2017-02-17 by petware@...
Hi folks,
I was wondering if there are any tutorials on how to use a densitometer to create correction curves for digital negatives. I am Mac based and have an xrite 810 to use. Are there conversion tables to convert my 810 readings to % readings. For instance, how do I know what 20% gray is supposed to be with my densitometer readings? Do I even have to convert the densitometer reading to % or can I enter the densitometer readings directly into the curve line n the qtr profile? I have been using my scanner to read the %densities but want to see if mt densitometer is more accurate, am I being foolish to try this instead of just using the scanner? Thanks for any tips anyone could offer!
Erik
2017-02-17 by Simone Simoncini
I have an 810 too. You can read the values and put them in the GRAY_CURVE= parameter.
I find it quite useless though. I use the build gray curve tool to adjust the curve in a rough way
Then I use charthrob to refine the midtones.
Much more convenient!
Have fun!
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Il 17 feb 2017 11:37 PM, "
petware@... [QuadtoneRIP]" <
QuadtoneRIP@yahoogroups.com> ha scritto:
Hi folks,
I was wondering if there are any tutorials on how to use a densitometer to create correction curves for digital negatives. I am Mac based and have an xrite 810 to use. Are there conversion tables to convert my 810 readings to % readings. For instance, how do I know what 20% gray is supposed to be with my densitometer readings? Do I even have to convert the densitometer reading to % or can I enter the densitometer readings directly into the curve line n the qtr profile? I have been using my scanner to read the %densities but want to see if mt densitometer is more accurate, am I being foolish to try this instead of just using the scanner? Thanks for any tips anyone could offer!
Erik
2017-02-17 by peterjbh@...
I'm switching from a scanner to an 810 myself but it doesn't get here until the 24th. Are you using the stouffer 31 step strip?
2017-02-17 by petware@...
I was planning to use just a print out of a 21 step 0-100% wedge, but I do have a stouffer 21 step wedge that I could use as well which I guess would give me known values but I'm not sure how to convert density readings to % readings if that's required. Qtr is still all new to me so I'm still in my learning stage.
2017-02-18 by sanking@...
Erik,
Most QTR profiles allow you to create correction curves either with the GRAY_CURVE= command, or with LINEARIZE.
The easiest way by far that I have found to do this is to insert the log density readings of the printed step wedge directly into the LINEARIZE command. You can also do this with a spectrophotometer, but the log density values are easier to manipulate if there is some inconsistency in your process. LINEARIZE will not install if the density readings are not constantly increasing/decreasing.
Assuming that your prints are not neutral black (and most alternative work is not) it is best to use a
color reflection densitometer for the readings. I use an X-Rite 408,
which has CMYK and visual readings.
If you have been using GRAY_CURVE and change to LINEARIZE you will probably see a significant increase in the IQ of your digital negatives.
Sandy
2017-02-18 by petware@...
Thanks Sandy,
I want to try the linearize and my densitometer has visual as well as rgb channels to choose from. One thing I'm not fully grasping is how to get the known values for the densitometer readings in order to create the curve. In percent readings it's easy for me to have a say known 5% patch that reads as 12% with a scanner and eye dropper tool so I know how to adjust the curve but I can't wrap my head around how to figure out what a known densitometer reading should be so that when I take a reading of my contact wedge that needs to be adjusted I'll know how to proceed. Is there a conversion chart or formula that you use to get known density readings? I must be missing some detail to the process? Is it possible to explain how you arrive at your target density vs actual density figures.
Thanks
Erik
2017-02-18 by sanking@...
Erik,
It is very simple. You just set your densitometer to read density (not Dot Gain), and then set it to Density minus paper so that when you read the 0% patch of your step wedge the density reads 0.00. Density is measured in log (logarithmn) units. If your densitometer does not have a Densithy - Paper setting you will need to measure the base denstiy of the paper, and then subtract that for all of your readings. For LINEARITY to work, Step 0% needs to be entered as 0.00.
