Reading mode: Light Dark

Gray card re-visited

Go to last post
photoave said:
BTW I have read numerous times that a styrofoam cup is an ideal
replacement for the standard grey card.
Click to expand...
I was under the impression that a styrofoam cup was ideal for deciding white balance not exposure.

Although I'm usually wrong about most things..........LM
 
I don't know the phototechnical background of an 18% gray card.

Assuming 18% gray is actually the same as 82% of white, then the value would be 210 for RGB for 210 for the percent brightness in Photoshop.

This gives a very light gray result which I don't know is even close to what is considered a "gray card"
Yehuda Katz said:
Hi friends,

I want to print by myself a gray card. Can you give me rhe R-G-B
value of 18% gray?

Thank you, Yehuda
Click to expand...
 
I might also recommend printing it out with a LaserPrinter on the whitest paper you have. This would avoid any color ink fading that others have warned about.

I tried it in Photoshop to my HP LaserJet and it produced a nice looking gray image. I guess you could do that and take it in to a photography shop to compare it against the real deal.
TurboTed said:
Assuming 18% gray is actually the same as 82% of white, then the
value would be 210 for RGB for 210 for the percent brightness in
Photoshop.

This gives a very light gray result which I don't know is even
close to what is considered a "gray card"
Yehuda Katz said:
Hi friends,

I want to print by myself a gray card. Can you give me rhe R-G-B
value of 18% gray?

Thank you, Yehuda
Click to expand...
--
Click to expand...
 
Yehuda Katz said:
Hi friends,

I want to print by myself a gray card. Can you give me rhe R-G-B
value of 18% gray?

Thank you, Yehuda
Click to expand...
Or you could just download the one I made a few weeks ago.

Figured it would work just the same. Printed it on both sides of semi-gloss photo paper so one side is matt and the other gloss.
http://www.protecon.net/download/grey_card.zip
People forget about base 16 with rgb.

Try it for yourself.

Alex
 
photoave said:
8x10? Wow that would be huge. Can you share the purpose for such a
large 'target'? I like the idea of being able to toss one in a
tiny camera bag>
Click to expand...
Remember that you have to fill the screen with it in order to use it properly.
Philly Wood said:
As for experimenting, if you are interested in
new 'research" on the effects of the subtley of whitebalance, why
not try different colors to see the effects of say a cranberry
colored "greycard' on the image.
Click to expand...
Nice idea.
Philly Wood said:
BTW I have read numerous times that a styrofoam cup is an ideal
replacement for the standard grey card.
Click to expand...
True, for setting the white balance, but not for light metering.

Yehuda
 
LM1 said:
If you take a picture of a white wall or white piece of paper
without any exposure compensation the result should be 18% grey and
you could just print that.

Or is there something wrong in my logic
Click to expand...
It seems logical for me, but it can't beat the proper RGB setting in PhotoShop.

Yehuda
 
LM1 said:
If you take a picture of a white wall or white piece of paper
without any exposure compensation the result should be 18% grey and
you could just print that.

Or is there something wrong in my logic.........LM
Click to expand...
In theory, I don't think there is anything wrong with your logic .

Reality, however, will throw a curve. The camera doesn't use black & white film (or sensors), but is a mosaiced RGB, which uses fancy math to 'guesstimate' the picture. While the result will be close, it will be slightly off. (You can see this by running the sample tool in PS over each pixel, and noting that the pixels are not identical to each other, and will NOT have equal RGB values.) It would seem much easier to simply use a paint program to choose a specific shade of grey (ie, 128,128,128) and then 'paint' the entire canvas in that single color.

My guess (purely a guess), would be that an RGB value of 128, 128, 128 would produce 18% grey. My reasoning is that, in B&W photography, 18% grey represents the middle tone between white & black. In the RGB world, 128,128,128 is halfway between 0,0,0 & 255,255,255.

An additional difficulty, however, is that even assuming 128,128,128 is 18% grey, does your printer know this? And as noted by a previous poster, your ink & paper must combine to produce a 'reflectance' of 18%. Unless you already have a sample of an 18% grey card, or some other way to accurately measure reflectance, and even with the correct RGB values, it may be difficult to know whether you've actually printed an 18% grey card.

