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Still trying to figure the grey level of an 18% grey card (was: If I shoot a grey card, should this end up as 127 grey?)

 
 
Alan F Cross
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      02-29-2004
I had several responses on this, all giving different answers!! I have
also seen three definitions of the meaning of 18% grey:

Definition 1: An analysis of 'average scenes' (whatever that may be)
showed they were 18% reflective. Can't imagine there is such a thing as
an average scene that would be in any way useful!

Definition 2: the mid point of a 5-stop scale. I can see that 100/18 is
around 2.5 stops. This sounds more plausible, but if you work on a
6-stop range, the point moves to 12.5% so there's little justification
for the figure.

Definition 3: It's just an arbitrary figure to which most exposure
meters are calibrated, by convention (and some do something else).

Perhaps the truth is a combination of these. But there must be a value
(between 0 and 255) that represents the same shade of grey as an 18%
card. I know that there is the gamma question, but surely this is to
essentially remove the non-linearity of the display tube, not to distort
the grey-scale itself.

A mid-grey that is scanned and then printed should look the same, no
matter how screwed up it is on the display. I would have thought that
the tube should *try* to display the same grey that is being scanned and
being printed (within the constraints of reflective vs transmissive
viewing).

So, the question remains: if I create a grey of 127 or 128, *should* it
look like an 18% grey card? If not, what level of grey should?

The root of my question is the following problem:
When I send mono work out for printing, I attach a grey scale of equal
steps (0, 16, 32, 48,.....240, 255). It is easy to recognise 0 and 255
when printed, but how do I tell them that their rendition of mid-grey is
too light or too dark?

I would need a full wedge to prove linearity across the range, and to
ensure that there is no black or white clipping. But I really want a
separate grey patch, labelled "this should look like and 18% grey card
when printed". Perhaps this is 127/128, or perhaps it's something else
(and that is the question!) The only reason to choose 18% grey is that
they would certainly have a reference card available to compare.

I'd appreciate any further comments on this perplexing (but essentially
simple) issue.

TIA
--
Alan F Cross
 
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Robertwgross
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      02-29-2004
Alan, shoot a Kodak 18% gray card and be done with it.

---Bob Gross---
 
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Don Coon
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      02-29-2004

"Alan F Cross" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> I had several responses on this, all giving different answers!! I have
> also seen three definitions of the meaning of 18% grey:
>
> Definition 1: An analysis of 'average scenes' (whatever that may be)
> showed they were 18% reflective. Can't imagine there is such a thing as
> an average scene that would be in any way useful!
>
> Definition 2: the mid point of a 5-stop scale. I can see that 100/18 is
> around 2.5 stops. This sounds more plausible, but if you work on a
> 6-stop range, the point moves to 12.5% so there's little justification
> for the figure.
>
> Definition 3: It's just an arbitrary figure to which most exposure
> meters are calibrated, by convention (and some do something else).
>
> Perhaps the truth is a combination of these. But there must be a value
> (between 0 and 255) that represents the same shade of grey as an 18%
> card. I know that there is the gamma question, but surely this is to
> essentially remove the non-linearity of the display tube, not to distort
> the grey-scale itself.
>
> A mid-grey that is scanned and then printed should look the same, no
> matter how screwed up it is on the display. I would have thought that
> the tube should *try* to display the same grey that is being scanned and
> being printed (within the constraints of reflective vs transmissive
> viewing).
>
> So, the question remains: if I create a grey of 127 or 128, *should* it
> look like an 18% grey card? If not, what level of grey should?
>
> The root of my question is the following problem:
> When I send mono work out for printing, I attach a grey scale of equal
> steps (0, 16, 32, 48,.....240, 255). It is easy to recognise 0 and 255
> when printed, but how do I tell them that their rendition of mid-grey is
> too light or too dark?
>
> I would need a full wedge to prove linearity across the range, and to
> ensure that there is no black or white clipping. But I really want a
> separate grey patch, labelled "this should look like and 18% grey card
> when printed". Perhaps this is 127/128, or perhaps it's something else
> (and that is the question!) The only reason to choose 18% grey is that
> they would certainly have a reference card available to compare.
>
> I'd appreciate any further comments on this perplexing (but essentially
> simple) issue.


Have you tried scanning a grey card?


 
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Tony Spadaro
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      02-29-2004
A grey card comes out at about RG&B 160 on a reasonably calibrated
monitor.

