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If I shoot a grey card, should this end up as 127 grey?

 
 
Alan F Cross
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      02-25-2004
Talking theoretically ...

If I take a shot of an 18% grey card, should I find this to be at level
127 (on an 8-bit grey-scale)?

Coming at the question from the other end, if I make a print of an
electronically-generated 127 patch, should this look like my 18% grey
card, or some other grey shade?

I know in practice things won't work out perfectly, but I want to
understand what I should be aiming for.

TIA.
--
Alan F Cross
 
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Lionel
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      02-25-2004
Kibo informs me that Alan F Cross <(E-Mail Removed)> stated
that:

>Talking theoretically ...
>
>If I take a shot of an 18% grey card, should I find this to be at level
>127 (on an 8-bit grey-scale)?


No.

In the unlikely event that there's any linear relationship, it'd be more
likely to be about 46.


>Coming at the question from the other end, if I make a print of an
>electronically-generated 127 patch, should this look like my 18% grey
>card, or some other grey shade?


Some other shade.

--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------
 
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Journalist-North
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      02-25-2004

"Alan F Cross" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> Talking theoretically ...
>
> If I take a shot of an 18% grey card, should I find this to be at level
> 127 (on an 8-bit grey-scale)?
>
> Coming at the question from the other end, if I make a print of an
> electronically-generated 127 patch, should this look like my 18% grey
> card, or some other grey shade?
>
> I know in practice things won't work out perfectly, but I want to
> understand what I should be aiming for.
>
> TIA.
> Alan F Cross


-------

Analysis of the image INFO in PS will tell you...Window>show>info

Mid-Gray (the 18% gray card value) will produce the following color
analysis:

R = 133-135 G = 133-135 B = 133-135 >>Lum = 127<< Sat = 160 Hue = 0

C = 0; M = 0; Y = 0; K = 54%

Journalist

 
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Roland Karlsson
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      02-25-2004
Alan F Cross <(E-Mail Removed)> wrote in
news:(E-Mail Removed):

> If I take a shot of an 18% grey card, should I find this to be at level
> 127 (on an 8-bit grey-scale)?
>
> Coming at the question from the other end, if I make a print of an
> electronically-generated 127 patch, should this look like my 18% grey
> card, or some other grey shade?


No and no

First - the digital 8 bit pictures uses a gamma to
improve the useful gradiation.

Second - the grey card can be any value - it depends
on your exposure.


/Roland
 
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bob
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      02-25-2004
"Journalist-North" <(E-Mail Removed)> wrote in news:4a2%
b.1911$(E-Mail Removed):

>
> "Alan F Cross" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)...
>> Talking theoretically ...
>>
>> If I take a shot of an 18% grey card, should I find this to be at level
>> 127 (on an 8-bit grey-scale)?
>>
>> Coming at the question from the other end, if I make a print of an
>> electronically-generated 127 patch, should this look like my 18% grey
>> card, or some other grey shade?
>>
>> I know in practice things won't work out perfectly, but I want to
>> understand what I should be aiming for.
>>
>> TIA.
>> Alan F Cross

>
> -------
>
> Analysis of the image INFO in PS will tell you...Window>show>info
>
> Mid-Gray (the 18% gray card value) will produce the following color
> analysis:
>
> R = 133-135 G = 133-135 B = 133-135 >>Lum = 127<< Sat = 160 Hue = 0
>
> C = 0; M = 0; Y = 0; K = 54%
>
> Journalist
>
>


In Photoshop 6.0, the values associated with the 15% and 20% grey (default)
swatches are:
R=G=B = 218 and 205

CMYK values are 13%, 10%, 11%, 0%; and 19%, 14%, 15%, and 0%

I think an 18% grey card, if exposed for 18% grey (auto meter) should come
up with an 18% exposure, which will print at 18%, and everything will be as
expected. If an 18% grey were recorded at 50%, and then printed at 50%, it
would be way too dark, wouldn't it?

Bob

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Alan F Cross
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      02-25-2004
In message <4a2%b.1911$(E-Mail Removed)>,
Journalist-North <(E-Mail Removed)> writes
>
>"Alan F Cross" <(E-Mail Removed)> wrote in message
>news:(E-Mail Removed)...
>> Talking theoretically ...
>>
>> If I take a shot of an 18% grey card, should I find this to be at level
>> 127 (on an 8-bit grey-scale)?
>>
>> Coming at the question from the other end, if I make a print of an
>> electronically-generated 127 patch, should this look like my 18% grey
>> card, or some other grey shade?
>>
>> I know in practice things won't work out perfectly, but I want to
>> understand what I should be aiming for.
>>
>> TIA.
>> Alan F Cross

>
>-------
>
>Analysis of the image INFO in PS will tell you...Window>show>info
>
>Mid-Gray (the 18% gray card value) will produce the following color
>analysis:
>
>R = 133-135 G = 133-135 B = 133-135 >>Lum = 127<< Sat = 160 Hue = 0
>
>C = 0; M = 0; Y = 0; K = 54%
>
>Journalist
>



How do you produce an 18% grey in Photoshop, then? It can't be a scan,
because that will be off by an unknown amount. Are you just holding up a
grey card next to the screen, and tweaking till they look similar? The
values will depend on gamma and to some extent colour space.

