How to use Grey Card?

[Matt30D]

I am confused about how to use a grey card? What is the procedure to due so, like in a wedding situation, or even an outdoor shoot with a senior?

[TheHoff]

Two uses... the first is correct exposure. For this you need a true 18% grey card.

Have your subject hold the card at their head level, in the same light, and you can aim your camera's (or handheld) spotmeter at the grey card to determine the exact exposure necessary. A camera's meter wants to average everything to that level of grey so that is what it expects to meter off of.

The second is white balance... getting a frame with a grey card (or pure white object) in the frame gives you a simple point on which to white balance when you process.

[Gentleman Villain]

Traditional gray cards are basically a relic of the past. I can't think of any reason to use them anymore. If you're interested in using a reference card for more accurate exposure and color balance then shoot RAW and use a Whibal card...

Video Tutorials explaining the use of a Whibal card:
http://whibalhost.com/_Tutorials/WhiBal/01/

I used a gray card for the Zone System (by Ansel Adams) to develop a personal system of exposure and processing of B&W film. I also used gray cards as a reference when printing color negatives. But gray cards aren't really accurate or consistent enough for modern digital workflow IMHO

[JoYork]

I seem to recall reading somewhere that we should all be using 12% grey instead of 18% grey, but the articles were long and my attention span short... :(

[Wilt]

Traditional gray cards are basically a relic of the past. I can't think of any reason to use them anymore. If you're interested in using a reference card for more accurate exposure and color balance then shoot RAW and use a Whibal card...

Video Tutorials explaining the use of a Whibal card:
http://whibalhost.com/_Tutorials/WhiBal/01/

I used a gray card for the Zone System (by Ansel Adams) to develop a personal system of exposure and processing of B&W film. I also used gray cards as a reference when printing color negatives. But gray cards aren't really accurate or consistent enough for modern digital workflow IMHO


That link says that the WhiBal is for White Balance reference, whereas gray cards are for Exposure reference! So if you need absolute accuracy of inherent brightness, not merely accuracy of White Balance, there is still a need for a brightness reference card--the traditional gray card (when an incident light meter is not being used, but the shooter is relying upon only a reflective light meter such as the one in the camera). The WhiBal card is the wrong level of reflectivity to be used as a target for reflected light meter, it is only useful in providing an absolutely neutral gray (RGB mix) surface for reproduction of color balance.

[Wilt]

I seem to recall reading somewhere that we should all be using 12% grey instead of 18% grey, but the articles were long and my attention span short... :(

Some authors (Ctein 1997) have argued that the calibrated reflectance is closer to 12% than to 18%.
You can again read the precise reason for 12% on the Wikipedia page



" Calibrated reflectance

It is commonly stated that reflected-light meters are calibrated to an 18% reflectance,[3] (http://en.wikipedia.org/wiki/Light_meter#cite_note-2) but the calibration has nothing to do with reflectance, as should be evident from the exposure formulas. However, some notion of reflectance is implied by a comparison of incident- and reflected-light meter calibration.
Combining the reflected-light and incident-light exposure equations and rearranging gives
Reflectance ζ is defined as
A uniform perfect diffuser (i.e., one following Lambert's cosine law (http://en.wikipedia.org/wiki/Lambert%27s_cosine_law)) of luminance L emits a flux density of πL; reflectance then is
Illuminance is measured with a flat receptor. It is straightforward to compare an incident-light measurement using a flat receptor with a reflected-light measurement of a uniformly illuminated flat surface of constant reflectance. Using values of 12.5 for K and 250 for C gives
With a K of 14, the reflectance would be 17.6%, close to that of a standard 18% neutral test card. In theory, an incident-light measurement should agree with a reflected-light measurement of a test card of suitable reflectance that is perpendicular to the direction to the meter. However, a test card seldom is a uniform diffuser, so incident- and reflected-light measurements might differ slightly.
In a typical scene, many elements are not flat and are at various orientations to the camera, so that for practical photography, a hemispherical receptor usually has proven more effective for determining exposure. Using values of 12.5 for K and 330 for C gives
With a slightly revised definition of reflectance, this result can be taken as indicating that the average scene reflectance is approximately 12%. A typical scene includes shaded areas as well as areas that receive direct illumination, and a wide-angle averaging reflected-light meter responds to these differences in illumination as well as differing reflectances of various scene elements. Average scene reflectance then would be
where “effective scene illuminance” is that measured by a meter with a hemispherical receptor.
ISO 2720:1974 (http://en.wikipedia.org/wiki/Light_meter#CITEREF_ISO2720:1974) calls for reflected-light calibration to be measured by aiming the receptor at a transilluminated diffuse surface, and for incident-light calibration to be measured by aiming the receptor at a point source in a darkened room. For a perfectly diffusing test card and perfectly diffusing flat receptor, the comparison between a reflected-light measurement and an incident-light measurement is valid for any position of the light source. However, the response of a hemispherical receptor to an off-axis light source is approximately that of a cardioid (http://en.wikipedia.org/wiki/Cardioid) rather than a cosine (http://en.wikipedia.org/wiki/Cosine), so the 12% “reflectance” determined for an incident-light meter with a hemispherical receptor is valid only when the light source is on the receptor axis."