I find the experiment interesting. It would be neat to know if 18% grey cards could be printed from an inkjet. Could be useful in an emergency, or to print up 'disposable' grey cards. Good luck.

Howdy.
 
TurboTed said:
I don't know the phototechnical background of an 18% gray card.

Assuming 18% gray is actually the same as 82% of white, then the
value would be 210 for RGB for 210 for the percent brightness in
Photoshop.

This gives a very light gray result which I don't know is even
close to what is considered a "gray card"
Click to expand...
I think that a better first approximation would be to put a real graycard next to the monitor and compare it with several "grays". In the meantime I've found a graycard at Ritz in our area, and I'll be able to carry out those experiments by myself.

Thank you, Yehuda
 
Maybe your right but somehow 18% just doesn't sound like a number that translates into a halfway point.

If someone has a grey card they could photo it and give us a rough idea of the RGB values so you know where to start.

LM
Howdy said:
In theory, I don't think there is anything wrong with your logic .

Reality, however, will throw a curve. The camera doesn't use black
& white film (or sensors), but is a mosaiced RGB, which uses fancy
math to 'guesstimate' the picture. While the result will be close,
it will be slightly off. (You can see this by running the sample
tool in PS over each pixel, and noting that the pixels are not
identical to each other, and will NOT have equal RGB values.) It
would seem much easier to simply use a paint program to choose a
specific shade of grey (ie, 128,128,128) and then 'paint' the
entire canvas in that single color.

My guess (purely a guess), would be that an RGB value of 128, 128,
128 would produce 18% grey. My reasoning is that, in B&W
photography, 18% grey represents the middle tone between white &
black. In the RGB world, 128,128,128 is halfway between 0,0,0 &
255,255,255.

An additional difficulty, however, is that even assuming
128,128,128 is 18% grey, does your printer know this? And as noted
by a previous poster, your ink & paper must combine to produce a
'reflectance' of 18%. Unless you already have a sample of an 18%
grey card, or some other way to accurately measure reflectance, and
even with the correct RGB values, it may be difficult to know
whether you've actually printed an 18% grey card.

I find the experiment interesting. It would be neat to know if 18%
grey cards could be printed from an inkjet. Could be useful in an
emergency, or to print up 'disposable' grey cards. Good luck.

Howdy.
Click to expand...
 
Proteus said:
Yehuda wrote:Alex,
Click to expand...
Proteus said:
I notice that you used rgb 80. Out of curiosity, how did you come to this value?
Click to expand...
Proteus said:
Yehuda
Click to expand...
And that's where I got 80.
But I like where you got 50 better.
Confused as usual.......LM
Proteus said:
LM1 said:
Proteus said:
Because 18% is 0.18 of a possible 1 in decimal. In hexadecimal its 50.

Alex
Click to expand...
And........that gets you to 80 how?
Click to expand...
Its not 80...

Alex
Click to expand...
 
Hi Yehuda,

Sounds like an interesting project. Let us know how it goes. As far as the RGB value I can answer that one with confidence. The 18% gray card reflects 18% of the light that hits it, looking to the eye to be a middle gray. If you photograph a gray card with proper exposure and white balance the histogram will occupy the middle portion of the histogram in a little column. So to recreate that in RGB you would use 128,128,128. As has been mentioned the reflectance of the paper used to print the gray card may not be correct so your results may not be very accurate, but you never know. Let us know how it goes please.

--
Shay

My Sony F707 Gallery: http://www.shaystephens.com/portfolio.asp
 
You must log in or register to reply here.

Keyboard shortcuts

f
Forum
m
My threads
Mobile site
About
Editorial content
Cameras & Lenses
Community
All content, design, and layout are Copyright © 1998–2025 Digital Photography Review All Rights Reserved.
Reproduction in whole or part in any form or medium without specific written permission is prohibited.
When you use DPReview links to buy products, the site may earn a commission.
©GPS Media - Guides, Products, Services.
Back
Top