Doug Kerr
Well-known member
A respected colleague has suggested elsewhere that I start a thread for a rant on gray cards. Sorry, rant is not my milieu.
But maybe it is time to give my semi-annual lecture on "the 18% matter".
We often hear that "[reflected light] exposure meters (or automatic exposure systems) are expected to be calibrated to 18% gray". Is that true?
Well, that statement doesn't really describe any identifiable technical situation, so it can't be either true or not.
But here's a situation that the speakers of this phrase might mean to refer to. Excuse me for stating it in a rather elaborate way; I need to be precise here, and the matter is a little complicated.
If we have a camera:
a. Whose automatic exposure system is "calibrated" as prescribed by the applicable ISO standard
and
b. Whose ISO sensitivity ratings are made in accordance with the applicable ISO standard
then, assuming the use of an "averaging" type of metering by the automatic exposure system:
If we have a scene whose average reflectance (over the field of view of the exposure metering sensor) is 18%, the camera exposure will be set so that a scene region with 100% reflectance will receive a photometric exposure on the sensor 1/2 stop below saturation.
[Photometric exposure is the property to which the sensor responds. For example, it doubles if we double the luminance of the scene region being considered, double the exposure time, or increase the aperture by one stop.]
A corollary is this:
If we use incident light metering by having a reflective light exposure meter (or the camera's automatic exposure control system) observe a gray card in the scene, then (for a camera meeting specifications a and b above) if that card has a reflectance of 18%, a scene object with a reflectance of 100% will receive a photometric exposure on the sensor 1/2 stop below saturation.
If we prefer a "no headroom" exposure strategy (where a 100% reflectance object will be given the saturation photometric exposure), then we will need to "bump" the exposure by 1/2 stop from that suggested by the meter (or use an exposure compensation of +0.5 Ev in the case of an automatic exposure control system).
Let me speak for a moment about "medium gray".
Sometimes a neutral body with a reflectance of 18% is referred to as "medium gray". That's pretty arbitrary, but is often rationalized by the fact that the L*a*b color coordinate system (in its original use, to describe "reflective" color) assigns an L* value of very nearly 50% to a color with reflectance 18%. Fair enough.
Some people than carry this the the realm of the digital representation of an image, and say that the digital representation of a gray color that corresponds to a photometric exposure 18% of that which would produce the highest possible digital representation should be considered "medium gray" (again, L*=50% if we consider the image in the L*a*b* coordinate system).
As a further cause of confusion here note that in a digital graphic arts context, such a color is called "82% gray" (0% gray there meaning not gray at all, that is, "white").
Best regards,
Doug
But maybe it is time to give my semi-annual lecture on "the 18% matter".
We often hear that "[reflected light] exposure meters (or automatic exposure systems) are expected to be calibrated to 18% gray". Is that true?
Well, that statement doesn't really describe any identifiable technical situation, so it can't be either true or not.
But here's a situation that the speakers of this phrase might mean to refer to. Excuse me for stating it in a rather elaborate way; I need to be precise here, and the matter is a little complicated.
If we have a camera:
a. Whose automatic exposure system is "calibrated" as prescribed by the applicable ISO standard
and
b. Whose ISO sensitivity ratings are made in accordance with the applicable ISO standard
then, assuming the use of an "averaging" type of metering by the automatic exposure system:
If we have a scene whose average reflectance (over the field of view of the exposure metering sensor) is 18%, the camera exposure will be set so that a scene region with 100% reflectance will receive a photometric exposure on the sensor 1/2 stop below saturation.
[Photometric exposure is the property to which the sensor responds. For example, it doubles if we double the luminance of the scene region being considered, double the exposure time, or increase the aperture by one stop.]
A corollary is this:
If we use incident light metering by having a reflective light exposure meter (or the camera's automatic exposure control system) observe a gray card in the scene, then (for a camera meeting specifications a and b above) if that card has a reflectance of 18%, a scene object with a reflectance of 100% will receive a photometric exposure on the sensor 1/2 stop below saturation.
If we prefer a "no headroom" exposure strategy (where a 100% reflectance object will be given the saturation photometric exposure), then we will need to "bump" the exposure by 1/2 stop from that suggested by the meter (or use an exposure compensation of +0.5 Ev in the case of an automatic exposure control system).
Let me speak for a moment about "medium gray".
Sometimes a neutral body with a reflectance of 18% is referred to as "medium gray". That's pretty arbitrary, but is often rationalized by the fact that the L*a*b color coordinate system (in its original use, to describe "reflective" color) assigns an L* value of very nearly 50% to a color with reflectance 18%. Fair enough.
Some people than carry this the the realm of the digital representation of an image, and say that the digital representation of a gray color that corresponds to a photometric exposure 18% of that which would produce the highest possible digital representation should be considered "medium gray" (again, L*=50% if we consider the image in the L*a*b* coordinate system).
As a further cause of confusion here note that in a digital graphic arts context, such a color is called "82% gray" (0% gray there meaning not gray at all, that is, "white").
Best regards,
Doug
