CHROMiX

Dot Gain - Basic Questions

Basic but somewhat confusing question for everyone in the know out there… If a press is supposed to have an ideal 20% dot gain given running to SID (gracol etc.) which can be read with a spectrophotometer, meaning a 50% patch imaged on a linear plate should read at 70% on the sheet. Alternatively, does this mean that a 5% patch should read 6% (5x20%=6) and a 25% patch should read 30 (25*20%=30) and 70 should read 84 and 90 would read %108%?

Is this sounding correct?

Nope. Can’t really print more than 100%, so with this formula, your shadows would be plugged to 100% around 83%. Further, apply this formula at the 50% patch (50 x 20%) add that value back to 50% and you get only 10% gain at 50%. Moreover, this isn’t uniform 20% gain throughout the tone scale. A common misconception about dotgain can be traced to the use of 20% gain at 50%. A more accurate statement might be a 20 point increase to the 50% patch…leaving the potential math out of it.

Given the variables of paper, ink, screening (dot shape), ect., I don’t think a mathematic forumula will cut it when attempting to estimate dotgain. FM screening has vastly different gain characteristics than conventional screening for example. A lack of a fully defined dot gain curve is a major shorting of printing standards, though ISO 12647-2 does take it further. Gracol’s G7 approach of defining a neutral print density curve goes a long way in clearing this up, despite printing variables.

mike

For commercial offset on a decent paper, the gain in this situation would probably be more like 14-16%. 20% dot gain would be more of a legacy gain from the films days (before CTP) where dot gain on top of 50% began with the film/plate exposure. I think most printers find it necessary to add a slight bump to the 50% patch imaged on the plate in order to hit 20% gain on the printed sheet, so the 50% patch on the plate measures 56%.

mike

THis is from an earlier thread but I have done some more testing and think I am looking great. I still have a little concern about the 8% dot and below ut I am not experiencing dot blow out.

  1. Calibrate my Delta to out put lineriar plates
  2. Entered a process curve to meet my goal dot gain
    a. Black 18%
    b. Magenta 14%
    c. Yellow 14%
    d. Cyan 16%
  3. Output test plates using this process
  4. Pressman ran press up to consistant densities that work best on her press.
    a. Black 1.70
    b. Magenta 1.35
    c. Yellow 1.25
    d. Cyan 1.35
  5. From this run I take dot gain reading from stock.
  6. I take these numbers back to my Delta in enter them in to a measured curve that will bring my gain down to my goal gains listed in # 2.
  7. My initial testing matched the proof well.

I also use Superior inks the final dot gain result was as follows fter all the testing

K = 19% +/- 2%
C = 18% +/- 2%
M = 12% +/- 2%
K = 12% +/- 2%

It was fun and interesting. I know I did not do the graycol7 exactly but I dont have some of the equipment to read lab and do some of the more indepth testing. My goal was to decide on a standard and expectation of printing. I choose the graycol because it gave me some goals. I may not be printing a Lab neutral gray but visually it looks neutral and gray.

good luck

I have a Heidelberg Delta 7 workflow. If anyone is interested in my simple approach to the graycol

Just wondering how you came up with these numbers to be your target dot gains and what do you use for a cmyk profile?

We’ve recently implemented G7 and it seems to be working well. We do have a way to measure Lab though because previously we were using a custom press profile. We only have one press that we print color on. I did find that to be the closest match I’ve had press to proof but as things changed at the press I was constantly tweaking the curves to match whatever gains they were getting at the time, we were having some press issues and had it worked on 5-6 times over the last 2 years. Anyway after testing the custom press profile some I found that it wasn’t as smooth as the “official” profiles (swop or g7) so what we’ve done is to implement G7 but I did pull the G7 data into Profilemaker and created a profile for our use with MaxGCR. We were using MaxGCR before and it seemed to make more stable color on press, use less cmy inks. Also, to my way of thinking, and I could be totally wrong, if anyone can tell me yea or nay please do, but using high GCR with the G7 method seems like it would make it even easier to control gray balance as you’re taking a lot of the cmy colors out of the equation and replacing them with just k, especially in gray\neutral areas?

The one thing I miss about SWOP seps or custom press profile is we all knew exactly what the dot gains at press were supposed to be: 22-20-20-18 kcmy. Where with G7 curves it’s not so obvious if something is off to see if the gains are ok.

The biggest thing regardless of what standard you choose to use is that it will make your printing more consistent if you’re watching and controlling everything the best you can in prepress and at press. Consistency equals repeatability, giving you confidence that the jobs going to press will match the proof if nothing else happens along the way and if it does now you’re familiar with all the variables and know what to look for to be able to correct the problem.

Because of inconsistencies I’ve seen from our press I began reading each proof, film and plate going out of prepress. Sounds excessive? Maybe, but I’ve learned exactly what our variations are and know for certain that the plates I’m sending out are exactly like the last ones that went out and matched the proof really well, so if the next set doesn’t, it’s not the plates and they can start checking out things at the press instead of me having to prove to them first that its not the plates. More peace of mind for me and less troubleshooting what almost without exception was a press variable that had changed.

Just my…probably .10 cents worth.

Terry