18% Grey Card

[aonsen]

I've got a few questions about grey cards... Being an amature

photographer, I've never considered using one, but as time goes on and

I get more and more involved in photography, I'm thinking of getting

some (or making some if they happen to be expensive)<br>

So my question is this:<br>

When you take a properly exposed picture (with a digital camera) of an

18% grey card, does the grey have a value of 209 (255-(255*.18)=209)

when I sample it in photoshop? <br>

<br>

And furthermore, if I get a 5x7 and/or 8x10 print made (at a local

photo shop) of a solid color grey of 209 intensity, will that be a

good cheap grey card?<br>

[coconutdaydream]

i'm way cheaper than you! i got an old piece of cardboard and spray painted it a light/medium gray. It works just fine for me, but I have no idea what your needs are.

[aonsen]

I guess what I'd also like to know is what should the histogram on my 300D look like when shooting a 18% grey card? <br>

When shooting my palm, and bringing the exposure down by 1 stop (as I read in other reference), I mainly get a spike centered in the left 2nd quadrant (from the left of the histogram, and there are 5 quadrants in total on the display). The spike width is almost (maybe 80%) an entire quadrant's width.

[michael_flaherty]

Do yourself a favor and simply buy a Kodak Gray Card set. They're relatively cheap and deadly accurate. Home made gray cards will likely be off and defeat the purpose of using them in the first place. I shoot gray cards when I need a custom white balance and also when I want an accurate Flash exposure lock when shooting with dedicated flash units in ETTL mode.

[sunilarch]

try the value #808080(hue=0, saturation=0, blackness=50%) which will give you the color but your display moniter and printer may not give you 100% accuracy.

 

the histogram of a perfect grey card will be a single line(spike) in the centre of the histogram!!!

Sps

[bobatkins]

18% grey is supposed to be a mid-tone, so in theory the histogram would be a single spike in the middle of the plot (at a level of 128).

 

Buy a grey card. They're dirt cheap (under $10).

[aonsen]

Ok.... I get the idea... <br>

I'll spend a few bucks on a grey card.<br>

<br>

But I'm still a little confused. Why (and how) is 18% a mid tone? Would 50% grey not be a mid tone, since the photoshop color is 50% grey, and the histogram shows a spike right down the center of the graph?<br>

I dont understand the relationahip between 50% grey color, and 18% grey card value... Can anyone shed some light on this?

[bobatkins]

Yes. It's a logarithmic scale, not a linear scale.

 

If you want another explanation, 50% of the objects in the world are lighter than 18% gray, 50% are darker!

 

The first explanation is the better one.

[m_.]

what 18%? same as why Myanmar was called Burmar, Beijing was called Peking, etc.. Some guy got that in his head and everyone followed. Now it is too late to change. Just use it.

 

You palm or cement pavement are some of the free 18% grey cards.

[jon_austin]

Bob:

 

What is the calculation that yields 18% grey = 50% tonality, 128 on an 8-bit grayscale, etc?

 

I ran a few logarithm calculations, and can't figure out how this is derived. I also did a Google search, but couldn't find an explanation.

 

Thanks,

 

Jon

[kevinconnery]

<i>What is the calculation that yields 18% grey = 50% tonality,

128 on an 8-bit grayscale, etc?</i><p>

That depends on the gamma of the color space used.

ColorMatch, a 1.8 gamma color space, will have 18% gray at

around 103. sRGB and Adobe RGB (1998), both 2.2 gamma

spaces, put it at around 125.

[john setzler]

The 18% figure describing gray cards is the amount of light they reflect. It is not indicitave of the actual shade of gray on the card.

 

I recently bought a pack of the kodak cards at my local camera shop. Two 8x10 and one 4x5 card for $15. The back side of these cards are 90% reflective white. They are good for setting custom white balance if you are shooting digital.

[les_meehan2]

Hi All

 

Don't assume that the camera light meter is calibrated by the manufacturer to 18% grey. Most aren't! You will have to recalibrate the camera to the grey card by testing to obtain the 'middle-grey' on your histogram. Zone system users will know what I mean.

 

Once calibrated it should be as others have said, a spike in the middle of the histogram (assuming the histogram is accurate!!).

 

Cheers

 

Les

[grahm_f]

The reason for finding a mid tone in a particular scene is so the exposure meter can find the proper exposure for that given scene (expose the mid tone properly and this should guarantee a proper exposure for *most* scenes). This is why you have to manual adjust the exposure for particular scenes which are made up of primarily black or white subjects - lack of mid tone. This is why the histogram should show a spike in the center of the graph -- this means proper exposure.

 

The 18% grey card stems from the idea that *most* scenes that average out to a mid tone reflect 18% of the light that falls on it (technically this number is now supposed to be closer to 13%, or so I've read).

 

An 18% gray card ensures a mid tone that reflects 18% of the light that falls on it.

 

Now beyond this I�m clueless to the math that is involved ;-) This is only my understanding of this, I hope it helps.

[mike_a]

I hope this is a more practical explaination of the 18% grey cards.

 

Have you ever shot a snow scene before, where the ground is completely covered in snow, as well as most of the scenery? Well, if you just snap the image, your snow will look dark, and the background darker. The goal of a light meter is to make most of the image balanced to normal, but since such an overwhelming ammount is white, the meter tells you to underexpose the image to compensate. If you meter from a 18% grey card instead of the snow (I've heard your hand works as well from my photo teacher), you get the balanced exposure everyone's been mentioning. Your snow is snow white, your shadows and points of contrast are as dark as they're supposed to be. I have heard of people who only use 18% grey cards in shooting for the balance it provides, but in my expierence, most scenes don't need it if your light meter is working properly and you bracket a stop or two each direction.

[vivid light photography]

Spend $15 and get a Kodak grey card. You get two 8x10's and a 5x7. If you were to use a photograph of one you most likely won't get the same proporties. Every photograph will have slightly different color casts and brightness. Also you may not get an 18% reflectance. Using something like this would most likely throw your exposure and WB off. The Kodak 18% grey card is an industry standard. Your camera meter (matrix, center weight, or spot) will do it's best to make your subject or scene an 18% (or 12% I've read) netral tone. This is why when using a spot meter your best to meter off a netral tone color on your subject or the color you want to be netral tone in your photograph. I recently did some shots of skiers on snow and noticed several things. First off, you can' always judge a correct exsposure using your histogram on your camera. Since your meter will try to expose the snow as a midtone it will also display it in the histogram as a midtone therefore it will be closer to the middle. Try taking a photo of both a dark blue and light grey solid surface. If both are exposed according to your camera meter they both should yeild a steep bell curve centered in your histogram. If you want either the dark blue or light grey to photograph as your eyes see them then you would need to compensate the exposure for both. I read about stopping up 1 or 2 stops when shooting on snow but did not because my histogram looked good. I was using my matrix meter mode and not my spot. 80% of my scene was snow and the rest was the skier which in turn represented only 20% of the pixels in the histogram. When I downloaded them later most were underexposed. Your best bet is to understanding how your various meters work, pay close attention to what is being metered in the scene or on the subject and make apropriate compensations based on your experience and desired outcome. BTW, I use a Kodak grey card for both exposure measurements and white balance.