--
http://www.chapelhillnoir.com
home of The Camera-ist's Manifesto
The Improved Links Pages are at
http://www.chapelhillnoir.com/links/mlinks00.html
A sample chapter from my novel "Haight-Ashbury" is at
http://www.chapelhillnoir.com/writ/hait/hatitl.html
"Alan F Cross" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> I had several responses on this, all giving different answers!! I have
> also seen three definitions of the meaning of 18% grey:
>
> Definition 1: An analysis of 'average scenes' (whatever that may be)
> showed they were 18% reflective. Can't imagine there is such a thing as
> an average scene that would be in any way useful!
>
> Definition 2: the mid point of a 5-stop scale. I can see that 100/18 is
> around 2.5 stops. This sounds more plausible, but if you work on a
> 6-stop range, the point moves to 12.5% so there's little justification
> for the figure.
>
> Definition 3: It's just an arbitrary figure to which most exposure
> meters are calibrated, by convention (and some do something else).
>
> Perhaps the truth is a combination of these. But there must be a value
> (between 0 and 255) that represents the same shade of grey as an 18%
> card. I know that there is the gamma question, but surely this is to
> essentially remove the non-linearity of the display tube, not to distort
> the grey-scale itself.
>
> A mid-grey that is scanned and then printed should look the same, no
> matter how screwed up it is on the display. I would have thought that
> the tube should *try* to display the same grey that is being scanned and
> being printed (within the constraints of reflective vs transmissive
> viewing).
>
> So, the question remains: if I create a grey of 127 or 128, *should* it
> look like an 18% grey card? If not, what level of grey should?
>
> The root of my question is the following problem:
> When I send mono work out for printing, I attach a grey scale of equal
> steps (0, 16, 32, 48,.....240, 255). It is easy to recognise 0 and 255
> when printed, but how do I tell them that their rendition of mid-grey is
> too light or too dark?
>
> I would need a full wedge to prove linearity across the range, and to
> ensure that there is no black or white clipping. But I really want a
> separate grey patch, labelled "this should look like and 18% grey card
> when printed". Perhaps this is 127/128, or perhaps it's something else
> (and that is the question!) The only reason to choose 18% grey is that
> they would certainly have a reference card available to compare.
>
> I'd appreciate any further comments on this perplexing (but essentially
> simple) issue.
>
> TIA
> --
> Alan F Cross



 
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mark_digital
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Posts: n/a
 
      02-29-2004

"Alan F Cross" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> So, the question remains: if I create a grey of 127 or 128, *should* it
> look like an 18% grey card? If not, what level of grey should?


A few years ago there was a discussion in this group along the same line you
seem to be asking. I'm sure it made a few go out and buy a gray card and
understand a bit better about nuetral gray and it's visual perception under
varying light sources and intensities.
If you printer and paper were capable of producing a sample that could
reflect 18% gray then your problem would be solved.


 
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JPS@no.komm
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      02-29-2004
In message <(E-Mail Removed)>,
Alan F Cross <(E-Mail Removed)> wrote:

>Perhaps the truth is a combination of these. But there must be a value
>(between 0 and 255) that represents the same shade of grey as an 18%
>card.


That would be (0.18^(1/2.2))*255 = ~117. Most cameras and RAW
converters target ~127, though.
--

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

 
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JPS@no.komm
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      02-29-2004
In message <(E-Mail Removed)>,
Alan F Cross <(E-Mail Removed)> wrote:

>So, the question remains: if I create a grey of 127 or 128, *should* it
>look like an 18% grey card? If not, what level of grey should?


I would think that the answer depends on the paper used. Paper is in
its own little world of low contrast, and blacks that are grey in the
real world. I don't think prints even attempt to center around a grey
card.
--

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

 
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JPS@no.komm
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Posts: n/a
 
      02-29-2004
In message <PAd0c.38058$%(E-Mail Removed)> ,
"Tony Spadaro" <(E-Mail Removed)> wrote:

> A grey card comes out at about RG&B 160 on a reasonably calibrated
>monitor.


Huh? Monitors don't have RGB values. They are passive analog devices,
displaying voltages created by the gfx chips. An average exposure will
be about 127 with most cameras and RAW converters. I've seen 117 as
well (which is what 18% grey should be) in one context, (but I don't
remember where).
--

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

 
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Tony Spadaro
Guest
Posts: n/a
 
      02-29-2004
Quite wrong. If you would take the time to compare a grey card to the greys
available in Photoshop you will find - that if your monitor is fairly well
calibrated you will get about 160. Don't talk theory when it takes vary
little work to see that your are flat out wrong. If you don't have a grey
card - buy one.

--
http://www.chapelhillnoir.com
home of The Camera-ist's Manifesto
The Improved Links Pages are at
http://www.chapelhillnoir.com/links/mlinks00.html
A sample chapter from my novel "Haight-Ashbury" is at
http://www.chapelhillnoir.com/writ/hait/hatitl.html
<(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> In message <PAd0c.38058$%(E-Mail Removed)> ,
> "Tony Spadaro" <(E-Mail Removed)> wrote:
>
> > A grey card comes out at about RG&B 160 on a reasonably calibrated
> >monitor.

>
> Huh? Monitors don't have RGB values. They are passive analog devices,
> displaying voltages created by the gfx chips. An average exposure will
> be about 127 with most cameras and RAW converters. I've seen 117 as
> well (which is what 18% grey should be) in one context, (but I don't
> remember where).
> --
>
> <>>< ><<> ><<> <>>< ><<> <>>< <>>< ><<>
> John P Sheehy <(E-Mail Removed)>
> ><<> <>>< <>>< ><<> <>>< ><<> ><<> <>><



 
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JPS@no.komm
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Posts: n/a
 
      02-29-2004
In message <kog0c.55216$(E-Mail Removed)> ,
"Tony Spadaro" <(E-Mail Removed)> wrote:

>Quite wrong. If you would take the time to compare a grey card to the greys
>available in Photoshop you will find - that if your monitor is fairly well
>calibrated you will get about 160. Don't talk theory when it takes vary
>little work to see that your are flat out wrong. If you don't have a grey
>card - buy one.


You're not making one bit of sense. Inability to communicate and
inability to detect failed communication and initiate a retry is an
epidemic disease, and you are infected.

Where is this "160" figure coming from?

A grey card's level in a room depends totally on ambient lighting.
--

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

 
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