exposure36 Photography

How Your Camera Sees the World

by Jim Altengarten, exposure36 Photography

 

In photography, how the camera sees the scene is much more important than how you see the scene, because it’s the camera that takes the final image.  In order for you to “see” the scene the way the camera will expose it, you need to understand how the camera’s exposure metering system works.  Once you select the right metering system, you’re on the right path to achieving a good exposure.

 

Many manual cameras and some introductory auto-focus models have only one metering system.  This makes it easy to figure out how exposure metering works.  If you have a camera that fits this description, don’t worry.  There are techniques that you can employ to simulate other metering systems.

 

We will use the image on the left to demonstrate how each of the metering systems operates.

 

CENTER-WEIGHTED METERING

 

Center-weighted metering is the default metering system for all manual cameras.  It’s also the default metering system for most auto-focus
 cameras when they’re used in manual mode, although it’s not the most sophisticated metering system available.  Manufacturers often provide center-weighted metering on auto-focus cameras to ease the transition from manual to auto-focus cameras. 

 

The center-weighted system operates just as its name implies.  It concentrates the exposure on the central portion of the viewfinder.  Typically, about 75% of the emphasis of the metering system is in the central portion of the viewfinder.  The remaining metering is feathered out toward the edge. In actuality, very few of the tonalities at the edge of the frame are even considered by this metering system.

 

The diagram at the right is a rough approximation of how this metering system works.  The black circle represents the central portion of the viewfinder, which is the location of the major emphasis of the metering system.

 

Center-weighted metering works best when your main subject covers the central portion of your viewfinder.  In this instance, you’re probably metering only one tonality.  Since the camera will make that subject a medium tone, you can have confidence you’ll get the tonality correct when you make any adjustment.  You can meter other areas of the scene besides your main subject if you want to find a medium tone to meter.  Just remember that the tone should cover the central portion of the viewfinder for best results.  After metering another area, just recompose, make sure you “lock” in the exposure values, and ignore what your meter indicates after re-composing.

 

Unless you’re metering a medium tone with the central portion of your viewfinder, you need to use some exposure compensation in order to get a good exposure.  This is a very good system for learning to recognize different tonalities.   If you metered a scene without adjustment and it came out too bright, then you metered a part of the scene that was darker than medium. (The camera says, “The scene is way too dark.  Add exposure.”)  Conversely, if the scene comes out too dark, then your meter was influenced by some bright element in the scene. (The camera says, “The scene is way too bright. Take away exposure.”)

 

Would the center-weighted system work with the sample image (left)?  In the image, you have a variety of tonalities--the white bird, the dark water of the river, and the tones in-between, such as the gravel along the riverbank and the grasses in the upper portion of the frame.  Since the center-weighted system works best if you have only one tone covering the central portion of the viewfinder, the sample scene might be a challenging one on which to use this particular metering system.

 

If you didn’t re-compose the scene, your center-weighted circle would cover both the white bird and the darker riverbank.  (right) The circle is too large to just cover the bird, even if you re-compose the scene.  When the metering system covers two tonalities, you can’t make any adjustments with confidence that the exposure will be correct.

 

Do you simply give up on this action shot?  No!  If you raise your camera to the grasses above the riverbank, you have one tonality in the central portion of your viewfinder.  Your next problem will be to decide whether the grasses are a medium tone. If they are, then there’s no need to make an adjustment.  Re-compose and take the shot.

 

The procedure sounds simple in hindsight, but how do you do all of that while the bird is landing in the water?  In this case, there were two birds that flew upstream, landed in the water, and floated back downstream.  They repeated these movements for over a half hour.  You might miss them on the first attempt, but at least you would be able to see that the birds are too small to meter with this system.  While they were floating downstream, you could establish your exposure on the grasses.

 

What if the bird landed only once?  Don’t take a chance on being able to meter just the bird.  Preset your exposure based on the grasses (or any other large element in the vicinity).  There’s nothing wrong with anticipating potential exposure problems and resolving them in advance.

 

 

SPOT METERING

 

A spot meter is usually found in higher-end cameras (both manual and auto-focus).  Spot metering works differently from center-weighted metering.  All of the emphasis of the exposure meter is devoted to a small section of the viewfinder, and everything else is ignored.

