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.

On-Camera Fill Flash

The small, built-in flash on your camera is not meant to be a powerhouse that will enable you to capture large groups indoors or throw light any significant distance outdoors. Most are good to about 10 feet maximum, though range can be boosted somewhat with higher ISO settings (with the increased noise price that you pay.) Some have a bit more power than others, but rarely do they go beyond a fairly short range. However, there are times when the small output can be used for adding just a taste of light to highlight a foreground subject and to bring lighting balance into a contrasty scene.

An opportunity for just that situation came up when photographing along the Rio Pueblo in Northern New Mexico last year, where the foreground sat in deep shadow. I knew I wanted to retain those shadowed forms to highlight the bright foliage, but after the first shot I thought that the shadows were too dominant (fig 1). Near my shooting position sat a small bush with the same yellow coloration of the background that did not record in my first shot.

I raised the small flash on my camera and took a shot. I liked what it did for the composition but the foreground plant looked too bright, a result of hitting it with flash from a few feet away. I then chose to use flash exposure compensation at -1.5EV, a good practice when working close like this. The resultant shot helped balance the composition and brought some visual interest into the foreground.

So next time you are working in strong contrast, and want to compose to maintain the shape and form of the shadows, consider using the small pop-up flash, along with flash exposure compensation, to add some extra visual “kick” to your images.

1
Exposure: Spot meter reading on bright bushes in background, f/16 at 1/125 sec at ISO 100; 24mm (equivalent) lens.
2
Exposure: f/16 at 1/125 sec at ISO 100, fill flash (on-camera) set at -1.5EV flash exposure compensation.

Text and images copyright George Schaub 2009, all rights reserved

Landscape and Nature Photography

“I was studying for a degree in environmental conservation at the University of Colorado in Boulder, taking classes in mountain ecology...My summers were spent backpacking in National Parks. My mother gave me a camera when I went away to school, and it seemed like a natural thing to take along. I wanted to document what I was seeing and what was exciting to me. Photography didn’t start out as my ultimate goal. After a couple of summers backpacking and photographing nature, the activity of photography grew to be more important than backpacking.”

William Neill, from an interview with Grace Schaub

The desire to incorporate the power and beauty of the natural world around us into our being is one of the prime motivations for making landscape and nature photographs. These photographs can then be shared with others to show where we’ve been and what we’ve appreciated. A landscape may depict clouds rushing over mountains in the wilds of the Rockies, or a barn or rustic farmhouse in mannered fields. Not all of nature is bucolic and sunny. Powerful landscapes can also show the power and, at times, fury of nature, even the devastating effects of man upon the natural world.

One of the keys to successful landscape photography is using visual and technical applications to capture a true “sense of place.” The aim is to record both the external visual record of the place as well as the internalized power and presence of the experienced moment in which it is recorded. The most powerful images are both visual and emotional records.

Landscape images may at times be a gift from a coincidence of sky, light, time of day, or the viewpoint offered by the road or trail. However, evoking a true sense of place usually demands patience, applied technique and a willingness to “feel out” an area prior to photographing. It also requires active seeing and contemplation on what framing, exposure and time of day will best communicate the power and beauty of a location.

One way to approach landscape studies is to leave your camera in your bag before you begin to shoot. Move through an area and make mental notes on framing, the direction of the light and the best point of view. While spontaneous moments of inspiration should not be denied (especially on days when the light is undergoing constant change), consideration of a number of photographic options prior to making pictures may be the best course. It also allows time for enjoying and appreciating the place.

Landscapes tend to be broader views of an area that encompass sky and ground or a lake with surrounding forest. Though the distinction may be slight, and the photographs may be made in the same locales, nature photography is generally on a more intimate scale. It may be photographs of wildlife or a clump of fall leaves caught in the glistening waters of a rushing stream. Nature photography often relies on chance, or serendipity, and pictures are found while enjoying a hike in the woods or a stroll down the beach. Nature can also be an excellent source of abstract forms; images made in that frame of mind become metaphors for a grander design, or touch emotions not usually engendered by the subject’s face value.

Landscape and nature studies have always been an important part of photography The earliest book of photographs by one of the pioneers of photography, Fox-Talbot, was entitled “The Pencil of Nature.” Nature as metaphor was a major theme of photographers such as Minor White, Walter Chappell and Edward Weston. The linkage of nature photography with conservation became the life’s work of Ansel Adams. Many activist-photographers carry on this work today.

