(after last publication)
More elaborate recommendations for a wider variety of light conditions can be found in the Kodak Professional Photo Guide.
How well does it work to simply read Kodak's instructions and follow them as best you can? Quite well with negative film; not well enough with slide film; not at all when using electronic flash.
Fundamentally, an exposure meter can be built in two ways. The first is to measure the light falling on the subject that you intend to photograph: incident metering. The second is to measure the light coming off the subject in the direction of the camera lens: reflected metering. The typical handheld accessory lightmeter gives the photographer a choice between these two methods. The typical in-camera meter can only measure reflected light. Both kinds of meters recommend a combination of aperture and shutter speed to the photographer who will then use that recommendation as a starting point when actually exposing film.
When using an incident light meter, the most important source of error of which the photographer must be aware occurs when the light is highly directional. The incident dome may not catch the light exactly the way the combination of the subject and camera lens.
When using a reflected light meter, the most important source of error is that the subject's reflectance may not match the meter's assumption about the subject's reflectance. Suppose that you're taking individual portraits of Alex and Mia (at right). You measure the light being reflected off Alex's white fur and set the camera to whatever the meter recommends. Repeating the image with Mia as the subject you find that much less light is reflected by her black and brown fur. So the reflected light meter recommends a wider aperture or a slower shutter speed than it did for Alex.
Does this make sense? With negative film, perhaps. Mia is darker and if you want to get her tones into the linear portion of the film's curve you'll need a longer exposure. But consider that if you'd used an incident light meter it would have recommended the same exposure for both dogs. After all, the same amount of light was falling on them. If you'd used color slide film and the incident meter's recommendation you'd get one slide with a white dog in it and one slide with a black dog in it. What if you'd used the reflected meter's recommendation with the slide film? You'd get two slides exposed with an identical amount of light and therefore both would be the same shade.
Exactly what shade do you get when you follow a reflective meter's recommendation? 18% gray. This is a tone midway between 0% gray (white) and 100% gray (black). Reflected meters are calibrated to assume that the average scene is 18% gray. The reflected meter couldn't know that Alex is a white dog and that Mia is a black dog. When you pointed it at Alex it assumed that the day had gotten brighter. When you pointed it at Mia it assumed that the sky had become cloudier.
Is this 18% gray assumption reasonable? If you take portraits of Caucasian people and meter off their facial skin you'll probably find that your slides come out a bit too dark. Typical Caucasian skin is about 1/2 f-stop lighter than 18% gray. So the reflected meter thinks that the subject is lit somewhat brighter than in reality.
Painful Details
There are some details that can make life painful when setting exposure. As you focus closer to a subject you are moving the lens farther from the film. The lens is throwing the same amount of light in a larger and larger circle of which the film intercepts a smaller and smaller fraction. For small format (35mm) cameras this effect is not significant until you get a macro lens and start taking pictures of things comparable in size to the 24x36mm frame itself. However, if you are taking macro photographs and following the recommendations of a handheld light meter you will find that your pictures are underexposed by 1 or 2 f-stops.
The handheld meter, whether reflected or incident, can't know what impediments there are to light reaching the film. The meter manufacturers assume an ideal lens. Your lens may be covered with a fine coating of dust. Your lens's internal elements will not be perfectly transmissive; some light will be lost each time it goes through a piece of glass within your lens. You may have stuck a filter in front of the lens.
A good way of sweeping away all of these details is the through-the-lens meter. Necessarily a reflected light meter, the metering cells are placed behind the lens and in front of the film, oftentimes built into the viewing system. These cells see what the film will see and therefore if light is getting blocked for any reason the meter simply sets the exposure as if there were less overall scene illumination.
Digital Cameras:
We've arrived at the last and least important chapter in the text.Your choice of camera will not have much effect on the final image. If you're going to be a working photographer, however, you should know what tools are available.
Here are the factors that go into the choice of a camera for a project:
1. What is the required final image quality?
2. At what magnification will the image be viewed?
3. How much weight can you carry to the subject?
4. How much time do you have to take the picture?
Suppose that your project demands high image quality and high magnification. For example, you are going to make a 40 x 60 inch enlargement and display it in a corridor where people can walk right up to it to check out fine detail. This requirement pushes you toward using high resolution high quality sensor for the original exposure. That high-res, high-quality sensor may come wrapped in a large relatively heavy camera, which gets us into Factor 3: "How much weight can you carry to the subject?"
Different cameras work at different speeds. With the latest Canon or Nikon autofocus systems, you might be able to capture an unanticipated event on a soccer field. A photographer using a view camera and digital back will not be able to do this; he will still be setting up the tripod, focusing, stopping the lens down to taking aperture, closing the shutter, cocking the shutter, attaching a laptop computer, etc. If your subject is a big mountain, you can probably afford to take your time making the image.
Now that we have the factors in mind, let's dive into the types of cameras available:
- point and shoot (subject of a separate article)
- single-lens reflex (SLR) cameras; the standard mass-market tool
- lens-shutter and rangefinder cameras; great values, terrible values, and nothing in between
- digital backs for medium-format and view cameras
- panoramic cameras
Sensor Sizes
Larger sensors offer lower noise at high ISO settings and are therefore essential for taking pictures in low light conditions. The best light for photography is typically fairly dim and therefore using a larger sensor is highly desirable. Unfortunately, the cost of manufacturing a sensor goes up exponentially with size and the largest sensors can cost more than a car. Here are the standard sizes:
1/1.8": 5x7mm, common size for point and shoot cameras
4/3": 13.5x18mm, the Olympus Four Thirds system- APS-C: 15.7x23.7mm, the standard for "small sensor" digital SLRs such as the Nikon D200, $1289 and Canon Digital Rebel XTi (Black), $545
35mm film: 24x36mm, Canon's better digital SLRs, starting with the Canon EOS 5D, $1890- 48x36mm, Hasselblad H3D and digital backs from companies such as Leaf, MegaVision, Phase One, Sinar
60mm line, scanning backs from Seitz
72x96mm, scanning backs from BetterLight
The scanning back, a line of CCD elements that are swept mechanically behind the lens, is a great idea, but it only works for static subjects since the minimum scan time is about one second.
to be continued………………
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