18% Gray Card

[PhilsterT]

I can't grasp this concept and it's really frustrating. I think it's creating a barrier for me not allowing me to produce better work. I can't understand how a metering off an 18% gray card allows black and white film to come out nicely (at the correct exposure). Why are all the colors perfectly in tune?

Sorry, this is my first post and I'm not sure if I put it in the right section.

[rent]

welcome to the forum!

we humans are wired to adapt to different lighting conditions and also use our experiences to tell us what color familiar objects are. it's like saying our visual system always tries to properly expose the scene in front of us, and it does a good job most of the times.

this is not the case with a light meter because it has no idea what it's looking at and needs a reference point. what a light meter is calibrated to do is assuming the scene it's metering is exactly halfway in exposure between pure white and pure black. you'd think that'll be 50% grey, but it's actually 18%.

and since this is the way light meters are "trained" to do, one way to get a proper exposure is to meter off of an 18% grey card, in a way, making it very easy for the meter to do its job.

-alex

[PacAce]

... you'd think that'll be 50% grey, but it's actually 18%.

Actually, it is 50% gray. The 18% is referring to the reflectance of the card which is the amount of light reflected back by a 50% gray card. And even with the 18%, you can get into a debate with some other people who will claim that the camera is really calibrated for a 12% gray. :)

[rent]

hahaha leo, you are absolutely right. i just didn't want to make it even more confusing for the Philster. yeah i heard about that 12% too, thanks kodak (sacarstically)! LOL!

-alex

[bokeh'ed]

tried scouting the above issue to address my questions, but not much success:confused:

anyway, the userguide says to shoot an 18% gray card obtain a better white balance and use this image to set as the reference CWB.

my question is, under what lighting shd the gray card be shot? fluorescent? tungsten? daylight? or set to the default in-camera AWB settings?

also, does it matter what ISO/shutterspeed/aperture value/partial metering is used to get the reference shot? should default parameter (sharpness/contrast/saturation/color tone = 0) settings be applied?

or is there any TIFF softcopy of 18% gray than can be copied into the memory card and get the camera to import the settings for greater accuracy (eliminate all the above variables)?

[Rigrider]

You shoot the card under the same conditions your shooting the rest of your subject. The idea here is to meter off the card in order to tell the camera what the current lighting conditions are. But now I'm a little confused, because while I'm not 100% sure, I THINK you're supposed to shoot a WHITE card to set your white balance, an 18% grey would be for metering the scean.

Again, I'm not 100% on this, since I shoot Camera Raw, so I don't worry about white balance until Post.

L8r,

[GoneFission]

All those above variables are exactly the reason why you would want to shoot a gray card! There is no "universal" WB setting. The light source and what the light is reflecting off of all affect the light spectrum and hence the colors.

If you use AWB, the camera will sample the entire picture, and assume that the average of it all is gray. In general, this works out pretty well. The other WB settings, such as tungsten, are basically hardwired shots of gray cards under those lighing conditions. However, different light bulbs emit slightly different spectrums, different colored walls change the lighting, etc. As such, the only way to tell the camera what "color" is actually white is to shoot a gray card in the conditions you will be shooting in.

[bokeh'ed]

ok, makes sense now. that's to say if i were to take an outdoor portrait shot, i have to take a shot of the gray card with the same settings as the portrait shot?

[jimsolt]

ok, makes sense now. that's to say if i were to take an outdoor portrait shot, i have to take a shot of the gray card with the same settings as the portrait shot?

You are dealing with two concepts here. You can use the gray card for both, but they are not the same.

Exposure-- use the card to meter for exposure. On the back of my gray card is a whole page of instructions, but basically you use it when the lighting conditions are "not average" like if there is a lot of bright sky. The card will serve as an "average" to meter exposure from.

White Balance (that's what tells the camera what the color of the light illuminating your scene is) -- use the card (I prefer white, but the 18% gray will work) to set your custom white balance. Follow the instructions for YOUR camera, but it will involve filling or almost filling the screen with the card in the SAME light as your subject and then setting a custom white balance -- good only for that same situation.

They are TWO DIFFERENT adjustments even though you might use the same card.

And before someone else says it, if you shoot RAW, its a new ball game with white balance. You can adjust it in post.

Jim

[chtgrubbs]

Unfortunately a gray card doesn't make B&W film come out perfectly, at least not without alot of experimentation on the photogrphers part. Film speed, metering technique, film processing time and printing all have to be adjusted to make B&W come out right. I've spent countless hours over the years doing so.