The Gray Card: Purpose and Use?

[crob2go]

I bought a gray card on the advice of something that I read. The article said

something to the effect that digital cameras try to meter or expose to 18 %

gray. Gray card suppose to solve this problem and give a better exposure

reading. For me I get same reading through camera whether using gray card or

the subject. Is my expectation of a gray card wrong in that it help one to

get better exposure. Any answers would be helpful. Thankyou ...Craig

[mikeseb]

All reflected-light exposure meters--which includes in-camera meters as well as handheld

and all spot meters--are calibrated to give the exposure settings required to render

whatever they are reading as middle (or 18%) gray.

 

If you take a reflected-light meter reading from an 18% card placed in the same light as

your actual subject, it will give you a good approximation of the "correct" exposure for the

scene. Brighter or darker values in the scene will fall where they should in the image.

 

This is essentially what an incident-light meter does (these meters often have a half-ping-

pong-ball dome over their sensors). It measures the light FALLING on the subject, not

reflected from it, and gives exposure values necessary to render a middle gray as a middle

gray.

 

Using an incident meter, or a reflected meter with a gray card, should give nearly the same

exposure of a given scene. Either technique is helpful to avoid exposure errors caused by

a reflected meter getting fooled by large areas of very dark or very bright values in its field

of view.

[steve_crist]

Using a gray card will allow you to get more accurate exposures when you shoot photos with a high content of white or black. The meter assumes an average tonal base (18% gray) so is you shoot a pale bride standing in front of white curtains, the meter assumes everything is 18% gray. This will cause an under-exposure. Conversely, if you shot a dark groom in a black tuxedo in front of black curtains, it will be overexposed since the meter wants to make everything gray. So if in these instances you have your subject hold an 18% gray card in front of them and you spotmeter off that card, your exposure should be pretty accurate. If you shoot enough scenes like these you get a feel for the compensation required and can forego the card.

[Ian Taylor]

Makes color balance a breeze, as long as you're shooting RAW.

[Ed_Ingold]

All of the above and more. A good, non-metameric grey card can also be used to set a custom white balance for a DSLR or professional DVcam.

[helenbach]

The ISO standard for digital camera exposure systems (ISO 2721) requires that a uniformly lit target (it doesn't matter what shade of grey it is, as long as it is even and it fills the frame) gets exposed at 12.8% of the maximum exposure that would record detail.

 

That means that if you used an unadjusted 18% grey card reading you would have half a stop of overexposure latitude over a 100% diffuse reflecting object in the same illumination as the grey card. You may or may not want that latitude.

 

This is much the same situation with film camera meters - if you read off a grey card then you can open up half a stop if you want. Most meters are 'calibrated' for 12.5% or 14% reflectance rather than 18%, but it's all open to discussion.

 

Best, Helen

[jim_doty]

A gray card reading provides a know standard of reference (reflectance) when your subject contains tonalities that vary from 18% reflectance (medium toned). Taking a reflected light reading of a gray card in the same light as your subject is like pointing an incident light meter at the light source. If your primary subject is significantly lighter or darker than medium toned, you will have to adjust your camera setting from the gray card reading to compensate. If you don't, you can burn out white subjects and lose black subjects.

 

More here:

 

http://jimdoty.com/Tips/Exposure/exposure.html

 

Jim

[curt wiler]

"The article said something to the effect that digital cameras try to meter or expose to 18 % gray."

 

Craig, don't believe everything you read. Helen got it right. For most people outside of a studio, the best use of a gray card is setting the white balance.

[crob2go]

First like to thank:Michael, Hellen, Steve, Ian, Edward, Jim, and Curt. I apprceiate you guys sharing your knowledge and also the taking of your valuable time to answer my question. Thanks again.....Craig

[pete_s.]

To say that the camera exposes for middle grey is not really correct because it would depend on the exposure mode.

 

If the camera is using matrix or evaluation mode it is trying to figure out what you want by comparing your picture to a large number of similar scenes that it has stored and choose the exposure of the scene that is the best match. It also uses the color information and probably also your focus selection. Exactly what goes on in the camera is proprietary information and will be different for different manufacturers and also for different camera models.

 

If it is a digital camera it will also try to adjust the contrast (tone curve) making dull objects higher in contrast and contrasty objects lower with the asumption that you want to capture as much detail as possible.

 

So if you for example shoot a backlit portrait against a window the "intelligent" camera will most likely overexpose the window to get more detail in the person because it recognize a backlit scene. It will also slightly change the tone curve and make some of the dark information brighter to get even more detail in the person.

 

If the camera was in spot exposure mode it would however try to make whatever you pointed you camera at middle grey because that is in the middle of the dynamic range the camera is able to capture.

 

Same thing in center weighted mode but it would be an average reading of the whole scene but where the center spot would be dominant and the camera would then try to expose the average middle grey.

 

So looking at our example with a backlit portrait the exposure would be quite different depending on the exposure mode and where we pointed the camera. It would also be different if the model was wearing a white t-shirt or a black dress. To take away these differences we can use a grey card as a known reference and use the camera to set the exposure off the card. That way we would get the same exposure no matter what our subject is wearing or the amount of light coming from the back. And that is the purpose of the grey card as an exposure tool - a known reference.

[w.smith]

Here's a nice wrap-up of White Balance Techniques in a 3.5MB PowerPoint file. Download

from http://download.yousendit.com/465F33C51E9A6A85. Covers grey cards, expodiscs,

coffee filters (...), etc. etc.

[helenbach]

An interesting presentation. One or two observations on it, in order:

 

Colour temperature is measured in 'kelvin', not 'degrees Kelvin', if you wish to use the correct terminology.

 

"lower values are cooler (blue) and higher are warmer (amber/yellow)"

 

Higher values are 'cooler' (blue) and lower values are 'warmer' (amber/yellow).

 

"you could use the back of your hand" [for white balance]

 

I would not recommend the use of the back of your hand for doing a white balance, no matter what colour your skin is, especially if you are still alive or only recently dead. I think that he has got confused between exposure measurement techniques and white balance techniques - that also applies to some other of his advice about grey cards.

 

You do not need to use an incident meter at the subject location, you just need to use it in the same light as that falling on the subject, and you do not point the meter at the light source.

 

I wouldn't recommend using the Expodisk in 'reflected' mode for white balance. If you have spent that much money on a piece of white plastic you should use it properly - on the light falling on the subject, not the light coming off the subject when doing a white balance.

 

He didn't mention the use of a polystyrene (styrofoam) cup instead of an Expodisk for white balance - they are slightly cheaper, and they are just as white.

 

Best, Helen

[w.smith]

DPCC, Disposable Polystyrene Coffee Cup technique is for advanced pros only.

[savas_kyprianides]

The white balance slideshow stated that the Expo Disc has many uses, ostensibly beyond the

white balance capacity. What are those many uses?

[helenbach]

As far as I remember it was originally designed so that cameras with TTL metering could be used as incident meters. You can also do that by using your camera to read off any tone that you know the reflectance of, such as the palm of your hand, a grey card or a white card, then adjusting the reading accordingly.

 

Best, Helen

[crob2go]

I would like to thank everyone who contributed answer after 22 Nov.......Craig