The Gray Card Topic Post...

[Alex]

Here are my questions about Gray Cards. If the camera is slightly out of focus when zoomed in on the card, how does that affect the reading?

I wonder if perfect focus may contribute to a false reading versus a slightly out of focus reading off of the gray card.

The fact that a gray card is 2-D, yet lighting is 3d, how does that affect the gray card reading one gets?

<font color="#a62a2a" size="1">[ October 15, 2002 02:59 PM: Message edited by: Alex ]</font>

[TA 152]

<font color="#a62a2a" size="1">[ August 28, 2003 05:07 AM: Message edited by: S8 Booster ]</font>

[Greg Crawford]

NO PROBLEM, DON'T WORRY BE HAPPY!
It might even help... say you have some smudges on your gray card it would help even things out. Gray is Gray... 18 % reflective zone V in the zone system.

[Actor]

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Here are my questions about Gray Cards. If the camera is slightly out of focus when zoomed in on the card, how does that affect the reading?
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It doesn't.
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I wonder if perfect focus may contribute to a false reading versus a slightly out of focus reading off of the gray card.
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They don't.
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The fact that a gray card is 2-D, yet lighting is 3d, how does that affect the gray card reading one gets?
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It doesn't.


*******(Alex here, I had put a wrong word in one of my quotes that Actor used above and it made my quote confusing, so I corrected the word in the original quote, and here)

<font color="#a62a2a" size="1">[ October 15, 2002 03:03 PM: Message edited by: Alex ]</font>

[Alex]

I didn't make my issue clear enough. I was refering to color temperature issues. I'm assuming the reading of a grey card by a metering device is affected by color temperature.

If that is the case, then a 2 dimensional grey card will block certain colors from hitting the card if the light source is not directly front lit.

[Actor]

Use of a gray card addresses two issues.

<ul type="square">[*]1. It lets you get the exposure right. Like it or not, light meters are calibrated using a gray card. Unless the scene in front of you averages to gray your meter will give a false reading if you do not use a gray card. Unless your TTL meter is a spot meter the card of will have to fill the frame to assure an accurate reading.

I once took stills of some notes written on a white dry erase board. When the film came back the board was gray. Fortunately I could still read the notes, which was all I wanted.[*]2. Color temperature is another matter. If you want to use a reversal film like K40 and project it yourself then you are pretty much stuck with deciding whether your light source is daylight, flourescent or tungsten, and filtering accordingly. This situation can be improved if you can afford a color temperature meter (several hundred dollars). The meter will tell you the exact temperature of your souce and knowing the temperature of your film will tell you what filter to use.

On the other hand, if you are getting a print or telecine then filming a few frames of gray card will help. Tell the timer or telecine operator to correct the entire scene to the gray card. Actually, with most NLEs you should be able to do that yourself.

On a calendar shoot a few years back I wanted to use Kodak Ektar 25. (now discontinued. Dam Kodak!) This meant that I would have to use a lot of lights. The Ektar was a daylight film but I did not want to give up any of my light to a filter. So I had the models hold a gray card for the first frame of the film and told the lab to correct off the card in that frame. I got endless lectures from the lab about how I should be using a filter or a tungsten film.[/list]<BLOCKQUOTE><font size=2 face="Verdana, Helvetica, sans-serif">quote:</font><table border="0" width="90%" bgcolor="#333333" cellspacing="1" cellpadding="0"><tr><td width="100%"><table border="0" width="100%" cellspacing="0" cellpadding="2" bgcolor="#FF9900"><tr><td width="100%" bgcolor=""><font size=2 face="Verdana, Helvetica, sans-serif">
I'm assuming the reading of a grey card by a metering device is affected by color temperature. </font></td></tr></table></td></tr></table></BLOCKQUOTE>
Yes, it is. With a gray card the meter gives a correct reading. Without it, it may not.
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a 2 dimensional grey card will block certain colors...
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The gray card does not block any colors, at least not selectively. I.e., the gray card is essentially a white card that reflects only 18% of the light striking it. <BLOCKQUOTE><font size=2 face="Verdana, Helvetica, sans-serif">quote:</font><table border="0" width="90%" bgcolor="#333333" cellspacing="1" cellpadding="0"><tr><td width="100%"><table border="0" width="100%" cellspacing="0" cellpadding="2" bgcolor="#FF9900"><tr><td width="100%" bgcolor=""><font size=2 face="Verdana, Helvetica, sans-serif">
...from hitting the card if the light source is not directly front lit.</font></td></tr></table></td></tr></table></BLOCKQUOTE>
There is no way in which the angle at which the light strikes the card could affect the color temperature of the light reflected from it. The advice to hold the card at an angle midway between the light source and the camera is a compromise between the area of the card seen by the camera and the amount of illumination from the source.

[Alex]

<BLOCKQUOTE><font size=2 face="Verdana, Helvetica, sans-serif">quote:</font><table border="0" width="90%" bgcolor="#333333" cellspacing="1" cellpadding="0"><tr><td width="100%"><table border="0" width="100%" cellspacing="0" cellpadding="2" bgcolor="#FF9900"><tr><td width="100%" bgcolor=""><font size=2 face="Verdana, Helvetica, sans-serif">Originally posted by Actor:
Use of a gray card addresses two issues.