[Gentleman Villain]

Yeah I understand all that....I just can't imagine that people need a gray card for exposure when there are in-camera light meters, LCDs and histograms. It just seems like a relic. But never mind. I should know better than to post about this stuff because I can't remember what it was like to be a newbie

[TheHoff]

Semantics at work... he asked "how do I use a grey card?" not "is a grey card useful?"

:D

[Wilt]

Yeah I understand all that....I just can't imagine that people need a gray card for exposure when there are in-camera light meters, LCDs and histograms. It just seems like a relic. But never mind. I should know better than to post about this stuff because I can't remember what it was like to be a newbie


You need to read Ansel Adams The Exposure, to bettery comprehend subject tonality and its influences on meter readings, then. :) In camera meters are the most easily FOOLED instruments in the photographer's arsenal! Put yourself on a football field or a soccer field, and have players run in and out from your frame, and watch how the meter brightness changes significantly...but the inherent brightness of the light which is striking the scene may be unaltered all during that (in the case of a bright cloudless sky, or in the case of a gray uniformly overcast sky, as two examples). With the gray card (or with the incident meter) you take a single reading, set the camera on M and follow the reading's values, and never alter exposure during the game. Finito!

[Gentleman Villain]

I was personally taught the zone system working in a lab under a man who printed for Yosef Karsh..

I KNOW ALL OF THIS STUFF already.... Oh god I hate this place sometimes

spare me the lectures on meters wilt...

[Wilt]

If you have all that knowledge, I don't comprehend how you could make a simplistic statement about no "need a gray card for exposure when there are in-camera light meters, LCDs and histograms". When you move exposure 'to the right', you inherently have lost the scene brightness in an effort to capture more gray levels in the shadow zones, due to the limitations of the digital sensor, so how would you know the right brightness levels to bring them all back to? :confused:

[Wilt]

Waitasec, you are not the only person reading the thread, there will be dozens (perhaps more) others who later read this thread with little to no photographic knowledge. It is for THEM that the additional comments are made, as a counterpoint to the opinion expressed by you. I am not trying to convert you to my religion or political party, I am merely trying to lay factual information on the table so that they may, too, make informed decision! (Now lay off the profanities, please!)

[Gentleman Villain]

Waitasec, you are not the only person reading the thread, there will be dozens (perhaps more) others who later read this thread with little to no photographic knowledge. It is for THEM that the additional comments are made, as a counterpoint to the opinion expressed by you. I am not trying to convert you to my religion or political party, I am merely trying to lay factual information on the table so that they may, too, make informed decision! (Now lay off the profanities, please!)

great...I hope they learn lots from you

[nicksan]

Is that what the grey card is for?

Well son of a gun! I was using it as a fan to cool myself off when it got hot while I was shooting. Does a damn good job at it too.

Damn man...is that why all my images have a rainbow cast to them?

I need to talk to my local lab I learned all this stuff from...the photo dude at my local CVS pharmacy.

;-)