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Halftone Screens

Elliptical, Round and Square dot shapes

continued from page 1
Dot Shape
There are three halftone dot shapes generally used today. The most common is the Elliptical Dot that gives smoother midtones especially when printed on a Litho Offset machine. By joining the dots below 50% on one axis only and then above 50% on the other axis, the change in this important tonal area appears less harsh than is otherwise experienced with the Square Dot.

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.

!00% - 0%

Halftone Dot Range
The maximum screen range possible is of course 100% - no dots in the whites of the picture, to solid ink in the darkest areas.

However this range is never used for most imaging because:
• The transition from 0% to the smallest dots can appear too harsh.
• There is always dot gain applied to the dots when printed.

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.

Normal image

Brighter Three Quarter Tones

Brighter Quarter Tones

Highlights, Shadows & Midtones
Highlight
is the correct name for the lightest ares of an image (and copy). Shadow is the name for the darkest image areas, and the Midtones areas are in between. Also there will be reference to the Quarter Tones and Three Quarter Tones that appear: 1. between highlights and midtones and, 2. between midtones and shadows.

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.

Brightness and Contrast samples

Brightness and Contrast
The top panel of this image has a normal midtone (would output 50%), but flat End Points. A small 'o' defines the 50% point and the Halftone Dot Range would be 85% to 15% = 70%.

The second and third panels have been altered in Brightness resulting in:
1. a lighter image overall and,
2. a darker image overall.
• The Tonal Range remains the same.
• The 50% point has moved accordingly.
• The Dot Range would be 70% - 0% and 100% - 30%.

The fourth and fifth panels have been altered by Contrast resulting in:
1. a reduced tonal range (flatter) and,
2. an extended tonal range (more Contrast).
• The brightness has not changed.
• The 50% point remains the same as the original.
• The Dot Range would be 70% - 30% and 100% - 0%.

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)

In Brief!
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).

Highlights - normal picture 2% - 3%, dark picture with catch lights (dusk or night scene) 0%.
Shadows - normal picture 92% - 96%, dark picture - consider reducing the Midtones 5% - 10% if there is a lot of important shadow detail.

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')

7 For Photocopier Artwork, try 65# to 70#.
A GOOD EXERCISE: Print out several images of varying tonal range with some flat Screened Tints (normal end points plus include a few images that are flatter at either / both ends). Try these on different photocopiers and assess them. The results can be quite amazing (read disappointing) and will quickly help you learn about tone control for output if you then make adjustments to each image, referring to the copier results, and reprint and assess them again on the same copiers to see if you have improved them. Because copiers lose Highlights and bung! in Shadows, learning to control images for output this way can make tones and detail jump out that you didn't think existed (make sure you only use around 60# - 70# screen ruling). This is an excellent exercise for students.

Normal image

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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