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Thread: Elliptical vs. Round vs. Square dot shape?

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    tmiller_iluvprinting's Avatar
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    Question Elliptical vs. Round vs. Square dot shape?

    We print most of our 4 color work at 200LS with an elliptical dot shape. I wasn't involved in this decision, and we have been printing like this for at least 10 years. I have been asking why we chose to do this, and of course, most of the responses have been , 'we've always done it like that'. The reading that I have done has explained that elliptical is used mostly for screen printing and other print processes with heavy dot gain. We have problems matching proofs that are made using the official GRACol press data which I believe was run at 175 LS, using a square or round dot shape. The problem that we have when trying to match the GRACol proof is that the 1/4 tones on the press sheet are heavier than the proof. My thought is this may be because of our 200LS and elliptical dot shape. Any thoughts on this? Thank you.
    Best regards,
    Todd

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    Hi Todd,

    This doesn't answer the question "why" but could you apply a curve in the 1/4 tones to lighten them up?

    Not sure if you've seen Gordo's blogspot on dot shape: The Print Guide: AM Screening Halftone Dot Shapes
    Doesn't sound like it would effect 1/4 tones.

    Greg
    Premedia Software Inc.

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    I found that we switched to elliptical when we installed our film based image setter years ago on the advice from someone that elliptical would lessen visible rosette and lessen dot gain. If we print 200LS I would think that the visible rosette between dot shapes would be splitting hairs. We continued with elliptical when we went to CTP because of legacy work. After reading Gordo's blog(which is an excellent reference-thank you Gordo, thanks Greg for the direction!) the dot shape may the root of some of our issues.
    Best regards,
    Todd

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    Maybe a different question, what is the best dot shape for printing 200LS, sheetfed, offset? I have a feeling our use of elliptical is making our job harder.
    Best regards,
    Todd

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    Quote Originally Posted by tmiller_iluvprinting View Post
    Maybe a different question, what is the best dot shape for printing 200LS, sheetfed, offset? I have a feeling our use of elliptical is making our job harder.
    Best regards,
    Todd
    Round dot (a.k.a. non-transforming round dot i.e. non-Euclidean).

    BTW it's the dot shape that was used for most (if not all) of the GRACoL 7 characterization press runs.

    best, gordo
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    Quote Originally Posted by tmiller_iluvprinting View Post
    I found that we switched to elliptical when we installed our film based image setter years ago on the advice from someone that elliptical would lessen visible rosette and lessen dot gain. If we print 200LS I would think that the visible rosette between dot shapes would be splitting hairs. We continued with elliptical when we went to CTP because of legacy work. [snip]
    The advice you were given was wrong and/or incomplete.

    Elliptical dots don't lessen the visibility of rosettes nor do they lessen dot gain (which can easily be compensated for with a curve applied to the plate). What the person was probably referring to when they said "dot gain" is the optical bump that occurs when dots start to touch. With a simple Euclidean dot (round/square/round) that bump occurse at the 50% tone and in press work can appear like a dark line at the 50% tone that might appear to be dot gain.

    Elliptical dots split that optical bump into two - half at about 40% when the dots first link up in one direction (first axis) and the other half at about 60% when the second axis of the dots start to touch. This moderates the appearance of the optical bump.

    But the bumps are still in the important tone areas. And the directional nature of the dots in an elliptical screen can cause other problems.

    Round dots bury the optical bump in the shadows at the 75% tone so it's not visible.

    best, gordo

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    Quote Originally Posted by tmiller_iluvprinting View Post
    [snip] The reading that I have done has explained that elliptical is used mostly for screen printing and other print processes with heavy dot gain.
    That makes no sense. One counters dot gain with a compensating tone curve - the dot shape has no effect - unless they are referring to the optical bump that happens when dots first touch (see my other post).

    We have problems matching proofs that are made using the official GRACol press data which I believe was run at 175 LS, using a square or round dot shape.
    175 lpi round dot. Square dots are very rarely used anymore.

    The problem that we have when trying to match the GRACol proof is that the 1/4 tones on the press sheet are heavier than the proof. My thought is this may be because of our 200LS and elliptical dot shape.
    Can't you fix the 1/4 tones with a tone reproduction curve applied to the plate?

    best, gordo

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    Quote Originally Posted by gordo View Post
    Can't you fix the 1/4 tones with a tone reproduction curve applied to the plate?
    Thanks Gordo for the information. I am using Curve2 Software to create our press curves and haven't done any adjusting of the curves coming out of Curve2(which I believe is needed to match a proof made with the GRACol data.) My predecessor always wanted to adjust the proof, but in doing so we always moved further away from the GRACol standard, which makes sense. I have always contended that the difference in line screen may be the reason we do not match the GRACol proof(and that plate curve adjustment is needed), and that the use of the elliptical dot shape is making our job harder. Thanks again for your insight, always much appreciated.
    Best regards,
    Todd

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    I really can't see how the use of the elliptical dot shape is making your job harder. The difference in line screen (200 lpi vs 175 lpi) would result in a very slight difference in dot gain in your 1/4 tones. Have you checked the optical brightening agent content difference between your press paper and your proofing paper? That can make a difference in the appearance in you 1/4 tones.

    best, gordo

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    Round is Sound, Elliptical is Cryptical ( that is what we used to say, but I have no idea as to why we said that )

    I am with Gordon on the above.
    Michael Jahn - Slightly used PDF Evangelist
    Simi Valley California


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