Originally posted by biz-engineer Interesting article.... .. using an incident light meter and camera in manual mode it works well to depict realistic exposures, although realistic isn't necessarily the best for an image to look good. Without a stand alone light meter, the camera light meter can be used when pointing the lens to a 18% gray card and locking the exposure (pressing the AE-L button). Exposure is a whole important topic, and raises questions: When to I exposure to preserve high light or should I get a correct exposure of the mid-tones and shadows while accepting burned high lights?
The 18% gray card method will work as long as everything in the scene has Lambertian reflectance (
Lambertian reflectance - Wikipedia) and is lit by the same very distant light source (e.g., the sun) as the 18% gray card.
If there are different light sources illuminating the scene (e.g., shade and sun), then things get a little messy. (Exposing with the gray card in the brighter light helps.)
If the light source illuminating the scene is at very different distances to different parts of the scene (e.g., a single room light or a photoflash), then things get more messy. (Exposing with the gray card in the brightest area of the light helps but sometimes the ratio between the brightest and darkest parts of the scene are too much.)
If some of the objects in the scene are shiny, then things get messy. (Chances are the reflections will blow-out.)
If some of the objects in the scene are the light sources for the scene, then things may be almost impossible. (Blow-outs are unavoidable: the disk of the Sun is about EV 32 or 17 stops brighter than the sunlit ground.)
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The more fundamental problem is that prints, slides, and computer displayed images all have a fixed and modest maximum brightness. The image of a "white" object in a scene needs to reflect, transmit, or display 100% brightness in order to look white. But if a scene contains both a white object and also the bright or reflected light source that illuminates that white object, then the print, slide, or computer image would have the impossible task of being BOTH 100% bright for the white object and then 1000%, 10,000%, or even 10,000,000 % bright to accurately replicate the image of the light source.