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The Round Dot also suffers from a fairly harsh transition as the dots get larger and start to touch, but this happens at a higher percentage than the square dots 50% point, and tends to reduce the visual effect slightly because it is in the darker areas of the image.
The method of printing, the plate type and product surface can all influence the preferred dot shape. i.e. Screen Printing, Flexo Printing, on plastic, tin, cardboard etc..
Your clients printer should always be consulted when dealing with special products but otherwise, stipulating elliptical dots will do for general print jobs.
this range is never used for most imaging because:
Therefore we have to make allowances when preparing our images for output. Think of an 80% dot that spreads to 90% on the printing press. Then the 90% dot would increase to 100% and all of the shadow detail that can make or break a good result would be lost. Suggestions will be made after looking at the other halftone dot definitions.
Shadows & Midtones
Referring to the images above, the top image is a normal full range picture that would print 50% in the right hand end panel.
The second image has been reduced in the midtones (Gamma adjustment) giving it less contrast in the quarter tones and more contrast in the three quarter tones. i.e. the picture detail in the darker areas will stand out more and the picture detail in the lighter areas will show less detail.
The third image has been increased in the midtones (Gamma adjustment) giving it more contrast in the quarter tones and less contrast in the three quarter tones. i.e. the picture detail in the lighter areas will stand out more and the picture detail in the darker areas will show less detail.
Therefore, image areas can be more contrasty, or become flatter.
second and third panels have been altered in Brightness
fourth and fifth panels have been altered by Contrast
With this understanding we can assess a scanned image and adjust it with the correct tools knowing how we will affect, or not affect, the all important Shadows, Highlights and Midtones. (The Flatbed Scanning pages include graphics on the use of these controls)
1 Unless perfect, always scan, or rescan, a greyscale image a little flatter at both end points and then maximise (often called 'Equalise') the image. Then adjust the midtones (Gamma) if needed before altering the end points to the following: (assuming standard good to best quality printing and stock - paper).
normal picture 2% - 3%, dark picture with catch lights
(dusk or night scene) 0%.
2 Unless very experienced, organise trade scans for CMYK work. Get contract proofs of them also (remember to include these costs in your quotes). If you want to try adjusting CMYK scans, NEVER maximise or 'Equalise' the channels, or adjust the Screen Dot ranges unless you really know what you are doing. Rarely are the highlight and especially the shadows the same value in each colour. Don't adjust by the monitor, it won't be any where near accurate for colour, learn to 'use the numbers' only (particularly in pastels etc, just a couple of percent out in only one channel (of cmyk) and the colour will change).
3Use the Elliptical Dots for most jobs. CMYK - consult the printer.
4 When scanning images that are very flat or faded (i.e. old photographs) and you want them to appear that way, obtain a 'Greyscale Step-wedge' from a photographic store or supplier. Scan the wedge with the photo and apply Note 1 above, but referring to the wedge as a perfect original. This should give you a close similarity to the faded original. A well protected Step Wedge can supply you with good 'Neutral Greys' for adjusting this important aspect of a flat bed scanned colour picture. Not all flatbeds are efficient in this respect.
5 The Industry Standard for good offset printing is 150#, quality Screen Printing uses 110# to 133# and Web Offset on good magazine stock uses 133#, and 85# for Newsprint.
6 Never adjust the Screen Angle values (CMYK) if optioned in a DTP program. (see 'Image Output and Grey Levels')
Photocopier Artwork, try 65# to 70#.
8 When adjusting 'Gamma' on any device or image, we are changing the midtones. If an image is full ranged, i.e. 0 to 255 in RGB terms, then the end points will not change, only the midtones. But if the end points values are not at maximum then keep in mind they will change slightly with the Gamma alterations. This is why a normal greyscale image should be maximised first, Gamma adjusted then set for white-point / black-point print output values.
Note: The image above can be used to alter the Brightness and Contrast controls of a monitor if you cannot visually define the three darkest steps.
9 When creating Vignettes (pronounced vin-nets) or Fountain fills that range to 0% and/or 100%, always set the value to 1% or 99% instead. Imagesetter output has a bad habit of producing an abrupt step to 0% and 100% and this small change seems to help - and don't forget Dot Gain on poor stock.
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