----- Original Message -----
From: <kpp@xxxxxxxxxx>
To: "List for Photo/Imaging Educators - Professionals - Students" <photoforum@xxxxxxxxxxxxxxxxxxxxxx>
Sent: Monday, September 12, 2005 3:00 AM
Subject: counting stops in Photoshop
> hi all!
>
> I'd like your advice on this...
>
> I need to raise the brightness of some images in photoshop exactly one f/stop.
>
>
>
> How can i achieve this?
>
>
>
> I am thinking that there are 256 units between black and white, that's 16x16. So if we assume that there also 16 stops between black and white would moving the slider in the levels window 16 units, give mea 1 stop worth of difference?
>
>
>
> just a moment I googled something out:
>
> powerretouche.com/Exposure_plugin_tutorial.htm
>
>
>
> Question still remains though, thanks, kostas
> ======================================================
> so... no matter what, CHEER UP MY FRIENDS! Life is too precious to jump the other side of the fence...
> kostas papakotas / Clenched Teeth Photography
> http://groups.msn.com/clenchedteethphotography
> http://groups.msn.com/clenchedteetharticles
>
>
>
> _____________________________________________________________________________________
>
> http://www.mailbox.gr ÁðïêôÞóôå äùñåÜí ôï ìïíáäéêü óáò e-mail.
> http://www.thesuperweb.gr Website ìå ÁóöáëÝò Controlpanel áðü 6 Euro êáé äþñï ôï domain óáò!
>
>
> I'd like your advice on this...
>
> I need to raise the brightness of some images in photoshop exactly one f/stop.
>
>
>
> How can i achieve this?
>
>
>
> I am thinking that there are 256 units between black and white, that's 16x16. So if we assume that there also 16 stops between black and white would moving the slider in the levels window 16 units, give mea 1 stop worth of difference?
>
>
>
> just a moment I googled something out:
>
> powerretouche.com/Exposure_plugin_tutorial.htm
>
>
>
> Question still remains though, thanks, kostas
> ======================================================
> so... no matter what, CHEER UP MY FRIENDS! Life is too precious to jump the other side of the fence...
> kostas papakotas / Clenched Teeth Photography
> http://groups.msn.com/clenchedteethphotography
> http://groups.msn.com/clenchedteetharticles
>
>
>
> _____________________________________________________________________________________
>
> http://www.mailbox.gr ÁðïêôÞóôå äùñåÜí ôï ìïíáäéêü óáò e-mail.
> http://www.thesuperweb.gr Website ìå ÁóöáëÝò Controlpanel áðü 6 Euro êáé äþñï ôï domain óáò!
>
I would like to apologize for using HTML (Rich Text) for this posting, but
it includes a table of density values that makes using a HTML's table
capabilities necessary.
How to increase the "brightness" of an image by one stop using
PhotoShop? Not an "easy" task because of the "Gamma" function inherent in the
image's digital encoding, but quite do-able.
We can learn quite a bit from the lowly old Kodak Q-13
Grayscale tablet.
First:
The density increases by 0.3 density units per step. According to the
commonly accepted formula, a density increase of 0.3 represents 1/2
the light hitting the subject, or an equivalent of one stop decrease in
exposure. Correspondingly, a density decrease of 0.3 units indicates twice the
light hitting the subject, or an equivalent of one stop increase in
exposure
Second:
The Kodak Q-13 Grayscale has 20 steps (0 to 19), each increasing in density
by 0.10 reflection density units, or 1/3 stop per step. It is usually assumed
that the "A" step has a density of (roughly) 0.05 density units ... for a paper
reflection of (roughly) 90%.
Third:
Step 7 ... otherwise indicated with the "M" nomenclature ... is roughly
middle gray, or a value of 127/128. Note that this is the same value (roughly)
as Kodak's 18% Gray Card, properly exposed.
Fourth:
Scanning a Kodak Q-13 Gray Scale gives us the values (for each patch) shown
below in the rather large table.
Fifth:
One stop "Increase in Exposure" would be one stop lighter. This would be
moving from step 7 to step 4, or an increase from an RGB or Luminosity value of
128 to 165 (roughly), or an increase of 37 in the pixel value(s).
Sixth:
One stop "decrease in exposure" would be one stop darker. This would be
equivalent to moving from step 7 to step 10, or a decrease in the RGB or
Luminosity pixel value of 128 to 99, or a decrease of 29 in the pixel
value(s).
Conclusion - To Increase the brightness of a digital image by
one stop:
1 - Select a point in the digital image that measures as close to
128/128/128 (RGB) or 128 Luminosity.
2 - Increase the value of that point (by using either the 'Levels,'
'Curves,' or 'Brightness/Contrast' commands) to 165/165/165 (RGB) or 165
Luminosity.
I have included the compete table of the Q-13 measurements below. In case
that you cannot find a point that measures 128/128/128 (RGB), you can select any
point that measures close to one of the values given, and increase that point's
value by three "steps" for an increase of 1 stop in the image's
brightness.
