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By default the computer will assume that it's shooting an 18% grey card and expose the picture accordingly. There are a basically two knobs you can tune, to change this behavior.

One, you can decide what parts of the pictures are used in this calculation and how they are weighted. These are your spot, matrix, and middle measurement modes.

Two, you can use a creative program, e.g., night photography. That will treat bright highlights differently, AFAIK.

However, the world isn't really an 18% grey card and how you expose the picture depends on your creative intent. The most obvious examples are high-key and low-key photos. In fact, photographers often use 18% grey cards to determine the light conditions and then lock the exposure and white balance down and shoot manual. This way, they have a precise reading and no surprises. I only do that when I do portrait photography because skin tones and therefore white balance are crucial. In most other cases, I look at the histogram to see if the exposure is correct and use auto white balance.

You can always adjust white balance when you shoot RAW in post processing. However, you cannot fix the exposure if you're missing the data. That's why I use manual.




You’re forgetting the third knob you can tweak - the exposure compensation dial. That’s arguably the most important control for the metering system!

If you understand how your camera’s metering system works (area selected, 18 percent gray etc) you can often work with the Metering system and compensate for when you know it’s likely to get it wrong - for example it’s not uncommon to dial a +1 if shooting a bright snowy scene, as most metering systems will be overly aggressive in trying to preserve highlight detail in the snow.

Similarly, if shooting in darker conditions dial in negative compensation.

Metering is pretty sophisticated in modern cameras, the exposure comp dial let’s you work with the tool and augment it’s measurements with your own human input.

Exposure comp plus histogram reviews with metering on is a pretty effective way to work a lot of the time.


All in all, I agree with your assessments. Its good to note that what you are trying to do is read the amount of light hitting the target. This could always be done with a ambient light meter placed in front of your subject (Studio).

The issue was the convenience of seeing it through the camera. The first automatic in camera meters were limited to high-key or low-key and tried to just make the picture's average lighting match the 18% grey card, a portion of it (center weighted or spot). Highly colored subjects could throw this off as the meters weren't always linear to all wavelengths. As technology advanced, matrix meters used multiple points to try and guess tricky lighting conditions. As more elements to the matrix were added, identifying where the subject became possible.

However, they all are just trying to guess at the level of light hitting the subject of the photograph.


Again, you are incorrect if you meant that multiple samples have fixed the in-camera meter foobar.

I am assuming you are well acquainted with colour management, so I will just leave a brief suggestion. If you further need more pointers, let me know and I would love to elaborate.

Read about reflectivity and their influence on metering.




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