Image and text copyright George Schaub 2009. All rights reserved.

Digital Infrared Black and White

Infrared light is by definition “invisible” light that resides above the threshold of human vision. It is not Kirilian, or aura photography, though it sometimes creates an ethereal, ghost-like representation that can be quit seductive. Like most things photographic that appear visionary, there is a bit of science behind the magic. This involves disabling the “normal” operation of the camera to allow infrared level light to record, something that is usually blocked by an IR cutout filter placed in the light path from lens to sensor. It turns out that without this internal filter the digital image would be “polluted” with IR, and would, for most people, create less than desirable image quality. In truth, digtal sensors often have considerably more infrared sensitivity than you might think.

In most cameras the filter is “hard wired” into the construction, which means that if you remove it you now have a fulltime IR capable camera that cannot be returned to normal use. This must be done by a service company that knows its stuff; it’s something you cannot do on your own. Some cameras come IR dedicated, mostly those used by law enforcement agencies for gathering forensic evidence. A very few can be converted in do-it-yourself fashion, such as the Sigma model DSLRs, where you can remove the lens and literally pick out the IR filter (which doubles as a dust filter), albeit very carefully, and reinsert it later. Those who are true IR fans would do well to investigate the cameras used by police departments; as of this writing Fuji makes a few such models.

Another step that needs to be taken to capture true IR images is placing a filter over the camera lens, something that is uncommon for most digital photography (as most filter effects can be added later in software.) If you shoot IR without any filters with an altered camera you will get a sort of reverse pollution of visible light, something that certainly diminishes the IR effect.

There are three types of basic filters that can be used, with one being quite expensive and, in my experience, unnecessary to gain the effect. The filters used include a red filter, something black and white film photographers might still have in their closet, (a Wratten 25A), plus two filters that block more and more visible light and do not allow for image recording below the infrared threshold. The two other filters, which go under various names and codes according to the filter maker’s markings, block light under 700 nanometers and 830 nanometers (and some higher) on the spectrum; in other words, progressively more IR light and much less visible light. The highest blocking filter (830nm and above) gets very expensive and is only for well-heeled purists and aficionados.

The effects achieved with the two blocking filters can be amazing but you cannot see what’s going on in the viewfinder, which means that you have to frame and focus prior to placing the filter over the lens and making the exposure. Some photographers view over the top of the camera for an approximate framing. The red filter, while not as “pure” lets you at least see what’s going on in the finder. If you are an IR fan then the blockers will be your choice—those who dabble in it, as I do, will find the red filter is fine.

Exposure is unusual as well. It has nothing to do with making readings and using metering patterns, since you are not dealing entirely or at all with visible light. It’s a strange concept, but that’s also part of the IR mystique—being out of normal bounds of having to read exposures and balance highlight and shadow. The best way to work IR exposures is to start at somewhere around f/11 at 1/125 second and then review the image after exposure and adjust accordingly. The view in the finder is quite different than you’d expect, so you will have to gain experience with what a processed IR image looks like according to a certain exposure level. This is the only way you will be able to make predictions about what the correct exposure might be, or at least what it should look like upon playback.

There is something in IR known as a “focus offset”, which means IR light bends a bit differently and may arrive at the sensor plane in different ways than visible focusing. This will become critical when doing close-ups, but does not have much effect, in my experience, for photos made beyond six feet. I often shoot at f/11 or narrower with a fairly wide angle lens just to take up that slack, changing the shutter speed accordingly.

The procedure some use for shooting is to purchase a filter that is a bit larger than the largest diameter lens they own (thus it can be used on all the lenses owned). They first frame and focus the tripod-mounted camera, then hold the filter over the lens when the exposure is made. This procedure takes care as it can result in some light leak from the corners, but that is usually eliminated with some practice in proper holding of the filter. Exposure is set manually. Some bold photographers shoot handheld in the same fashion, viewing over the top of the camera to yield an approximate framing and shooting a bit wider than they normally would for a “fudge” factor. They figure they can apply a more specific cropping later.

IR black and white has always been near and dear to landscape photographers, and now that high-speed IR black and white film has been discontinued by major film makers digital seems the only way to go.