10 step grayscale target

[Ronald...@adobeforums.com]

In the current issue of Photoshop User, there is an article by Taz Tally on
using a 10 step grayscale target in the image shot by a digital camera, and
Photoshop channels to do color correction.

I've been using a 3 step home brew target - Black, White, and Gray (from a
Kodak 18% grayscale) arrangement for a while now, and it works quite well.
But I'd like to go the next step, and get a calibrated 10 step target.

I've searched in vain, for the 10 step target that Taz Tally metnions -
"Taz's 10-step-target", but have not been able to find it. Does anyone know
where it can be obtained? Or, if not Taz's target, where can a similar
calibrated grayscale target be obtained.

I did purchase an IT-8 target from Wolf Faust last year, and that does have
a 21 step grayscale target on it. But the files that define the swatches are
not defined in R-G-B values, which I want to have for a target that I can
use.

Ron Hirsch

[Gary_H...@adobeforums.com]

Stores that sell professional photo equipment and supplies usually have the target sets from Kodak. They come in at least a couple of sizes. I use the small one with photos on the scanner. As far as using 21 step targets in the field, my experience is that a good home brew target works just about as well, and if the wind blows it in the lake, you are only out about $0.25.

As for using targets with supplied calibration data, that data is typically in LAB units. Since the LAB colorspace includes RGB and a lot more, you can safely convert to LAB from RGB and back without fear of degradation.

Gary

[Ronald...@adobeforums.com]

Gary,

Thanks for your reply.

I looked over the data file for the IT-8 target I have - remember this is a
paper target, not a film target - and I did see the LAB data there.

But the problem is that I can't easily identify which of the data are for
the grayscale, and, even if I could, how would I take that data, and convert
it to RGB? Conversions are easy for an image opened in Photoshop, but the
only way I can get such an image is to scan the IT-8 target. Then of course,
I'd have the scanner profile to contend with. Then if I printed that out,
I'd have the printer profile to contend with.

Even though all my "stuff" is profiled, I'd still be off in what I'm looking
for.

I haev a goodly assortment of "test" image files, but what I'd like to get
is a 10 step grayscale with RGB values defined in advance by the supplier.

Taz Tally did refer to his 10 step target. But I searched high and low in
Google, but couldn't find it. I also wrote to Taz, and asked about it, but
I"ve not heard back yet.

Ron

[Gernot_...@adobeforums.com]

Ron,

below are the CIELab values for the bottom line grayscale in MY ACTUAL
reference file. Note: these are not accurately gray (a=0, b=0) but the diffe-
rences for a,b are not relevant in a photo.
If you take a photo of this IT8-Scanner target then you KNOW the L values
and you can ADJUST your photo by Curves in Lab mode for approximately
the same results (theoretically ...).
W.Faust offers also camera targets. You will easily find the relation between
patch identifiers like GS0 in the target and in the reference file.

