
My definition of a perfect exposure is having the brightest part of the scene captured at just under the maximum RGB value (e.g. 4096 in a 12-bit RAW). If the exposure is greater or less than this, I have less fidelity of my scene. And how can a light meter tell me where that brightest part of a scene is if it is measuring the whole scene, instead of various zones of the scene as the camera does? It cannot. The best it can do is figure out the EV of that one scene and hope that the dynamic range is not too small or not too large so that when captured to a digital sensor, neither the highlights get clipped, nor the exposure too low so that the more accurate bits go unused.
Blue text is unfortunately overly simplistic definition that ASSUMES that 'the entire range of brightness in a scene can FIT within the dynamic range of film/sensor'. So what happens when the DR of the scene is 10-11EV and only 8-9EV will FIT?!
- Then the photographer's brain (we are back to the need to THINK...metering is only a guideline!) decides if the highlight details are more important, or if the shadow details are more important.
- Perhaps the brightest details are the sun's own reflection in the chrome bumper...do you even CARE to capture an image of the sun in the chrome of the bumper?!
- Perhaps the shadow details matter far more than wanted details in the brightest part of the scene.
- Perhaps the brightest details are the sun's own reflection in the chrome bumper...do you even CARE to capture an image of the sun in the chrome of the bumper?!
One of the nice aspects of the one-degree SPOT meter is that it can measure the brightest area, and the darkest area, and show you if the DR of the scene exceeds the capture range of the film/sensor (and the DR of a digital capture is less at very high ISO than it is an lower ISO, and it is important to keep that in mind!) Then the photographer can decide what part of the DR is important to keep, and what is merely a 'nice to have'.
That is why product photographers shooting for publication on offset press will use the incident meter to set the base exposure, but also use a reflective spot meter to make sure it fits within the even more compressed DR limitations of the offset press image.