Center-Weighted Metering Pattern

Center-weighted averaging is a bit more “old fashioned” in that it is how light was read, for the most part, before advanced microprocessors got into the mix. The light is read from all parts of the viewfinder, with 70% of the light reading coming from the center of the frame and the remaining 30% of the calculation from the edges of the frame. It is called “averaging” because it takes in all the various brightness levels and then averages them to what is called a “middle gray” exposure reading. This is the basis for much of how metering and translating light value works, so is worth some consideration.

When the metering system receives light from the scene it attempts to average the exposure values so that the bright areas record as bright and the shadows as dark, in essence arranging the light values along a scale of light and dark. Let's say you are working with a scene where there is a range of values, from the bright white in clouds to the deep shadow of a valley. If you read the clouds alone with a spot meter they might read f/11. The edge of the valley might read f/8; the shadow area reads f/5.6. An averaged exposure would be f/8. This “places” the brightest clouds as a highlight, the edge of the formation as the middle value and the deep shadow as quite dark. Thus, exposing at around f/8 places the brightness values as they appear in the scene.

Because the pupil of the camera (the lens opening) is fixed at the moment of exposure, there is no leeway for adjusting to various levels of brightness within the frame. This means that one exposure time has to handle all the lights and darks in a scene, and try to get detail from them all. This is, as you can imagine, a delicate situation. How it is handled is to arrive at an exposure that allows in just enough light to bring detail into the dark areas (the shadows) and not get overwhelmed by the bright areas (the highlights.) This is usually an average of the two intensities of light. The average reading sorts out the lights and darks accordingly so some records as brighter than the average and some darker than the average, as it should be.

Although the exposure system is quite sophisticated you too have to do your part. In essence, the information you “feed” the brain of the meter is the information it acts upon. In exposure that means making a reading by pointing the camera and sometimes locking exposure values to get it right.

For example, let’s say light values EV 11, EV 7 and EV 9 exist in the scene. A change in 1 EV (exposure value) is one stop difference. The average of these three readings is EV 9, which we’ll call f/11 at 1/125 second. That’s the reading the meter will recommend and set for you. EV 9 then becomes the middle gray of the light to dark brightness value, or tonal scale of that image. In essence, the meter has read and set up a range of tones that will be recorded. With EV 9 as the middle gray, EV 11 records as a brighter value and EV 7 as a darker value.

If, however, the reading was made incorrectly and the middle gray was set at EV 7 (which you would get if you just read the shadow areas) then the EV 11 (quite bright) value would record as very bright, and result in overexposure. Conversely, if the middle gray were EV 11 (created if you just read the highlight area) then the EV 7 reading would become much darker than it appears to your eye. All the values work in lock step, so making a bright value a middle gray makes all the dark values darker (and perhaps underexposed, where no detail is seen) and making a dark value middle gray can cause all the bright values to become quite overexposed.

A sunset scene is a classic shot for center-weighted metering. The intent is not to get detail in the ground but to use it as a form that offsets the sky and defines the horizon line. The simplest approach to sunset shots where you want to have a rich sky is to use center-weighted metering, aim the camera at the sky (not the sun!), lock exposure and shoot. Use this method and you’ll never miss a dazzling sunset again.

This might seem quite confusing in the abstract, but working with the camera and making readings exclusively from certain brightness values in the scene, and observing results, will quickly show you how this system works. In fact, you can even make use of this knowledge to create very expressive exposures.

The key to center-weighted reading is to "bias the exposure" towards the highlight. In other words, point the camera towards the highlight area (including other areas as well) and make the reading from that area. This is especially true if the highlight area sits at the corner or out of the center of your framing.

Photo and text copyright George Schaub 2010. In this scene the camera was set on center-weighted metering pattern, pointed towards the brighter area in the upper right, and then exposure was locked and the image reframed to the compositon you see here. Matrix or evaluative would have undoubtedly overexposed the highlights in this scene.

Basics: Lens Aperture

Exposure is controlled by the aperture and the shutter speed settings. The aperture setting also influences depth of field, thus plays a major part in creative focusing decisions. Aperture settings are called f-numbers, and are expressed by "f/" followed by the number of the aperture set.

Aperture settings are indicated on a lens by a series of numbers; with some cameras and lenses there is no aperture scale on the lens barrel and the settings appear in the camera's viewfinder and/or LCD panel. A typical aperture scale might read: 1.4, 2, 2.8, 4, 5.6, 8, 11, 16. Each number indicates the ratio of the actual diaphragm opening to the focal length of the lens in use, thus any same aperture on any lens always allows in the same amount of light. As these are actually fractional numbers the smaller numbers signify larger openings. Thus, f/2 (or 1/2) represents a wider opening (or greater value) than f/16 (or 1/16).

As mentioned, f-numbers represent the same light value regardless of the lens or format in use. Thus, f/2 on a 50mm lens for a 35mm camera delivers the same amount of light through the lens as f/2 on a 200mm lens for a medium-format camera, even though the diameter of the openings themselves are different. If this wasn't the case the entire light control system in photography simply wouldn't work.

Each subsequent number in an aperture scale represents a halving or doubling of the amount of light that the aperture allows through the lens. (You can calculate the next higher number in any one-stop-step scale by multiplying the previous number by 1.4). Each step in the scale (say, from f/2 to f/2.8) is called a stop. Thus, every time you open up or close down the lens by one stop (opening means going to the next lower number, or wider opening; closing down means going to the next higher number, or narrower opening) you are changing the amount of light entering by the power of 2. A one stop change (say, f/8 to f/5.6) is a 2X difference; a two-stop change (sat f/8 to f/4) is a 4X difference; and so forth.