 

This diagram at the right is a rough approximation of how the spot meter system works.  Only the portion of the scene covered by the black circle is used by the exposure meter to determine exposure.

 

Although some people use spot metering for every situation, it’s best used only when either the scene contains a wide range of tones, or when you need to be very precise with your metering.  In the latter situation, for example, you may be photographing a person and want to make sure you expose the face properly.

 

Spot metering is valuable to determine the tonal range in a scene.  Your film has a certain exposure range.  Spot meters allow you to check small portions of the scene to see how many stops of difference exist between the brightest and darkest areas of the scene. 

 

In-camera spot meters typically measure about 2-3% of the viewfinder area.  Some Nikon models evaluate 1%.  Most spot meters in digital cameras are a little larger than average.  You can obtain a hand-held spot meter with a 1% field-of-view in a variety of price ranges.

 

A spot meter checking a part of a scene is similar to a center-weighted metering system checking one large tonality.  It makes it easy for the photographer to expose the scene, since he or she simply has to adjust for that tonality (if needed).  Since spot meters cover such a small area, it’s a rare scene where your spot meter can’t isolate one tone.

 

If you have a camera with multiple focus points, check your manual for the operation of your spot meter.  Some cameras will automatically link the spot meter to the active focus point.  Other models will keep the spot meter at the center focus point, even if it’s not the active one.  Usually, these latter cameras have a custom function that allows you to link the spot meter to the active focus point.

 

How does a spot meter system handle the sample image?

 

                                                                            

The spot meter area is small enough, so the bird is the only tonality included in the exposure measurement.  To shoot the image, you would use the following procedure:  Before the bird starts its landing, preset your camera to spot metering, and add the desired exposure compensation.  Since you know the bird is lighter than medium tonality, add one to 1.5 stops of exposure.  As the bird lands, all you have to be concerned with is placing the bird within the spot meter (on the active focus point) and getting the right shutter speed, which you should do in advance to ensure sharpness.

 

Another way of shooting the scene is to pre-expose on the grasses--the way you did with center-weighted metering.  The spot meter area is small enough, so you shouldn’t have a problem isolating the grasses (see the image on right). Be sure to change your focus point after exposing off the grass, unless you want that to be the area in focus.  One way of locking in the exposure settings from the grass is to note the settings, switch your camera to manual mode, and dial in the settings.

 

Spot meters differ from center-weighted meters in that they completely ignore anything outside their area.  In contrast, center-weighted meters place a small amount of emphasis (usually 25%) on the area outside the center zone.  

 

Spot meters are great, but don’t start grumbling if you don’t have one.  You can simulate a spot meter using one of the following techniques:

 

1. If possible, you can walk closer to your subject so the central portion of your viewfinder covers only one tone.  Meter off the tone and make any adjustments.  Return to your original location, and use the camera in Manual mode.  (This would probably not work in our sample image since the river was over 100 feet wide.)

2.  Change to a longer focal length lens. The focal length should be long enough to allow you to isolate one tone in the central portion of your viewfinder.  After metering, change lenses back to the original one, and use the camera in manual mode. 

 

Here is a word of warning about using a spot meter and a wide-angle lens:  These lenses include a lot of the scene in the final image.  Therefore, many of the elements in the scene are fairly small.  When you spot meter with a wide-angle lens, it’s easy for your metering area to cover more than one subject.  Precise metering is essential with the spot meter. 

 

PARTIAL METERING

 

Partial metering is identical to spot metering except that it covers a larger area of the viewfinder.   Partial metering systems are common in most intermediate-level cameras and are appearing more often in entry-level cameras.  Partial metering is included with spot metering in the advanced and pro-level cameras.  Since spot metering is so precise, you’ll probably choose it rather than partial metering.  

 

The image on the left show the partial metering area directed toward the grasses.  The bird is probably too small to meter with the partial system.  At least two tonalities would be covered by the partial metering area.

 

Using partial metering, the best exposure strategy is to establish your exposure values in advance, just as you do with other metering systems.  There is one big advantage of setting your exposure (and compensation) in advance:  There is less to worry about when the time comes to press the shutter button to take the photograph.