Found this Q&A on Google:

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

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
(199, both 2.2 gamma spaces, put it at around 125.

So clearly - it depends!!
--
Alan F Cross
 
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JPS@no.komm
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Posts: n/a
 
      02-25-2004
In message <(E-Mail Removed)>,
Alan F Cross <(E-Mail Removed)> wrote:

>Talking theoretically ...
>
>If I take a shot of an 18% grey card, should I find this to be at level
>127 (on an 8-bit grey-scale)?


Most cameras (JPEG and TIFF) and RAW converters aim for 127 or 128, some
for 117 (I'v seen this value, but I don't remember where).

>Coming at the question from the other end, if I make a print of an
>electronically-generated 127 patch, should this look like my 18% grey
>card, or some other grey shade?


Make a computer-generated grid of 256 squares, with each value from 0 to
255 (or skip 0 to 70, or so, to save ink). Print it, then see which one
is the same as the grey card. Using the same printer and paper, and
driver settings, print a whole page at that value, and you should get
18% grey. Don't use glossy paper, and don't expect the print to stay
the same over time.

>I know in practice things won't work out perfectly, but I want to
>understand what I should be aiming for.
>
>TIA.


--

<>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
John P Sheehy <(E-Mail Removed)>
><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><

 
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Journalist-North
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Posts: n/a
 
      02-26-2004

"Alan F Cross" <(E-Mail Removed)> wrote in message
news(E-Mail Removed)...
> In message <4a2%b.1911$(E-Mail Removed)>,
> Journalist-North <(E-Mail Removed)> writes
> >

(snip)
>
> How do you produce an 18% grey in Photoshop, then? It can't be a scan,
> because that will be off by an unknown amount. Are you just holding up a
> grey card next to the screen, and tweaking till they look similar? The
> values will depend on gamma and to some extent colour space.
>
> Found this Q&A on Google:
>
> What is the calculation that yields 18% grey = 50% tonality, 128 on an
> 8-bit grayscale, etc?
>
> 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
> (199, both 2.2 gamma spaces, put it at around 125.
>
> So clearly - it depends!!
>
> Alan F Cross

--------

Actually Alan the K value of CMYK is the target (see notes below), as well
as the indicative Luminance value (127) as the RGB gray value has you
working in RGB colour space rather than a black and white colour space (e.g
Greyscale = Grey1.8G or Grey2.2G). As a colour photo will necessarily be
working in a colour space then the following Adobe procedure can be used to
correct the colour (using the mid tone grey as the reference)

You can construct / simulate mid grey in Photoshop by creating a step wedge
and measuring the mid grey colour values as generated on screen; as received
from a scanner; and compared visually to the output from a printer, as well,
against a standard 18% grey card in ANY uniform light condition.

SEE PHOTOSHOP HELP: Eliminating unwanted colour casts

--------

This is quoted from the help screen:

If your scanned image contains an unwanted colour cast, you can perform a
simple test to determine whether the cast was introduced by your scanner. If
it was, you can use the same test file to create a colour-cast correction
for all images scanned with the scanner.

To identify and correct a colour cast introduced by a scanner:

1) Make sure that your monitor has been calibrated. (See Creating an ICC
monitor profile.)
2) Open a new Photoshop file, and use the linear gradient tool to create a
blend from pure black to pure white.
3) Choose Image > Adjust > Posterize, and posterize the blend using 11
levels.
4) Print the 11-step gray wedge on a black-and-white printer, and then scan
it into Photoshop.

Note: You can also perform this test using an 18-percent neutral gray card
or an 11-step gray wedge from a photography store.

5) Open the Info palette, and read the RGB values on-screen for each of the
gray levels.
>>> Uneven R, G, and B values indicate a colour cast. <<<

6) Use Levels or Curves to correct the colour cast, and then save the dialog
box settings. (See 4. Adjust the colour balance..)
7) Open the scanned image you want to correct, reopen the dialog box you
used to correct the cast in step 6, and load the saved settings.

