Archived posting to the Leica Users Group, 2004/04/19

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Subject: [Leica] Photoshop Questions
From: saganicc at MSKCC.ORG (Saganich, Christopher/Medical Physics)
Date: Mon Apr 19 13:59:26 2004

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: [] 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 

> 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
Deep Fried Films, Inc

John Brownlow
Deep Fried Films, Inc

Leica Users Group.
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