Re: Enlarger lens options.

On 5/10/2008 8:02 AM Richard Knoppow spake thus:

>     In making half-tone plates the iris is imaged by the 
> half tone screen as an array of spots or dots. By using a 
> square aperture the intestices of the dots are at the 
> corners so the variation is smoother. For color work each of 
> the images is photographed using an iris with a 
> lozenge-shaped aperture at a different angle. I can't 
> remember now if this is to prevent moir but I think it is. 

Yes, the screens are angled to prevent moiré.

By the way, the standard screen angles for 4-color (CMYK) printing are:

   K (black): 45°
   M (magenta): 75°
   Y (yellow): 90°
   C (cyan): 105°

So far as I know, these have been in use as long as 4-color halftone 
printing has been around.


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