Photography - Exposure and Camera Controls

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    Exposure

    Exposure simply means allowing light to strike your film. The tricky part is knowing how

    much light you need and how to control the amount of light reaching the film. The former

    is taken care of by a light meter, usually built in to the camera, and the latter is achieved

    by means of the aperture and shutter controls on your camera.You control the exposure by allowing light to pass through the aperture for a given

    amount of time. Right now we are going to have a look at the way apertures and shuttersare used to control exposure but not the other important functions they perform.

    Aperture and f-numbers.

    The aperture is just a hole whose size can be varied to allow more or less light to pass

    through it. The size of apertures are expressed in f-numbers. You can calculate an f-number, if you are keen or don't have much of a life, by dividing the lens focal length by

    the diameter of the aperture. The range of f-numbers follows a standard sequence with

    each f-number being half as bright, passing half as much light, as the previous one. Atypical aperture range may look like this:

    f 1.4 f 2 f 2.8 f 4 f5.6 f 8 f 11 f 16 f 22 f 32

    There are smaller and larger f-numbers but the actual numbers used are always the same

    and will maintain a constant value over different lens focal lengths. This just means that

    f-8, for instance , will always pass the same amount of light no matter what camera orlens you may be using. Similarly, f-16 will pass half as much light as f-11 and f-4 will

    pass twice as much as f-5.6. The difference in value between one full f-number and the

    next is known as a 'stop'. If you change aperture from f-8 to f-5.6 you will give your film

    one stop more exposure.The smaller the f-number is then the larger the aperture is and the more light it will pass.

    The f-number is also used as a guide to the light gathering abilities of a lens. Lenses with

    large maximum apertures ( small f-number ) are described as being 'fast'.Generally the aperture will always be held open at its maximum irrespective of whatever

    value you may have set it to and will not actually close down until the moment of

    exposure. The main reason for this is to produce the brightest image possible onto thefocusing screen. To see the aperture in operation you will have to remove the lens, unless

    you have a preview control, and look through the lens while turning the aperture control

    ring.

    Shutter and Shutter Speeds.

    The shutter prevents light from reaching the film until the moment of exposure, when it

    opens for a predetermined time allowing light passing through the lens aperture to reach

    the film. Unlike the aperture, which is always in an open position the shutter is alwaysclosed. Like the aperture, shutter values or 'speeds' follow a standard sequence with each

    one being half that of the next, allowing half as much light to pass through. A typical

    shutter speed range may look like this

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    interesting image. You can also add more light to a scene, by using flash or a reflector, to bringthe contrast range of the subject within that of the film.

    Types of film

    Negative film will hold more detail in the highlights, so in general it should be exposed for the

    shadows in the scene. This usually means over-exposing the image by about a third to half astop, or taking your exposure reading from the shadows. Digital cameras will not record anythingin the highlights if they are over-exposed, so it is best to expose for the highlights, which meansunder-exposing by about a third to half a stop to preserve detail in the brighter parts of the image,or taking your meter reading from the highlights.

    Some photographers adjust the film speed or ISO/ASA of their meters to make this calculationautomatically by setting the ISO/ASA speed slightly more for slide and slightly less for negative.So they might set 400 ISO/ASA negative film at 320, or 100 ISO/ASA slide film at 125.

    Over and under exposure

    A photographer can deliberately over- or under-expose a scene to make it more interesting or

    dramatic, or to make a point. Sunsets, for example, almost always look better under-exposed, asa little under-exposure tends to increase the saturation of colors and make them strongerwhereas over-exposure has the opposite effect, making colors washed out and less intense. Youcan make a high-key image, which is predominately light and bright, by over-exposing or a low-key image, which is dark and dramatic, by under-exposing. An everyday scene can betransformed by careful exposure.

    Metering in difficult light

    1. Compensating for average readings.The meter in your camera takes what is called a reflected light reading, which means it measuresthe light reflected back from the subject into the cameras lens. This is usually averaged tobalance the effect of light and dark areas to give an average exposure, which is fine in mostsituations. However, camera meters find it hard to deal with light subjects against a darkbackground, or vice versa, and you have to make allowances for this. A meter will read the darkbackground and try to turn it into 18% gray, which will over-expose both the background and thelighter subject in front of it. To prevent this, manually reduce the exposure by between half andone-and-a-half stops depending on the degree of difference between light and dark. For a darksubject against a light background, increase the exposure by the same amount.

    2. Faces and skin tonesDark-skinned faces can lead to over-exposure so, depending how dark the subject's skin is, closedown by half a stop to one stop. Another way to deal with this kind of difficult lighting is to use thespot meter facility on your camera if it has one. This means that the meter will measure only thelight falling on a small part of the scene in the middle of your frame when viewed through thecamera. This technique can be useful but be careful with it, as you have to work out thedifference between what you are metering and 18% gray to get the correct exposure. The best

    ways are to take meter readings from the skin tones of people's faces, or to take a reading of thebrightest and darkest parts of the scene you want to record and average them.

    3. Incident and reflected light readingsAnother way to meter is to use a hand meter to take an incident light reading rather than thereflected light reading that your camera meter takes. An incident reading measures the lightfalling on the subject rather than the light reflected back from it, and you can use it like a spotmeter to take accurate readings from different parts of a scene.

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    4. BracketingMany photographers faced with difficult light will make a series of exposures at half- or third- stopintervals either side of the basic exposure, then choose the best one after the pictures aredeveloped. This technique is known as bracketing and many cameras today have a program thatcan be set to do this automatically.

    5. F16 at 1/250thWhen all else fails, you can use the f16-at-1/250th rule, which states that for 400ASA film in brightsunlight the exposure will be 1/250th of a second at f16. In overcast, cloudy conditions it will be1/250th at f8. With practise, you will be able to guess the right exposure for a variety of light frombright sunlight to candle light.