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a 2 dimensional grey card will block certain colors...
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The gray card does not block any colors, at least not selectively. I.e., the gray card is essentially a white card that reflects only 18% of the light striking it. <BLOCKQUOTE><font size=2 face="Verdana, Helvetica, sans-serif">quote:</font><table border="0" width="90%" bgcolor="#333333" cellspacing="1" cellpadding="0"><tr><td width="100%"><table border="0" width="100%" cellspacing="0" cellpadding="2" bgcolor="#FF9900"><tr><td width="100%" bgcolor=""><font size=2 face="Verdana, Helvetica, sans-serif">
...from hitting the card if the light source is not directly front lit.</font></td></tr></table></td></tr></table></BLOCKQUOTE>
There is no way in which the angle at which the light strikes the card could affect the color temperature of the light reflected from it. The advice to hold the card at an angle midway between the light source and the camera is a compromise between the area of the card seen by the camera and the amount of illumination from the source.</font></td></tr></table></td></tr></table></BLOCKQUOTE>


Any light that is "off axis" meaning either behind the gray card, or off to the left or right at an extreme angle of the gray card, will never reach the gray card.

I find that a flaw in using a gray card. Perhaps applying a certain angle to the card can take that issue into account.

[Actor]

I have a feeling that I really don't understand what you're getting at but I'll respond anyway.
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Any light that is "off axis" meaning either behind the gray card, or off to the left or right at an extreme angle of the gray card, will never reach the gray card.
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If you are metering a gray card you are metering reflected light, not the source. The card should be held just in front of the subject.

If the source is off to the right, e.g., the source is east of the subject and the camera is south of the subject (pointing north) then the card should be held so that a perpendicular to it points southeast.

If the source is behind the subject then I would choose to meter off the card. This would provide correct exposure for the subject while washing out the background in white light, similar to what you get when you select "BACK LIGHT" on a video camera. Or you could elect to meter the background (without the card) leaving the subject in shadow or in silhouette. A third option would be to add additional lighting to the front of the subject or reduce the amount of light behind the subject.

By the way, exactly what do you mean when you say that light is 3d? Objects are 3d and images are 2d but it seems to me that light (unless you want to discuss its electromagnetic aspects) is 1d, that dimention being the direction in which it propagates.

[Alex]

By light being 3d I mean light bounces, or can be coming from more than one source, yet a 2D card is not really reflecting the reality of the scene.

I think a hexagonal shaped gray card would be better.

[Actor]

Recording "the reality of the scene" depends on our instruments and our understanding of how to use them. We want to know the intensity of the light reaching our film. If it's color film we also want to know the color temperature.

Using a light meter can be misleading. Although the following was written in terms of B&W negative to print still photography, it is still applicable and instructive.
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Imagine that you are photographing a large checkerboard mosaic pattern of black and white squares. If the mosaic has an equal number of black and white squares, the average reflectance will correspond to middle gray. If you were to step far enough away so that your eye could not isolate the individual squares, you would see a uniform gray surface of 18-percent reflectance.

A reflected-light meter reading of the entire mosaid would average the black and white tones and wold be exactly the same as a reading taken from a gray card held against the mosaic. An exposure based on either reading would result in a negative that when printed would reproduce the checkerboard pattern of alternating black and white tones.

If the pattern of the mosaic were altered so that there were many more black squares than white ones (with no corresponding change in lighting), the average reflectance of the surface would decrease in proportion to the number of black squares added. A reflected light meter would give a lower reading (based on an accurate observation and measurement), indicating that you should increase the exposure of the mosaic. This increase, however, would clearly be wrong, since the reflectance of individual squares in the moasic would not have changed, and the exposure should be independent of the number and distribution of tones among neighboring squares. A negative exposed with settings based on a reflected-light reading of the newly patterned, disproportionately black mosaic would be overexposed, with the black squares appearing as gray tones.

If you were to take a reading from a gray card placed before the new mosaic, on the other hand, you would find that the suggested exosure settings would be the same as they were in the first case. A negative exposed with the same settings as before would produce a print that mirrored the new scene.

If you reversed the experiment and arranged the mosaic so that there were far more white squares than black ones, a reflected-light meter would indicate that the average reflectance had increased (which would be true) and that you should allow less exposure. That conclusion would also be wrong, since the reflectance of the individual black and white squares would, once again, remain the same. If you were to use the exposure settings indicated by the meter, your negative would be seriously underexposed. A gray card reading would confirm that the intensity of illumination had not changed, and that the exposure should again be the same as in the first example.

The point of all this discussion is to reemphasize that a light meter always assumes that it is reading an average subject, with a reflectance of 18 percent, and that the print you want will average out to middle gray. If the distribution of tones within a scene is not average, the meter will not compensate, and your judgement must come into play.

John P. Schaefer, Basic Techniques of Photography; Little, Brown and Company; Boston, New York, Toronto, London; 1995; pp 160-161.
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The fact that the light on your subject may come from many diffenent sources really makes no difference as long as these sources all hit the card. Also the fact that the card is flat is exactly what we want. The card is a calibrating standard. The three dimentionality of objects in the scene is revealed by highlight and shadow, i.e., deviations from the flat gray of the card. To try and mimic the 3D aspects of the subject by somehow imposing a 3D shape would ,first, probably not result in any significant difference in the reading. Second, if it did show a significant difference it would probably err in the wrong direction, resulting in an incorrect exposure and defeat the purpose of revealing the 3D aspects of the subject.