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  1. #1
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    Default Yet another half tone question. re: photoshop and single point light source

    In reading about getting good half tone results, I see people disparaging the photoshop half tone filter.

    I assume that they are referring to the process that I have been using, which involves going into grey scale mode, then bit map mode, then choosing type of dot (ellipse, circle, etc), then choosing lpi

    Iíve spent a lot of time and endured a lot of frustration trying to work with half tones.

    There has been some success with 75 lpi with a 305 screen. But there have been a lot of failures as well.

    But if thereís a better method than photoshop, Iíd very much like to know what it is and how to use it.

    Also, I read about someone who said that his halftones really popped when he switched from a home made halogen setup to a better constructed exposure table that used multiple florescent tubes.

    The shop I work out of has had such a table for years, but I, having been told that a single point light source was very important to get good detail, have used a halogen, space bag/vacuum cleaner set up to have single point. This insistence has been the source of bemusement/amusement for my fellow shop users. And the spacebags tend to be troublesome after a few sues when it comes to sitting flat against the screen and positive.

    Any clarifications here would be welcome.

    Thanks,


    Jim

  2. #2
    gaffler's Avatar

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    Default

    the best way to do halftones is to draw them by hand on a sheet of cardstock and x-acto that crap.

    the halftone filter is different than what you are doing. (if you want to mess stuff up, check out the filter menu -> sketch -> halftone pattern.)
    Your photoshop method sounds pretty good to me; talk a little more about the nature of these failures and frustrations.

  3. #3
    squeegeethree's Avatar

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    Default

    Bitmap halftones aren't true halftones that's why they look bad to anyone who knows the difference.
    Download a halftone rip. There are several to choose from.
    You can download a free trial here and see the difference.
    Freehand Studio | AccuRIP

  4. #4
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    "There has been some success with 75 lpi with a 305 screen. But there have been a lot of failures as well"

    75 lpi is really high. It can be done, but it takes a lot of variables being exact. 35-55 lpi is much easier to expose and to hold the dots during printing. I generally use 55 lpi, round dots @ 22 degree angle. There's a ton threads about this. See: AndyMac and Richie Goodtimes.

  5. #5
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    Default

    The main issue is with moire patterns.

    this can be mitigated with the angle choice on the bitmap conversion; I finally came up with 19 degrees for 75 lpi on a Ryonet 305 screen, using a cross shaped dot. ( the only reason I chose the cross shaped dot was that it was the first halfway decent mostly non moired result I got after a lot (imagine the word lot in all caps here) of printing and testing.

    I also wonder if the angle chosen will vary for screens form different manufacturers, or even different rus from the same manufacturer.

    I really like the way the diffusion dither looks better, but I could never get a print without moire in it. My reading had led me to believe that if my ppi were less than 1/2 my screen mesh count, I should be able to succeed with dither, however I did not find that to be the case.

    Jim

  6. #6
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    patoakes's Avatar


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    If your mesh count divided by your lpi equals 4....you're asking for moire problems. If you reduce your mesh count to 230 or reduce your lpi, those problems will diminish. Angle, tension, and dot shape all factor into moire too, but lpi to mesh count is a big one.

  7. #7
    squeegeethree's Avatar

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    Default

    4.4 is what we divide by (inches but guess metric might be the same) and we don't have lpi moires. But patoakes is right, the bigger the dots the less of a moire.

  8. #8
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    Andymac's Avatar

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    it's all math. even a 65 line screen would be better.

    75x4 = 300 = virtually the same as 305 plus or minus a newton or two.

    re the dither, the same holds true. You never want a frequency of anything dividing into anything evenly.

    I would recomend figuring out what mesh you want to run, then make some squares of a range of halftones, burn them all, print them all, see what works best.

    It's better like they said, to have lower halftone count and have it all show crisp, than to have a higher count and have it splotchy or with moire.
    Andymac

    services www.squeegeeville.com
    equipment www.tmiscreenprinting.com

    Todo es empezar.

  9. #9
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    Default

    Thanks for the input.

    Any opinions on single point light source versus multi tube when it comes to detail and undercutting?

  10. #10
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    depends on the exposure unit. Vacuum is a must. good film is a must. high resolution /high solids emulsion is a must. coating is a must.

    then look at your exposure unit.
    Andymac

    services www.squeegeeville.com
    equipment www.tmiscreenprinting.com

    Todo es empezar.

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