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What is the IRE Rating of an18% Grey card


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#1 Bob Hayes

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Posted 09 March 2006 - 11:40 PM

If I have a front lit properly exposed HD image and I place an 18% grey card in the shot how many IRE will it read?
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#2 Clive Tobin

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Posted 09 March 2006 - 11:44 PM

If I have a front lit properly exposed HD image and I place an 18% grey card in the shot how many IRE will it read?


I'm not positive but it seems to me it would be about 50% of the way between setup and 100% white.
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#3 Hal Smith

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Posted 10 March 2006 - 01:26 AM

I'm not positive but it seems to me it would be about 50% of the way between setup and 100% white.

I'm rusty on my TV operating engineering but I believe that since black is IRE 7.5% and white is 100% IRE that an 18% gray card should be 18% of the way between 7.5 and 100 therefore = IRE 24.15 ((.18 X (100 - 7.5)) + 7.5 = 24.15).

Edmond, OK
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#4 Bob Hayes

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Posted 10 March 2006 - 02:13 AM

I'm asking the IRE when shooting HD where black is zero % and the scope tops out at 108 IRE. It appears to me that the Grey card runs at 35 to 40 percent.
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#5 Stephen Williams

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Posted 10 March 2006 - 02:27 AM

If I have a front lit properly exposed HD image and I place an 18% grey card in the shot how many IRE will it read?



Bob,

45 IRE.

The gammer & density color control charts are good in this respect as the patches are marked with IRE value.

http://www.gammaandd...ucts/index.html

Stephen
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#6 Bob Hayes

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Posted 11 March 2006 - 01:10 AM

Stephen. Thanks for the 45 IRE hint.

It appears to me that an 18% KODAK grey card properly exposed also gives me 45 IRE. That said, when I do that and look at my image, it seems like the picture is 10 IRE over exposed or about ¾ of a stop. So I have been exposing my grey card at 35 to 40 IRE. Exposing a grey card when shooting HD? Yup. I use my KODAK grey card, the one with white and black corners as my expose calibrating tool. I find that the large 18 % grey area and white and black corners also are easy to find on the vector scope. It also gives me a reference to see in dailies.
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#7 Michael Nash

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Posted 12 March 2006 - 07:31 PM

Stephen. Thanks for the 45 IRE hint.

It appears to me that an 18% KODAK grey card properly exposed also gives me 45 IRE. That said, when I do that and look at my image, it seems like the picture is 10 IRE over exposed or about ¾ of a stop. So I have been exposing my grey card at 35 to 40 IRE. Exposing a grey card when shooting HD? Yup. I use my KODAK grey card, the one with white and black corners as my expose calibrating tool. I find that the large 18 % grey area and white and black corners also are easy to find on the vector scope. It also gives me a reference to see in dailies.


The wonders of video gamma!

As I'm sure you're aware the highlight response in video cameras (including HD) is poor compared to the midtone and shadow response, causing you to want to underexpose slightly. If your ENTIRE image appears overxposed when the waveform shows a gray card at 45 IRE, you may want to check that your monitor is properly calibrated. I'm sure this part is not new to you.

But setting your exposure to the IRE value of a gray card is completely relative to the gamma value set in the camera. Raising or lowering the master gamma (without changing exposure) will change the IRE value of a gray card while the highlights and shadows remain relatively unchanged . Similarly, compensating your lens exposure after adjusting the gamma will result in the highlights and shadows becoming brighter or darker relative to the gray card (when exposed for a constant IRE value).

A gamma of .45 is considered "normal," but there are plenty of times where you may want to change the value to respond to the light conditions. For example, in harsh contrast situations (like sunlight and deep shade), I often go with a gamma of .35 to slightly compress the highlight response and stretch the shadow detail some (sometimes raising the black stretch as well). Then I underexpose slightly to bring the mids back in place and voila -- I've got a "normal" appearing contrast in a high-contrast setting. This can be used with or without post correction, when used judiciously. It's kind of like switching to a lower-contrast film stock for shooting outdoors.

Keep in mind that the camera always captures less detail in areas above approx. 60 IRE than in the rest of the image, regardless if it's from exposure or raised gamma. It's always better to err on the side of underexposure with video.
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#8 Bob Hayes

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Posted 13 March 2006 - 09:26 AM

Michael:

Good points. I?ve been looking at an image in the post house and they recommended I stop down a little more. I?ve been using a KODAK grey card as a reference so when I go into post I know what I set my exposure at and where. When I set the grey card at 3 IRE the whites seem to fall at about 80 IRE and the blacks at about 10 IRE.

You have another good point about the changing Gamma hugely affecting the mid tones. So if I set my grey card for 38 IRE at one Gamma setting and then change the gamma it would greatly change my exposure.
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