The best way to see how aperture settings effect light transmission is to take the lens off the camera, hold it up to the light and click through the aperture settings. (Note: Some lenses do not allow for aperture changes on the lens itself, but rely on the camera to change apertures,) You'll see that the maximum aperture, say f/2, is the widest opening. As you click through the scale, you'll see the diaphragm in the lens getting smaller. Think of water as it flows through a pipe. Given that the water will always fill the pipe, a larger diameter pipe will allow more water through. This can be applied to light flow and the aperture diameter.

Most lenses and cameras today allow for partial stops, like older lenses with click stops, or detents, between the aperture settings. On older lenses these are half-stops, and though not marked indicate a halfway point between the two aperture numbers on the ring. On newer lenses you can have 1/3, 1/4 or other fractional spreads. These step-less aperture settings means any value can be set, such as f/9.7 or f/11.3. These values are indicated in the camera viewfinder and/or the LCD panel on the camera.

Aperture rings are usually inscribed with all the available full stop settings on the lens, from the maximum, or widest, to minimum, or narrowest lens opening. The range of the aperture scale may differ depending upon the construction of the lens and its focal length. One scale may read f/2, 2.8, 4, 5.6, 8, 11, 16, while another may read f/4, 5.6,8, 11, 16, 22, 32 (the latter is more typical of zoom or telephoto lenses.) The lowest number in the scale (thus the widest opening) is called the maximum aperture; the highest number on the scale (thus the narrowest opening) is called the minimum aperture.

Photo and text copyright George Schaub 2010. Aperture settings allow you to control the depth of field, what appears sharp and unsharp in your photo. To get focus from foreground to background here an f/16 setting was used.

Color and Light

The overall quality of the light source can have a profound effect on color perception. Light and dark tints of color that in flat light would show as one hue become more differentiated in bright light--the effect of color contrast. Yet, if that light is too bright and the surface is glossy we will get greater interference, thus some of the color that we might see in flat light becomes "washed out" or replaced by white. If the surface is matte the reflection becomes more diffuse we see more color. Thus, the greater the surface reflection the less the color richness or saturation we perceive. Rough surfaces throw off all sorts of reflections that can vary the color in many ways.

Atmospheric effects also alter color. If you look at a range of mountains from a distance, for example, you see them as blue. When you walk or drive closer to them, however, you see them as green, or red or whatever color they might be.

The same goes for the color changes subjects seem to undergo throughout the day. The inherent color, if you will, of sandstone formations do not change but we all know that photographing those formations late in the day, on a clear day, will yield the most spectacular results. Those afternoon colors are influenced by the prevailing light. Their amber tint results from the color bias of the light as it travels longer distances later in the day.

The color of any one thing does not exist in a vacuum. It is influenced by the color of subjects around it and how those subjects absorb and reflect light. It's as if we exist in a world of color mirrors and reflectors that bounce light from one subject to another. This sets up the world of color relationships and creates many of the color enhancing vibrations and associations we see around us.

In short, the way we see color is almost subjective--it is certainly conditional. Just as brightness is influenced by a host of factors, color itself is always changing and being affected by the energy around it.

The color mood of this image is affected greatly by atmospheric conditions. Photo and text copyright George Schaub 2010

Looking at Scene Contrast

The main issue in making good exposures in high contrast scenes is learning the difference between how your eye “sees” and handles contrast and how the sensor “sees” and records brightness values. Contrast is defined as the difference between the brightest and darkest areas in a scene. In photography the areas that define a usable contrast range are those in which you can see and record detail and tonal values; the compositional decisions often involve how you treat those brightness areas that fall outside this range.

For example, if you photograph a white car in bright light you would want texture and tonal value in the car body and details and perhaps even in the tire tread. But you might not care about the details in the asphalt that sits in the shadow of the car. Or, if you’re taking a portrait in bright light you’ll want good skin tones values in your subject but may not care about information (details) in the shadow he or she casts. When we talk about a usable contrast range we are talking about those areas that you want to record and not those that may also be in the scene but that can fall into tone without detail, like a deep shadow. We can call this usable range of values the "significant" tones, with the brightest in which you want texture the significant highlight and the darkest in which you want detail the significant shadow.

If you take an exposure reading of just the significant highlight you are placing that highlight on the middle of the recording scale—-in essence, you are telling the exposure system that you want the highlight to record darker than it appears in the scene.

And, if you take a reading of just the significant shadow area (like in the image shown here) you will be recording it as brighter than it appears in the scene. This throws off the balance of brightness values in recordings where there are both bright and dark values. If you make a reading of and record the darker areas alone it will cause the brighter areas to “burn up” and become overexposed, just like the side of the building here.

If you want to make a quick test of how making readings from the lighter or darker parts of the scene affects your results, set up a bracketing sequence at +/-2 EV and take three pictures of a brightly lit scene with shadows and highlights. One exposure may average the two values, one will expose for the highlights and one for the shadows. You’ll see how making exposures for just a certain part of the brightness scale affects the other areas.

Photo and text copyright George Schaub 2010. Exposure here was read from the shadow areas. The result in a high contrast scene such as this is fairly substantial overexposure of the highlights, never a good thing in digital (or film) photography.