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.  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 understand how the camera sees the scene, you can select the appropriate metering system.  Once you select the right metering system, you are on the right path to getting a good exposure.

 

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

 

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

 

CENTER WEIGHTED METERING

 

This is the default metering system for all manual cameras.  It is also the default metering system for most auto focus cameras when they are used in Manual mode. 

 

In auto focus cameras, it is not the most sophisticated metering system available.  Manufacturers often provide center weighted metering on autofocus cameras to ease the transition moving from manual to auto focus cameras.  One student of mine switched from a manual Olympus camera to a high-end auto focus camera.  He was having difficulty figuring out exposure situations.  I suggested that he operate the camera in center weighted metering until he was comfortable with the other metering systems.  He had about 20 years of experience with center weighted metering with his old camera, so he was able to get great shots while taking his time to test the other metering systems on his new camera.

 

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 little 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 are probably only metering one tonality.  Since the camera will make that subject a medium tone, you can have confidence when you make any adjustment to get the tonality correct.  You can meter other areas of the scene besides you 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 are 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 (camera says: “the scene is way too dark, I need to add exposure”).  Conversely, if the scene comes out too dark, then your meter was influenced by some really bright element in the scene (camera says: “the scene is way too bright, I need to take away exposure”).

 

Would the center weighted system work with our sample image?  In the image we have a variety of tonalities i.e. the white bird, the dark water of the river, and the tones in between (the gravel along the river bank, and the grasses in the upper portion of the frame).  Since our center weighted system works best if we can have only one tone covering the central portion of the viewfinder, this might be a challenging image for this particular metering system.  If there is a problem, then we need to find a way to still get the image.

 

         original                                                     center weighted

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

If we did not re-compose the scene, our center weighted circle would cover both the white bird and the darker riverbank.  The circle is too large to just cover the bird, even if we re-compose the scene.  When the metering system covers two tonalities, we can not make any adjustments with confidence that the exposure will be correct.

 

Do we just give up and not try to take this action shot?  No!  If we raise our camera to the grasses above the riverbank, we can have one tonality in the central portion of our viewfinder.  Our next decision is 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.

 

It sounds simple in hindsight.  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 one-half hour.  You might miss them on the first attempt, but at least you will 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 about a situation where the bird is landing 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 is nothing wrong about anticipating potential exposure problems, and resolving them in advance.

 

 

SPOT METERING

 

A spot meter is usually found in a higher-end camera (both manual and autofocus).  Manufacturers build cameras this way because they believe that beginning and intermediate photographers would get confused with the more sophisticated techniques of metering with this system.

 

Spot metering works differently than 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 is best used 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 that the face is properly exposed.

 

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 difference exists 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.

 

When a spot meter checks a part of the scene, it is similar to a center weighted metering system checking one large tonality.  It makes it easy for the photographer to exposure the scene since he/she only has to adjust for that tonality (if needed).  Since spot meters cover such a small area, it is a rare scene where your spot meter can not 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 is 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 our spot meter system handle our image?

    

            original                                              spot meter

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The spot meter area is small enough so the bird is the only tonality included in the exposure measurement.  If you were shooting the image, here’s the 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 concern your self 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 just the way you did with center weighted metering.  The spot meter area is small enough that you should not have a problem isolating the grasses.  This is illustrated in the image on the right.  Be sure to change your focus point after exposing off the grass unless you want that area as the one 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.  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.  If your camera uses a metering system other than a spot meter, you can still simulate a spot meter using the following techniques.

 

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.  Let’s look at our second option.  Change to a longer focal length lens. It 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 metering with a wide angle lens, it is very easy for your spot metering area to cover more than one subject.  Precise metering is essential with the spot meter.  Be careful with wide angle lenses.

 

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 of the 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 will probably choose it rather than partial metering.   On my cameras, I disabled the partial metering since I always prefer to use spot metering for my precise exposure 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 establishing you exposure values in advance, just as we have done 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 to actual press the shutter button to take the photograph.

 

 

 

MULTI-SEGMENTED METERING

 

This metering system is found only on auto focus cameras.  Since it is the most sophisticated system, it is 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 have different names.  They all work in the same manner.

 

As seen in the image to the right, all of the viewfinders are divided into a certain number of zones.  Depending upon the camera, there can be as few as 3, and as many as 35 (as of 2003) separate zones.  The image on the right 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 it’s 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!!!!!!!!  Canon would like you to think so.  J   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 distinguished those two cameras is associated with the superior algorithm and database.

 

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 is the most sophisticated system.

 

Multi-segmented metering is best used when there is not 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 did not dominate the scene, the metering system was able to correctly expose the scene.  As long as there is at least one fairly large element that is a different tone than 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 not be sure how it is weighting each one.  Be conservative on using exposure compensation with this system.

 

There are two situations where exposure compensation is always acceptable with Multi-segmented metering systems.  First is if the scene has only tone tonality in it, or is predominantly one tonality other than medium.  Since there is 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.

 

The other scenario is when the subject is very small.  Often in this situation there is another tonality covered by the focus point and often the camera has difficulty determining which tone to use as its base tone.  Again, this is often a problem with wide-angle lenses.

 

Doing exposure compensation with a Multi-segmented metering system is the same as previously explained.  If the scene is light, add exposure.  If the scene is dark, take away exposure.

 

One exception to the above is back lighting (light coming from behind your subject).  Older autofocus 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 lighted 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 using fill (partial) flash to illuminate your subject properly.