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What are the things that effect the exposure of our film? The photographer must know these like their own name....very closely.
First lets look at the variable that we have the least control over -- Brightness and ASA. Brightness is measured by the light meter. It is measured in foot candles or the amount of light one candle would give on a 1 foot square one foot from the candle. This is usually only seen as a meter needle moving or lights that light up. The importance thing is that it can be exactly measured by an electric meter. Second in our vairable that we can not change much is the ASA of our film. The ASA or film sensitivity is a fixed number and must be considered when we pick which film we want to purchase. The ASA can be pushed to double its number with special developer, but contrast is changed and shadow detail is lost.
Next we have the variable in the camera. The first control we have is the shutter. The shutter is selected to stop the motion of the camera or the subject so a photo sharpness is in pricise control. The general rule is to use as high a shutter as possible. The second control of importance is our aperture which effects the background focus or depth of field. This must be adjusted in co-ordination with the other variables from brighness to shutter. IF we have a fast film like Tri-x with an ASA of 400 and we are in a bright scene we will need both a fast shutter and a small aperture (which is a large f-stop number). IF we have the same film and are in the dark then the shutter can stay at a fast speed while the aperture gets wider (smaller number).
The variable are: 1) Brightness
2) Film Sensitivity
3) Shutter Speed
4) Aperture
All of these work together to give the film just the EXACT amount of light needed to record an image which is a neutral gray average of the scene. A good negative should have textured detail in the light as well as dark areas with the clear areas almost completely free of grey fog. Contrast is an important part of photography and it represents the RANGE of colors or shades of gray visible in a print or negative.
OVER - UNDER EXPOSURE Accurate exposure will result in accurate information on the final photo. Accurate information means that we can see DETAIL in both the shadow area of our print and the highlight area of our print. When we under expose a print we drop out the detail in the shadows causing them to turn DARK or black in our final print. Over exposure will cause the details in the shadow to look lighter than they should and it will cause the detail in the HIGHLIGHT area to drop off and become white or washed out. Exposure is the one most important factor in getting a good exposure. It is the light meter that will determine exposure.
Metering and Exposure Determination
The handheld meter remains one of the most valuable tools for a photographer. Two considerations are of importance in metering:
1. Meters provide a reading designed to reproduce a medium brightness level (referred to as 18-percent reflectance neutral gray) in the resulting photograph. Aim the meter at a bright highlight, and it will read it as a mid-tone. Aim the meter at a dark shadow, and it will also read it as a mid-tone. Handheld meters are single reading meters.
2. You can set only one exposure at a time. If you take a reading of several areas of a scene, and the readings differ considerably, then you have to interpolate or decide on one reading as a compromise. The usual strategy is to choose the area of the scene that most closely appears to be a mid-tone, and to use the reading for that area.
With these two principles in mind, and with some experience of how the various tones of the world compare, you can make correct exposure very nearly foolproof.
For example, medium Caucasian skin should reproduce as about a stop lighter than neutral gray. So, after taking a closeup reflected reading--or a spot reading--of your subject's skin highlight, use an exposure one stop more than your meter indicates. Average African-American skin, on the other hand, should be rendered about one stop darker than a reading for neutral gray. So stop down one stop from a close-up reading.
Another commonly used strategy is to meter the brightest area where you want to maintain detail, and the darkest shadow in which you want to maintain detail, and calculate the midpoint. It helps to have a meter with an analog scale of some sort so you can quickly eyeball the midpoint between high and low readings. Some high-end meters will calculate the midpoint (or average of several points) automatically.
A small investment that will improve your handheld metering skills is a photo gray card, available in photo stores and catalogs for a few dollars. By placing the gray card in the scene you want to photograph--for example, by having your portrait subject hold the card--you can make a direct measurement of a mid-tone either close up with a reflected meter, or with a spotmeter. After a while you'll be able to internalize your gray card: you'll look at a tone in a scene and decide it should be, for example, two stops darker than that for a gray-card exposure.
Over the years a number of systems have been devised for the proper "placement" of different brightness levels in the exposure depending on how you want the final result to look. The best-known is the Zone System, developed initially by Ansel Adams. If you are interested, get a copy of The New Zone System Manual, by White, Zakia, and Lorenz.
TYPES OF METERS
Reflected meter: A cell on the meter reads light reflected from the subject, at an angle that can vary from about 20 to 60 degrees. It should be used close to the subject, with the meter port pointed at the tone you wish to read. If you hold the meter close to a gray card, take care not to shadow the card.This is the type of meter found in most modern cameras. These meters will measure the total area inside the camera frame with a stronger emphasis on what is found in the center of the viewfinder.
Incident meter: With its cell behind a white plastic dome or panel, it's designed to read the light falling on the subject, not reflected from it. Typical acceptance angle is 180 degrees or a bit more. The correct method is to hold the meter at subject position and aim the white dome directly at camera position. It's fast and accurate in many situations.
Spotmeter: A specialized type of reflected meter with an extremely narrow acceptance angle (the best are around 1 degree) and a monocular viewfinder. You must pay careful attention to how the tone you read compares to a gray card, and set exposure accordingly. Some modern SLR cameras will allow the photographer to select spot mode and thus set the exposure on a small detail in the subjects brightness range.
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