Digital BW, The Print

Paul 

Sorry, I made a mistake when I did the calculations below.  A normalised
pixel value of 0.5 equates to a density of 0.6623 in a gray gamma 2.2 space
(via the math previously outlined).  Not 0.61. So yes if you are getting a
density of 0.61 for your 50% step then you are printing lighter than the
screen. 

Another way of looking at this is to open your step wedge in PS and run the
eye-dropper over it while looking in the Info Tab.  At 50% you have a pixel
value of 0.5 (shown by K=50%) and a LAB value of L=54. Carl Schofield
previously provided the following formula for calculating density from LAB's
L value.

If L>8 then Y = ((L+16)/116)^3

Else Y = (L*27)/24389

And Density = -Log10(Y)

With L = 54 this would yield a density of 0.6581

To be honest I have no idea why the two formulas end up with a different
value...

Either way I would expect your 50 step to have a density of 0.66 and if it
is showing 0.61 then I would say it is perhaps not surprising people think
it is printing lighter than on screen.

Cheers

Steve


> From: Steve Kale <stevekale@...>
> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Date: Fri, 03 Dec 2004 17:21:24 +0000
> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
> Subject: Re: [Digital BW] Tonal range and linearization
> 
> 
> Sorry, is this meant to be a subjective empirical test? Ie which do I
> subjectively prefer?
> 
> I use a workspace of Gray Gamma 2.2.  Someone who knows the intricacies of
> this space better than I may correct me, but I would say that if:
> 
> Luminance = C * value^gamma + black level
> 
> (taken from Norman Koren's site)
> 
> And we can usefully set C and black level to 1 and 0 respectively, then a
> normalised pixel value will have a luminance of 0.2176. If
> 
> Density = - log10(luminance)
> 
> Then it will have a density of -0.61.  Given it is a reflected density this
> then is 0.61.  That is, to match the workspace I would say it should print
> with a density of 0.61.
> 
> I thought I had read somewhere that PS uses LAB as the intermediate colour
> space when converting from one space to another.  I also thought LAB had a
> gamma of 2.2.  Perhaps I am wrong on this.
> 
>> From: Paul Roark <paul.roark@verizon.net>
>> Reply-To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
>> Date: Fri, 3 Dec 2004 08:51:49 -0800
>> To: <DigitalBlackandWhiteThePrint@yahoogroups.com>
>> Subject: RE: [Digital BW] Tonal range and linearization
>> 
>> 
>> Steve,
>> 
>>> Yes you are right because you in effect manually set the 50% grey slot
>>> (presumably with some iteration).  My comment was made in relation to a
>>> programme such as QTR where the midpoint is a function of linearization...
>> 
>> Yes, it is an iterative process.  The driver is a bit of a black box, but
>> once one gets used to it, it's quite predictable.
>> 
>> But, back to the simple question, would L=50 be a better midpoint?  If you
>> print a 21-step test strip, like one at
>> http://home1.gte.net/res09aij/Test-files.htm, would it match the monitor
>> better if the midpoint were more like density 0.73 (a bit lighter than the
>> current 60%) than the current 50% = 0.61?
>> 
>> Paul
>> www.PaulRoark.com
>> 
>> 
>> 
>> 
>> 
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> 
> Please visit the Group Homepage to check the Files, and other resources as
> they are often being updated.
> 
> http://groups.yahoo.com/group/DigitalBlackandWhiteThePrint
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> If you wish to receive no emails or just a daily digest, or you wish to
> unsubscribe, please edit your Membership preferences by visiting this same
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> Please follow these basic guidelines:
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> them short.
> - Good manners are required at all time. No personal attacks or flames.
> Hostile, aggressive or argumentative users may be removed from the membership
> without notice.
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> printing. Users who persistently make off-topic posts may be removed from the
> membership.
> - By posting on this forum you agree to abide by the group rules and
> guidelines, and to abide by the actions and decisions of the group Owner and
> Moderators. See �Group Topic, Rules and Guidelines� in the Files section:
> http://groups.yahoo.com/group/DigitalBlackandWhiteThePrint/files/
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> BY PARTICIPATING IN AND/OR POSTING MESSAGES TO THE DIGITAL BW, THE PRINT
> YAHOO! GROUP YOU EXPRESSLY UNDERSTAND AND AGREE THAT THE �OWNER� AND
> �MODERATORS� OF DIGITAL BW, THE PRINT YAHOO GROUP SHALL NOT BE LIABLE TO YOU
> FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, CONSEQUENTIAL OR EXEMPLARY
> DAMAGES, INCLUDING BUT NOT LIMITED TO, DAMAGES FOR LOSS OF PROFITS, GOODWILL,
> USE, DATA OR OTHER INTANGIBLE LOSSES (EVEN IF THE  �OWNER� AND �MODERATORS� OF
> DIGITAL BW, THE PRINT YAHOO GROUP HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH
> DAMAGES), RESULTING FROM: (i) THE USE OR THE INABILITY TO USE THE DIGITAL BW,
> THE PRINT YAHOO GROUP; (ii) UNAUTHORIZED ACCESS TO OR ALTERATION OF YOUR
> TRANSMISSIONS OR DATA; (iii) STATEMENTS OR CONDUCT OF ANY THIRD PARTY ON THE
> DIGITAL BW, THE PRINT YAHOO GROUP; OR (iv) ANY OTHER MATTER RELATING TO THE
> DIGITAL BW, THE PRINT YAHOO GROUP.
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