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
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
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)
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
wrote in message
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
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.
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