Below is an actual profile for one of my carbon tissues that shows how to enter the density values in the profile. Once these are entered you simply save the profile, then run Install. The process takes less than two minutes to complete. Way, way, way easier and faster, and more accurate, then creating a curve with a scanner. Believe me, I know from experience.
If you don't understand log standard I suggest you review it as it applies to photographic sensitometry. Basically, one stop is equal to log 0.30, each step of a 21 step wedge is 1/2 stop, or log 0.15. Density, as used in sensitometry, is the common log of opacity, which in turn is the reciprocal of transmittance (ability of a material to allow light to pass). Opacity of course is the ability of the material to block light. For example, at 50% transmitance the opacity is 2, and the denisty is log 0.3.
But you don't need to understand any of that, just set the densitometer to read density, then set it to read Denisty minus paper white, and place the readings directly into the LINEARIZE command.
Sandy
#PRINTER=QuadR4880
#CURVE_NAME= Carbon100 #Based on Carbon Tissue #238, 7-8 grams/liter of Cal Tint II Lampblack with tissue coated to a wet height of 0.9mm.
#Profile when printing a step wedge gives a DR (density range) of approximately log 2.20.
#With Tissue #238 sensitize with a 4% potassium dichromate solution, diluted 1+1 with acetone, approximately 10 ml per 8X10 sheet, applied in two coatings with a roller. 650 ULF units.
GRAPH_CURVE=YES
N_OF_INKS=8
DEFAULT_INK_LIMIT=100
LIMIT_K=26
LIMIT_C=1
LIMIT_M=1
LIMIT_Y=31
LIMIT_LC=1
LIMIT_LM=1
LIMIT_LK=44
LIMIT_LLK=1
GRAY_HIGHLIGHT=1
GRAY_SHADOW=1
GRAY_GAMMA= 1
CURVE_K= "0;0 100;100"
COPY_CURVE_C=K
COPY_CURVE_M=K
COPY_CURVE_Y=K
COPY_CURVE_LC=K
COPY_CURVE_LM=K
COPY_CURVE_LK=K
COPY_CURVE_LLK=K
LINEARIZE=“0.00 0.03 .13 .23 .35 .43 .53 .65 .69 .76 .85 .92 1.00 1.08 1.14 1.24 1.32 1.40 1.46 1.54 1.60”
2017-02-18 by sanking@...
Erik,
BTW, you should probably set the densitometer to read Visual, that way it will read all of the colors and average the density. Unless you are making something like a cyanotype, in which case you probably should set it to cyan.
Sandy
2017-02-18 by richard@...
Use this: http://www.bwmastery.com/qtr-correction-curve-tool
2017-02-18 by petware@...
Ok, Sandy now I get it. Thanks! I was assuming I needed input output values like the gray curve needs. Indeed it does seem simple compared to what I was thinking needed to be done. Am I to assume that the linearize is added after an initial gray curve is added to the profile to fine tune the profile or do you use linearize instead of a gray curve? Do you use the 21 step wedge that is included with QTR for your readings which rads in 0-100% or do you get your readings from a stouffer 21 step wedge which is 1/2 step density or does it matter?
Thanks again!
Erik
Richard, I tried to open your link but it said the server stopped responding. I'll try again later, thanks!
2017-02-18 by sanking@...
Erik,
I use both the 21 step wedge in the iOne folder that is included with QTR, and also the 51 step wedge.
Also use the 51 step wedge in the iOne folder, but for alternative printing you must have great control of process to linearize with the 51 step wedge. I find it much easier to do this with the densitometer than with a spectro since it is easy to see the values and adjust them slightly to get good increasing/decreasing values.
I use the Stouffer 21 step wedge only for analog testing, for digital work it is better to print negatives from the 21 or 51 step digital wedges where the steps are at increment of 5% or 2%.
