[Leica] Photoshop Questions

[mark at rabinergroup.com (Mark Rabiner)]

On 4/19/04 1:59 PM, "Saganich, Christopher/Medical Physics"
<saganicc@mskcc.org> wrote:

> Thanks John,
> 
> OK, if I scan a neg which was made using source of luminescence 16 c/ft2 I
> would have a neg density of 1.2 and the histogram should show then 127 peak
> with 8 bit, if I account for the screen gamma.
> 
> Since the input and output values are linear (between 0 and 1) how is this
> related to negative density?  Is there a mathematical relationship between the
> log base 10 density the input/output.  Or perhaps I'm trying to compare apples
> and oranges?
> 
> Chris Saganich
> 
> 
> -----Original Message-----
> From: lug-bounces+saganicc=mskcc.org@leica-users.org
> [mailto:lug-bounces+saganicc=mskcc.org@leica-users.org] On Behalf Of Johnny
> Deadman
> Sent: Monday, April 19, 2004 2:27 PM
> To: Leica Users digest
> Subject: [Leica] Photoshop Questions
> 
> That depends on a lot of things, not least the gamma with which the
> image is encoded, which changes the grey point.
> 
> Don't take this as gospel, but my understanding is:
> 
> Output Value = Input Value ** Gamma
> 
> Where both input and output values are between 0 (black) and 1 (white).
> 
> The higher the gamma, the darker the midtones.
> 
> What this means is that 50% grey (127 in 8-bit images) represents the
> following grey values, depending on gamma:
> 
> 50% grey in linear gamma (Gamma = 1)
> 
> 28% grey in Gamma 1.8 (Mac standard)
> 
> 22% grey in Gamma 2.2 (Windoze standard)
> 
> 18% grey in Gamma 2.5 (often used as a standard for uncorrected CRTs)
> 
> 
> On Apr 19, 2004, at 11:26 AM, Saganich, Christopher/Medical Physics
> wrote:
> 
>> Can anyone explain the units used in the grey histogram, specifically
>> the "Level", (O- 256) and how this might relate to opacity, density or
>> luminescence from a densitometry or sensitometry perspective?  I can't
>> see a relation but there should be one.  For example how does the 18%
>> grey relate to the number 127?
> --
> John Brownlow
> 
> 


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Mark Rabiner
Photography
Portland Oregon



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