It's the stereotypical image of a press operator - bent over the presswork and examining it under a loupe. People attending a press approval often wonder what the press operator is examining so closely. In fact there's a great deal of information that the press operator can determine from a close up view of the press work.
The first thing that's checked is registration.
On the left, dot centered rosettes indicate that the presswork is out of register by one half row of dots. On the right, clear centered rosettes shows the presswork is in register.
Misregistration can also be determined by examining the edge of images to see if one of the process colors is extending beyond the image edge. Click on the above image to enlarge.
Next is halftone dot formation.
Halftone dots can reveal issues such as dot doubling that may be the result of loose or incorrect blanket packing.
Here speckles of ink indicate there may be a problem with the plate processor. Also, in this case, the black printer is, because of the elongation of the dots, showing a problem with slurring.
The amount and type of halftone dot distortion can reveal problems with ink water balance as well as the condition of the fountain solution.
When halftone dots are the same size and should be the same approximate density as in the case of Cyan and Magenta - but aren't - this can signal a problem with the solid ink density or an ink/water imbalance.
Pinholes forming inside halftone dots can signal a calcium carbonate issue.