= = = = = = = = = = = = = = =
The following is a reading of the values (Raw values - in an un-assigned
color space) from a scan of a relatively good condition Kodak Q-13 Grayscale
target. The Scan was made using an Epson Perfection 4870 scanner and SilverFast
AI scanning software.
| Nominal | Nominal Stops | Measured | Mathematical | Delta from Gray | |||||||||||
| Patch | Density | From "M" | Red | Green | Blue | Luminosity | Red | Green | Blue | Grayscale | Grayscale | R | G | B | |
| 0 | A | 0.05 | 2.33 | 240.25 | 239.43 | 240.08 | 239.75 | 94.2% | 93.9% | 94.1% | 94.0% | 94.1% | 0.13% | -0.19% | 0.06% |
| 1 | 0.15 | 2.00 | 219.44 | 218.34 | 219.05 | 218.77 | 86.1% | 85.6% | 85.9% | 85.8% | 85.9% | 0.19% | -0.24% | 0.04% | |
| 2 | 0.25 | 1.67 | 198.98 | 198.08 | 199.28 | 198.49 | 78.0% | 77.7% | 78.1% | 77.8% | 78.0% | 0.08% | -0.27% | 0.20% | |
| 3 | 0.35 | 1.33 | 184.20 | 183.20 | 184.42 | 183.66 | 72.2% | 71.8% | 72.3% | 72.0% | 72.1% | 0.10% | -0.29% | 0.19% | |
| 4 | 0.45 | 1.00 | 164.92 | 164.50 | 166.04 | 164.83 | 64.7% | 64.5% | 65.1% | 64.6% | 64.8% | -0.09% | -0.26% | 0.35% | |
| 5 | 0.55 | 0.67 | 152.49 | 151.84 | 153.28 | 152.25 | 59.8% | 59.5% | 60.1% | 59.7% | 59.8% | -0.02% | -0.27% | 0.29% | |
| 6 | 0.65 | 0.33 | 140.01 | 139.57 | 140.83 | 139.86 | 54.9% | 54.7% | 55.2% | 54.8% | 55.0% | -0.05% | -0.22% | 0.27% | |
| 7 | M | 0.75 | 0.00 | 128.48 | 127.96 | 129.09 | 128.33 | 50.4% | 50.2% | 50.6% | 50.3% | 50.4% | -0.01% | -0.22% | 0.23% |
| 8 | 0.85 | -0.33 | 117.58 | 117.46 | 118.73 | 117.66 | 46.1% | 46.1% | 46.6% | 46.1% | 46.2% | -0.13% | -0.18% | 0.32% | |
| 9 | 0.95 | -0.67 | 108.01 | 107.96 | 109.29 | 108.15 | 42.4% | 42.3% | 42.9% | 42.4% | 42.5% | -0.16% | -0.18% | 0.34% | |
| 10 | 1.05 | -1.00 | 99.42 | 99.28 | 100.63 | 99.50 | 39.0% | 38.9% | 39.5% | 39.0% | 39.1% | -0.14% | -0.19% | 0.33% | |
| 11 | 1.15 | -1.33 | 92.06 | 91.52 | 92.79 | 91.85 | 36.1% | 35.9% | 36.4% | 36.0% | 36.1% | -0.02% | -0.24% | 0.26% | |
| 12 | 1.25 | -1.67 | 85.22 | 84.89 | 86.17 | 85.16 | 33.4% | 33.3% | 33.8% | 33.4% | 33.5% | -0.08% | -0.21% | 0.29% | |
| 13 | 1.35 | -2.00 | 78.62 | 78.10 | 79.60 | 78.45 | 30.8% | 30.6% | 31.2% | 30.8% | 30.9% | -0.06% | -0.26% | 0.32% | |
| 14 | 1.45 | -2.33 | 74.28 | 73.33 | 74.46 | 73.75 | 29.1% | 28.8% | 29.2% | 28.9% | 29.0% | 0.10% | -0.27% | 0.17% | |
| 15 | 1.55 | -2.67 | 68.35 | 67.95 | 69.35 | 68.25 | 26.8% | 26.6% | 27.2% | 26.8% | 26.9% | -0.08% | -0.24% | 0.31% | |
| 16 | B | 1.65 | -3.00 | 63.83 | 63.47 | 64.70 | 63.73 | 25.0% | 24.9% | 25.4% | 25.0% | 25.1% | -0.07% | -0.21% | 0.27% |
| 17 | 1.75 | -3.33 | 59.83 | 59.66 | 60.82 | 59.86 | 23.5% | 23.4% | 23.9% | 23.5% | 23.6% | -0.11% | -0.17% | 0.28% | |
| 18 | 1.85 | -3.67 | 55.38 | 55.29 | 56.24 | 55.44 | 21.7% | 21.7% | 22.1% | 21.7% | 21.8% | -0.10% | -0.14% | 0.24% | |
| 19 | 1.95 | -4.00 | 53.77 | 53.92 | 54.83 | 53.99 | 21.1% | 21.1% | 21.5% | 21.2% | 21.2% | -0.16% | -0.10% | 0.26% | |