--------

The values I quoted as

R = 133-135 G = 133-135 B = 133-135 >>Lum = 127<< Sat = 160 Hue = 0

C = 0; M = 0; Y = 0; K = 54%

are nearly correct (in absolute terms) for mid grey, that is, the mid point
in the grey scale or on the step wedge constructed above, between pure white
and pure black, and working in an RGB colour space

You will note that all the RGB values are the same (per Adobe they are
perfectly balanced though slightly denser than 127 in all channels -
possibly due to my calibration settings); they are not shifted by the hue
control (value is zero) but the luminance value is 127 which is dead centre
between the white (255) and black (0) COLOUR values. Thus, I do have a 3 way
colour balance and the Luminance is the mid grey / 50% reflectance value.

The K (black ink) value I quoted is "near" to 50% (50% black ink density
printed on a pure white paper = mid grey) but the exact value I measured
(54%) may be off a bit, again due to my calibration settings. The K value is
a printer's reference value for mid grey.

If you also look at the step wedge extremes you will see that white has
values of R = 255; G = 255; B = 255; and Lum around that value (typically
240+ depending on your calibration) = 100% reflectance; whereas the black
end will have values of very near R = 0; G = 0; B = 0; and Lum = 0 / = 0%
reflectance

OTHER NOTES:

Note 1) As a side note...for commercial printing to paper, or for that
matter printing to your ink-jet which in fact actually prints in CMYK as
well, the K value is NOT the ONLY reference point as "absolute" black can be
improved (see note 2, below, on 4 colour B&W printing with 4 black inks) in
how it appears in on the page in a publication by slightly shifting other
colour values; but, this correction is almost NEVER made (in Photoshop or
any other image editor) before the image is referenced to a layout
application (such as QuarkXPress - where the colour values can be further
adjusted in the pre-press workflow). These corrections will not be seen on
screen, or even noticeable, but can be observed at the output of a printing
press and printers describe this as the "blacker than black" effect. That
is, the slight colour shift introduced (usually a bit of a CMY component
shift towards magenta) actually produces, visually to the human eye, a
blacker black on the page than just printing with "black" ink. So I am told,
it is more akin to matching the appearance of the paper printed image to the
non-linear gamma value of the human eye - more an arcane printer's "art"
than science at work here - and, of course, it depends to some degree on the
paper being used taking account of the whiteness and the non-white colour
and reflectance values of that against the semi-transparent inks being laid
down by the press - it also allows for "black" to be overprinted on, or
joined to, other colours and still look "black."

Note 2) 4 colour B&W ink-jet printing with all black ink cartridges
installed. Some ink-jet printers can be fed 4 black toned inks for only
printing photos in B&W. but allowing a wide gamut of black/grey toning; this
allows for almost the same thing, in fact it is the exact same thing, as the
printers are doing above by shifting from hard blacks to softer velvety
tones or vice versa. They are all "black" inks in the cartridges, yes, but
with slightly different colour shifts in their formulations. Ordinary paint
works the same way as printer ink - go into any paint store and ask for the
formula of a darkish shade of "charcoal grey" - more then likely you will
find a formula that looks something like this: "mix black 90% with white
9.9% and add yellow 0.1%." That ever so slight yellow shift does wonders
when the paint is viewed in daylight conditions. You could never tell the
difference between the above mix in the paint bucket and a mix of 90/10 B&W,
only, in a second bucket...but you sure can see it once it is painted on a
building.

Journalist

 
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Robert E. Williams
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      02-26-2004


Alan F Cross wrote:

> Talking theoretically ...
>
> If I take a shot of an 18% grey card, should I find this to be at level
> 127 (on an 8-bit grey-scale)?
>
> Coming at the question from the other end, if I make a print of an
> electronically-generated 127 patch, should this look like my 18% grey
> card, or some other grey shade?
>
> I know in practice things won't work out perfectly, but I want to
> understand what I should be aiming for.
>
> TIA.
> --
> Alan F Cross


Remember that an 18% grey card isn't NECESSARILY neutral in hue.
It is designed to have a standard REFLECTANCE equal to a middle grey
tone.

It was originally designed to help a photographer obtain proper exposure
,
not proper white balance.
I have two standard 18% grey cards, one by Kodak and one by MacBeth and
they have noticeably different hues.
However, they both give identical EXPOSURE readings on my camera because
they both have the same reflectivity.

Computer wise, we have 256 shades of grey, so I interpret "Middle Grey"
as
R=128, G=128, and B=128.
I create a sheet in Photoshop and fill it with a swatch that is
128,128,128, but I then convert it to Greyscale to remove ANY color.
Then I print it on Matte paper with Black ink just to make sure that my
printer does not introduce any color artifacts. This print CANNOT have a
color bias because it has NO color at all. The resulting sheet gives the
same reading as my Kodak Grey card and should also be perfect for White
Balance, but I have never tried it for this purpose.
Bob Williams
 
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