Luminous Landscape Forum
Raw & Post Processing, Printing => Colour Management => Topic started by: Edalongthepacific on January 23, 2011, 01:08:24 pm
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Digital camera meters are calibrated by manufacturers to what is called 13% gray. Does this mean light that is reflected from a card that is tinted 13% gray? That seems too bright to me?
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Why does it seem too bright to you?
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The 13% is the reflectance of the card, not the amount of gray. It is actually quite dark - L* somewhere around 40, or if you prefer AdobeRGB about 95, 95, 95
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Then is 18% gray even darker? How can that be if that reflectance represents midtones?
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Then is 18% gray even darker? How can that be if that reflectance represents midtones?
No, 18% is lighter. It is the percentage of light reflected. 18 > 13
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I THINK it works like this. Black and white photographers meter using an 18% gray standard. An 18% gray card, reflects more or less light depending upon the brightness of the light reaching it from any scene. Digital meters are set to a 12% gray standard. A 12% gray card (there are some out there) reflects more light, in the same scene, than an 18% card (1/2 stop). Publishers also use the 18% standard so that black and white type print,in fact, black and white. Black and white photographers want a complete 10 zones from pure white to flat white. Color digital photographers want detail in highlights and saturation in midtones.
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Sorry to say so, but it does not work like this at all. The idea behind the type of exposure metering that you are discussing is to place the midtone into the mid-point of the useful input range of the photosensitive material. 18% grey is midtone, but the card is flat. To get from a flat card readings that are representative for a 3D real-life object an exposure meter should apply correction. But it does not know what type of the surface it is. The difference in reflectiveness of two equally grey surfaces, one being flat, and the other being a sphere, is about 0.5 EV. 18% divided by square root of 2 (1 EV is a 2x change in light, so 0.5 EV is sqrt(2) ) is 12.7% grey. For a flat grey card to work as it should for exposure metering with a non-flat receptor it must be at least be placed at an angle to the lens axis, not perpendicular.
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So if an 18% gray card is properly placed and metered, the digital camera will accurately record neutral gray?
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More or less so, depending on how you define proper exposure (or real ISO sensitivity value).