ISO: which dot shape?

[Dario]

Hi all,
please do you know which dot shape is recommended by ISO standards for offset printing (12647-2) ?
...and WHY?

Thanks in advance.

 

[Dario]

Ok, so...

here are the 'which':
4.2.5 Dot shape and its relationship to tone value (film or printing forme)
Circular, square or elliptical half-tone dot shapes shall be used. For half-tone dots with a principal axis, the first link-up shall occur no lower than at 40 % tone value and the second link-up no higher than at 60 % tone value.

...and here is the 'why' (almost):
http://the-print-guide.blogspot.it/2009/02/am-screening-halftone-dot-shapes.html

 

[gordo]

Ok, so...

here are the 'which':
4.2.5 Dot shape and its relationship to tone value (film or printing forme)
Circular, square or elliptical half-tone dot shapes shall be used. For half-tone dots with a principal axis, the first link-up shall occur no lower than at 40 % tone value and the second link-up no higher than at 60 % tone value.

...and here is the 'why' (almost):
http://the-print-guide.blogspot.it/2009/02/am-screening-halftone-dot-shapes.html

Let me unpack that.

"Circular, square or elliptical half-tone dot shapes shall be used." By "Circular" they mean Euclidean - round/square/round. This term for dot shape is not consistently used so it's good to get clarification when discussing to avoid misunderstanding.
"For half-tone dots with a principal axis," i.e. elliptical

Note they don't include FM screening in their trinity. Possibly because when ISO was originally published FM was not well adopted and ISO was based on a film to plate workflow which made FM more difficult to implement than AM. Also it's high frequency (lpi) meant that its dot gain was out of their linear thinking specifications. While dot shape has an impact on printability, artifacts, etc. it has very little impact on color. (Frequency/lpi/size of dot does have an impact on color)

"...and here is the 'why' (almost)"

What do you mean by "(almost)"

 

[Dario]

I meant I don't know if ISO meant the same YOU meant.

Do you (know)?

 

[Macmann]

darioluca - The Print Guide is Gordo's. He is an outstanding resource.

 

[Dario]

I meant: they = ISO

 

[gordo]

I meant: they = ISO

I think I've lost the thread of your post. The "specification" you quoted was from ISO 12647-2. That's what I was referring to. What you read on my blog about the pros and cons of different dot shapes comes various sources including logical inference and testing that I participated in when I was involved with halftone screening development.
I am not a member of ISO if that's your concern.

 

[Dario]

I'm sorry, it's my fault: these are probably the limits of my English!
I firstly meant: does anyone know why ISO chose only those dot shapes?
Then I said 'almost' because I didn't know if Gordo's pros'n'cons were the same for ISO.
That's all. Sorry for bothering.

 

[gordo]

I'm sorry, it's my fault: these are probably the limits of my English!
I firstly meant: does anyone know why ISO chose only those dot shapes?
Then I said 'almost' because I didn't know if Gordo's pros'n'cons were the same for ISO.
That's all. Sorry for bothering.

Those dot shapes are the basic and standard. My pros and cons apply to the ISO dots.

 

[Dario]

Thank you all