I have used Richard's tool kit for smoothing multi-gray value QTR profiles for digital negatives and it worked great. Have not gotten around to using the spread sheet he suggests here for curves, but plan to.
One point that should be mentioned. The GRAY-CURVE= command applies to gray values, whereas LINEARIZE impacts all of the values. This may or may not be relevant to your work, but it should be mentioned.
Sandy
2017-02-18 by sanking@...
"Am I to assume that the linearize is added after an initial gray curve
is added to the profile to fine tune the profile or do you use linearize
instead of a gray curve?"
Erik
Sorry, failed to address this in my previous message.
I use LINEARIZE as the primary method of equalizing input/output, but I also use GRAY_CURVE= for specific features.
For example, I may use LINEARIZE for a specific process, like carbon, and GRAY_CURVE = for specific tissue formulas, etc. You can use one over the other, which can be very useful.
Sandy
2017-02-18 by petware@...
Many thanks Sandy, I believe I have a better understanding now. Your help is greatly appreciated!
2017-02-18 by petware@...
Richard, that's a very useful spreadsheet thanks! One question please, do you zero out your densitometer on paper white before taking the 100% ink reading or do you use the actual reading. Just a clarification as Sandy zeroes out for his linearize process and I want to make sure for a gray curve if I should of not. Thanks
Erik
2017-02-18 by richard@...
I use the actual density readings without zeroing out on paper white. You can get the majority of the correction done with the gray curve and then do a second linearization for different papers or slightly different chemistry mixes with the LINEARIZE= line. (Also using the exact density non-zeroed readings)
2017-02-18 by petware@...
Thanks Richard, I'll give your method a try. That spreadsheet is very helpful
2017-02-19 by sanking@...
Richard,
Would you explain the rationale for not zeroing to paper base the density readings?
The targets for the iOne have a BEG and END zone that seems designed to do this, and I believe that I get much better correction curves when I do so, both with the spectro and with a densitometer.
Sandy
2017-02-19 by Keith Schreiber
Hi Sandy,
I'm sure Richard can explain this better, but here is my practical experience.
It's the math. Gray_Curve values are given in percent. Richard's digital neg curve tool converts from Density to Lightness (L*), goes through some other fancy stuff, probably averaging to smooth any kinks out of the readings, and calculates the string of input;output pairs to paste into the Gray_Curve. Using L* rather than DEN provides visual linearity. That alone makes the whole process much easier. Sure wish I had known that years ago.
I've been using PiezoDN myself for going on a year now, but when I make profiles for friends I use Richard's spreadsheet to make the Gray Curve for the first step, and the QTR-Linearize-Quad for the second step. On a couple of occasions I've needed to tweak one of the linearize numbers manually.
With PiezoDN, there's probably even fancier math going on in the hidden part of the spreadsheet tools. There are no standard QTR text files. We begin with Master quad files that come with the download. QRT-Linearize-Quad is used with 129-step targets to perform the linearizations from L* readings. Two iterations is usually all it takes.
I too was so used to zeroing my old X-rite 400 densitometer on the paper base that when I switched to taking LAB readings (or Density for that matter) with my SpyderPrint my instinct was to figure out how to zero it on the paper base - you can't, but it doesn't matter. The zero function on my densitometer actually stopped working a few years ago, but I found I could get Density measurements from the SpyderPrint, I just couldn't zero it on paper - only on the calibration base. Eventually I got used to it. Your i1 and its software may work differently though.
Cheers,
Keith
Keith Schreiber
jkschreiber.com
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> On Feb 18, 2017, at 7:21 PM, sanking@... [QuadtoneRIP] <QuadtoneRIP@yahoogroups.com> wrote:
>
> Richard,
>
> Would you explain the rationale for not zeroing to paper base the density readings?
>
> The targets for the iOne have a BEG and END zone that seems designed to do this, and I believe that I get much better correction curves when I do so, both with the spectro and with a densitometer.
>
> Sandy
>
>