 

 MULTI-SEGMENTED METERING

 

Multi-segmented metering is found only on auto-focus cameras.  Since it’s the most sophisticated system, it’s the default system on all entry-level cameras.  Multi-segmented metering is a generic name.  It is called “matrix metering” on Nikon cameras, “honeycomb metering” on Minolta cameras, and “evaluative metering” on Canon cameras.  Other manufacturers use different names, but all the systems work in the same manner.

 

 As seen in the image (right), all of the viewfinders are divided into a certain number of zones.  Depending upon the camera, there can be as few as three and as many as 35 (as of 2003) separate zones.  The image shows a very simple grid as the metering zones.  Most have a different pattern.  Typically, your camera’s manual will diagram the zones for your camera in the section where multi-segmented metering is explained.

 

In simple terms, the camera’s computer evaluates the lighting pattern in each zone and uses an algorithm to search its database to find a similar pattern.  When the camera finds the pattern in its database, the associated f-stop and shutter duration appears in the viewfinder and LCD panel of the camera. All of this takes place in the milliseconds after you press the shutter button halfway.  How big are the databases?  The Nikon F5, a professional-level camera, has about 105,000 entries in the database.

 

The Nikon F5 has 6 zones, the Rebel Ti, Canon’s beginning level camera, has 35 zones.  Does that make the Rebel better?  NO!  However, the database size and search algorithm on the F5 is vastly superior to those components on the Rebel.  Part of the large sum of money that distinguishes those two cameras is associated with the superior algorithm and database of the F5.

 

That was the simple description.  The multi-segmented meter takes into account such things as the active focus point, lighting level, front/back lighting, subject size, and color. (No wonder it’s the most sophisticated system.)

 

Multi-segmented metering is best used when there isn’t a wide range of tones in the scene.  If everything in the scene is medium tone or averages out to medium, this system will not fail you.

 

The image on the left was shot with a multi-segmented metering system.  Note the medium tone of the blue sky, the very light tone of the snow, and the dark tone of the trees on the side of the mountain. 

 

Since the light and dark tones didn’t dominate the scene, the metering system was able to correctly expose it.  As long as there is at least one fairly large element that’s a different tone from the rest of the scene, the multi-segmented metering system will probably get a good exposure.

 

Exposure compensation with the Multi-segmented metering system can be tricky.  The camera searches the database to find a pattern similar to what it sees in the viewfinder.  If you make any compensation, you might be doing something that is counter-productive to what the camera is doing.  Since this sophisticated system acts on several variables, you can’t be sure how it’s weighing each one.  Be conservative when using exposure compensation with this system.

 

There are two situations in which exposure compensation is always acceptable with multi-segmented metering systems:

1. If the scene has only tone tonality in it, or is predominantly one tonality other than medium.  Since there’s no point of reference for the camera’s meter as to what is light, dark, or medium, it will make the entire scene medium.  If the scene is either high key (predominantly light) or low key (predominantly dark), you’ll end up with medium gray.

 

2. When the subject is very small.  Often, in this situation, there’s another tonality covered by the focus point, and the camera has difficulty determining which tone to use as its base tone.  (Again, this is frequently a problem with wide-angle lenses.) 

Doing exposure compensation with a multi-segmented metering system is the same.  If the scene is light, add exposure.  If the scene is dark, take away exposure.  One exception is back-lighting (light coming from behind your subject).  Older auto-focus multi-segmented metering systems will have problems and tend to underexpose the subject.  Newer models seem to do better.  The advanced and professional-level cameras should get it right most of the time.  Do a few experimental shots with your subject back-lit to see how your metering system reacts. If it underexposes, use your spot/partial (or simulated) metering to get the subject correct.  Be aware, however, that the background will probably be completely washed out (no detail).  Another alternative is to use fill (partial) flash to illuminate your subject properly.

 

Jim Altengarten is the owner of exposure36 Photography.  He focuses on photographic education through workshops, classes, private lessons, and classes on CDs.  Information about these products is available at the exposure36 Website (www.exposure36.com) or via telephone at 1-866-FOTOS36 (toll-free).  He specializes in Canon EOS cameras (digital and film) and teaches several classes and workshops each year that enable EOS users to understand all of the functions of their cameras.