cameramanben
Well-known member
Hi Sam,
middle gray, I'm afraid, isn't the middle of the recording (or at least very rarely is). To quote Wikipedia, 18% gray is :
but that is more about how it looks when captured or when displayed; it has no absolute relationship to the level of the recording. In a pure scene linear recording (with floating point values such as raw or ACES), 18% gray has a value of 0.18, where 90% white has a value of 0.9. With the two most common basic gammas applied, sRGB (for stills) and Rec709 (for video), the relationship places 18% gray around 48% and 43% IRE respectively, with 90% white card then coming out at 100% IRE in each. In reality, Rec709-based gammas frequently sit a percent or two lower, to help with the knee at the top end, but low forties is the most likely place to find 18% gray on a WYSIWYG video camera setup.
Most of the time going to 50% IRE on a Rec709-based look designed for 43% is only going to put you about 1/2 stop hot; with the knee at the top end and providing you haven't clipped it's not going to be the end of the world, but with lower-contrast looks - particularly those with even lower nominal 18% values - such as hypergammas or log, 50% could put you a stop or more hot. Mind you, with a log curve, you could claim that as rerating the camera.
Different transfer functions or gammas can specifiy nominal 18% where they like; eg Arri LogC - 38%, S-Log3 - 41%, C-Log - 33%.
Sony specifies 18% gray nominal values in the full names of their hypergammas, eg HG7 is actually HG8009G40 (800%, extended range and 18% at 40% IRE) and HG8 is HG8009G33 (800%, extended range and 18% at 33% IRE).
The 90% white level also changes with different curves, generally due to knees to compress wide dynamic range within the available integer range.
If you expose for the nominal 18% (or 90%) value under one 'gamma', you will still be correctly exposed when converting to any other,
Ben
middle gray, I'm afraid, isn't the middle of the recording (or at least very rarely is). To quote Wikipedia, 18% gray is :
but that is more about how it looks when captured or when displayed; it has no absolute relationship to the level of the recording. In a pure scene linear recording (with floating point values such as raw or ACES), 18% gray has a value of 0.18, where 90% white has a value of 0.9. With the two most common basic gammas applied, sRGB (for stills) and Rec709 (for video), the relationship places 18% gray around 48% and 43% IRE respectively, with 90% white card then coming out at 100% IRE in each. In reality, Rec709-based gammas frequently sit a percent or two lower, to help with the knee at the top end, but low forties is the most likely place to find 18% gray on a WYSIWYG video camera setup.
Most of the time going to 50% IRE on a Rec709-based look designed for 43% is only going to put you about 1/2 stop hot; with the knee at the top end and providing you haven't clipped it's not going to be the end of the world, but with lower-contrast looks - particularly those with even lower nominal 18% values - such as hypergammas or log, 50% could put you a stop or more hot. Mind you, with a log curve, you could claim that as rerating the camera.
Different transfer functions or gammas can specifiy nominal 18% where they like; eg Arri LogC - 38%, S-Log3 - 41%, C-Log - 33%.
Sony specifies 18% gray nominal values in the full names of their hypergammas, eg HG7 is actually HG8009G40 (800%, extended range and 18% at 40% IRE) and HG8 is HG8009G33 (800%, extended range and 18% at 33% IRE).
The 90% white level also changes with different curves, generally due to knees to compress wide dynamic range within the available integer range.
If you expose for the nominal 18% (or 90%) value under one 'gamma', you will still be correctly exposed when converting to any other,
Ben
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