-----------------------------------------------L-------a------b---------------------------
GS0 79.89 83.34 69.25 93.17 -0.93 -0.42 1.02 204.45
GS1 67.25 69.62 57.09 86.81 0.29 0.37 0.47 51.57
GS2 59.61 61.48 50.29 82.63 0.80 0.50 0.94 31.45
GS3 52.61 54.18 44.25 78.56 0.96 0.55 1.11 30.38
GS4 46.49 47.91 39.41 74.77 0.84 0.17 0.86 12.13
GS5 40.23 41.56 33.94 70.56 0.50 0.51 0.72 43.18
GS6 34.25 35.39 28.58 66.05 0.46 1.02 1.12 65.71
GS7 29.25 30.23 24.70 61.86 0.40 0.46 0.60 48.81
GS8 24.72 25.59 20.82 57.65 0.16 0.60 0.63 73.45
GS9 21.06 21.86 17.51 53.88 -0.15 1.20 1.21 96.02
GS10 17.13 17.85 14.40 49.31 -0.40 0.86 0.95 114.56
GS11 14.23 14.82 12.01 45.39 -0.44 0.66 0.79 125.28
GS12 11.55 12.07 9.87 41.33 -0.65 0.30 0.72 156.43
GS13 9.30 9.65 7.79 37.22 -0.08 0.68 0.70 95.23
GS14 7.43 7.70 6.27 33.34 0.02 0.40 0.41 81.22
GS15 5.51 5.74 4.71 28.74 -0.22 0.14 0.50 149.31
GS16 4.28 4.44 3.63 25.07 0.01 0.25 0.44 169.99
GS17 3.19 3.33 2.79 21.28 -0.37 -0.37 0.55 220.66
GS18 2.27 2.38 1.98 17.33 -0.52 -0.25 0.59 202.79
GS19 1.58 1.66 1.38 13.59 -0.50 -0.19 0.54 200.72
GS20 1.04 1.11 0.93 9.83 -0.65 -0.23 0.69 199.88
GS21 0.84 0.88 0.75 7.93 -0.41 -0.33 0.54 216.21
GS22 0.63 0.66 0.64 5.89 -0.07 -1.90 1.90 268.03
GS23 0.54 0.57 0.61 5.16 -0.42 -2.73 2.76 261.17

Sorry - no formatting here.
We have 9 columns. L-a-b are the columns 5-6-7

Best regards --Gernot Hoffmann

[Philo_...@adobeforums.com]

Since RGB is not one colour space, it is unlikely that you'll get defined RGB values. As mentioned above, you might try getting LAB values. These are defined for a GM colour checker chart. (and that gives neutral greys as well as colour swatches)

[SimRacer]

<Ronald...@adobeforums.com> wrote in message
news:EB0787B9AA560BC8...@in.webx.la2eafNXanI...

If you do find it, be sure to let us know. His method (and target) look
pretty user friendly, I'd like to get ahold of one of those targets myself.

[Ronald...@adobeforums.com]

Gernot,

Thank you for the info you presented. I'm afraid that I just recently read
you reply of January 8th, as I had neglected to mark that thread as
"watched".

After going through lots of searching, and gathering info in past weeks,
I've come to the conclusion that using Lab mode is the way to go for what I
want to do.

I do have one of Wolf Faust's IT-8 targets, and the associated file that
came with it. (You were the one who referred me to Wolf) It has the Lab
values listed for my card. They are similar to yours, so I assume that I
should use the values I have. I didn't really know which set of values were
the greyscale, but since you referred to GS0 to GS23, it then became obvious
that "GS" stands for "greyscale".

Is the workflow that I should follow as I've noted below? I'm using
Photoshop CS in Windows XP

1. Open the digital image in Photoshop, that includes the IT-8 target.

2. My camera uses a color space of sRGB - should I convert to Adobe RGB
(1998) when opening it, or does this not matter - I would suspect not?

3. Convert to Lab mode.

4. Go to a given swatch on the target, and use the color sampler tool to
record that sample - to keep a set of L-a-b values displayed on the screen
for me to use.

5. Bring up curves. Start off with the "L" curve. Place the sampled point in
question on the L curve by CTRL+clicking on that sample in the image.

6. Here's where it's a bit confusing - lets' say this is a middle point that
shows as 52/52 in the coordinates. The value of "L" in the text file is
50.3. Exactly how do I correct that point on the L, a, and b curves? Do I
merely change the output value so that it is "50.3"? Then do I repeat this
process for "a" and "b" using the same approach?

7. Once I've done that point, go on to other grayscale points and do the
same thing??? Continue on until as many points as I want have been done.

8. When I'm done, I assume that I should convert back to RGB mode, and then
save the file.

So, do I have things reasonably understood? If not, any corrections would be
appreciated.

Thanks again for your help.

Ron Hirsch

[careyj...@gmail.com]

Check the Taz Tally web site -- taztallyphotography.com -- and click on Online Ordering.