There are two primary methods of determining exposure. Understanding them and when to apply them will make it possible for you to more accurately exposure your files. When the overall image content is important, such as in landscape photography, we use the primary exposure method. When a subject, such as a portrait, is more important than the surrounding values we use the subject method.
The most used method for most photographers is to base the exposure on the highest important values in the image, ignoring specular highlights. This follows the concept of ETTR or "Expose To The Right" placing the brightest values to the far right on the histogram without clipping those values (overexposure). This is based on how linear digital capture works in your camera. It makes the best use of the full dynamic range of the camera and minimizes noise generation in the file. It is excellent for landscape photography and many other types of photography where overall exposure of the scene is the primary concept.
This practice can be overly simplistic as image content can exceed the dynamic range of the camera. In some high contrast circumstances the highest values like the sky, and important lower values can be further apart than desired for optimal exposure of both areas. In these circumstances it may be necessary to make more than one exposure, exposing once for the best high value area, and again for the lower value area. This will provide better source material for post processing but will require more complex processing techniques. One option is using HDR (high dynamic range) software, but that often results in less than the best image quality. Another option is to blend the two exposures using masking which is a bit more complex but not unrealistically difficult and can be mastered with a little practice. The blending technique is my preferred solution to the problem.
The most important part of producing good exposures is learning to use the histogram to help you understand what your exposure decision is providing you. There is no such thing as a perfect histogram as it is simply a graphic representation of the values captured. How the values lies within the histogram is what is important, especially how the values lie in relation to the left and right end. Exposures that result in the histogram hitting the wall on either end are not desireable, but can be informative in determining exposure settings.
The camera histogram can be a bit deceptive as it is based on the camera processed jpg and not the actual raw file. Lowering the sharpening and contrast settings in the picture style section of the camera menu will help. It also helps to train yourself to not expect the preview screen image to accurately represent the finished image.
The second method is basing the exposure on the primary subject in the scene without consideration for the overall values in the capture. This is best used for portrait and wedding photography or other images where the background may be secondary to the subject. The primary subject of any photograph should be where good exposure lies, but in this method the primary subject and the background surrounding the subject may vary in values. If the primary exposure for the subject also renders the entire scene well, you are golden.
Subject exposure can be done two ways: incident metering, or reflected. Incident metering requires a hand held meter with an incident dome that reads the light falling on the subject. The reading should be taken at the subject with the dome pointed at the camera. Pretty simple. No corrections. This is the way professional photographers have been working for decades. It works with ambient light, and can work with flash added to ambient light. In fact, with added flash the incident meter is pretty much a necessity. Once your meter is calibrated to the camera so you trust the readings it is about as simple as it gets. The added benefit is that it is very consistent from shot to shot. In the case of added flash the exposure for the subject can be balanced against an initial exposure setting for the entire scene.
Reflected readings take a bit more mental effort. You can use the spot meter in the camera to read a surface such as clothing or skin, or a gray card at the subject. The gray card will be consistent as it a known value. A caveat is that some gray cards will tend to give you about 1/2 under exposure depending on their actual reflectance, so the technique should be tested. Reading a surface requires compensation. Reading a white shirt or blouse will require added exposure as the reading will make the subject gray, not white. When photographing people I try to use the surface plane of the face that is parallel to the camera, but avoid highlights that may be present depending on the lighting. A single stop is added to compensate for caucasian skin. Note that this is compensation not overexposure. The compensation is to overcome the meter default to middle gray exposure values. This is all part of the process known as the zone system which is placing the exposure value where you want it to be relative to middle gray (Zone V). It is also important to consider the diffuse value of the surface, ignoring specular reflectivity.
The zone system can be applied to digital photography if you test your camera to know the dynamic range of the sensor and where certain exposure values will fall in the histogram. This is a bit more complex than the average shooter is willing to take on but simply having a few target values in mind for highlights and shadows is often sufficient. Most commercial photographers prefer incident metering using a hand held meter. Hand held incident meters read the amount of light on the subject ignoring reflectance values. This generally results in capturing exposure values accurately as the subject does not alter the evaluation. Candid photographers usually use a substitution method, reading a particular value in the scene using the in-camera meter. This is less consistent, but faster, and can be used creatively in interpreting where subject values will be placed.
Exposure is a relationship between the various tonal values in the image. The differences in reflectivity of areas of the scene is what creates the image and sets the mood of the photo. If the differences are great the image is said to have a high contrast and may require special consideration such as exposure bracketing, HDR or exposure blending to reign in the contrast. A subject with less inherent contrast will have a flat appearance coming out of the camera and may require expansion of the differences in tonal levels to look like you want it to.
It is important to understand that setting highlight exposure should be the primary consideration in digital capture. Shadows fall in relation to the highlights and in digital capture underexposed shadows are an invitation to noise in the image. If the shadows are too dark relative to an appropriate highlight exposure the contrast of the scene exceeds the dynamic range of the camera. Note that this also should be tested as dynamic range varies with cameras.
This image was taken on the side of the road in New Mexico. The lighting was fairly soft as a result of the cloud cover. Despite some high values in the sky area, the vast majority of the foreground was limited to the bottom half of the exposure scale. If you roll the mouse over the image you will see the original color capture before Photoshop adjustments. You will see how the wood structure is not rendered well and the sky contrast is rather flat as well.
Initial exposure was based on the high values in the sky, placing them as high as possible without clipping. An overall exposure based on the camera’s meter would have blown out the sky. I find that determining exposure based on the highest values works best for me. Once that is done the shadow areas are checked relative to the highlights.
I find that my camera typically wants to underexposure most average scenes by as much as a full stop, so I check the highlights and the overall histogram carefully after each capture. In addition I use spot metering to check specific areas of the image and "place" the values where I want them to be. This is the zone system.
In the zone system you use the meter to determine the exposure value of an area of the image. The meter tells you what the exposure should be if you want the area to be middle gray. You then modify the exposure in order to move the value up or down and expose according to the desired level. In the case of highlights you increase the exposure by about two stops depending on exactly how bright you want the area to record. That sets your exposure and the shadow values are recorded relative to the determined highlight exposure. This is the simplest method for setting primary exposure.
Setting the exposure based on the highlights seems to be to be the best option as clipped highlights and blank white areas are not what I want in my images. Second to that is the shadow area exposure which will be rendered in relation to the highlights. All that remains then is a quick check of the histogram to determine if the scene falls within the desired range and unimportant areas are not clipped at the bottom of the exposure. Losing highlight detail is not an option for me, while losing shadow detail is less important depending on the image but is easily remedied by a second exposure if needed.
The histogram confirmed that the scene was within the dynamic range of the camera, meaning the exposure I established for the sky did not force the shadows into clipping at the lower end. There was a substantial amount of foreground detail available as the left end of the histogram did not “hit the wall”. This gave me a good exposure for post processing with a single raw capture. The hump at the left of the big dip in the histogram represents the foreground information and the hump at the right represents the sky area.
In post processing there was contrast added to the sky, which showed values from slightly below middle gray to the high 240s. There was also contrast added to the foreground, and in particular to the wood structure. In both cases the corrections were made using masks to limit the corrections to specific areas. Brightening the wood by raising the contrast in curves significantly increased the foreground interest and made the image come to life. The color version was interesting, but the image to me needed to be black and white. That conversion was pretty easy since the primary dynamic changes had been done in the color version, but final tweaks to the wood structure needed to be applied to the black and white version to get the real impact where it needed to be.