Making 'Lith' negatives for photopolymer etc.

In another post I wrote about using photopolymer material to make printing plates.  Unlike inkjet printers, the letterpress method uses just one colour of ink to make the image. The printing block image therefore needs to be ‘1-bit’ = that is pure black or white.
Traditional printmaking uses Lithography (or ‘lith’) film which is super-contrast. Properly processed it’s either totally clear or dense black, with no mid tones.  Lith or ‘line’ films are still available and used to make images from larger or smaller artwork by photographing them in a copy camera. These huge beasts (sometimes the size of a room) were made so very large negatives could be made optically.  Most printers make their negs digitally now.

Part of the interest in this process is making photographic images which look different from the usual darkroom or inkjet prints. Reducing a picture to just black and white changes it radically, and some judgement is needed to get images which work well. 

A great example is right here on my coffee mug:

It’s a lith rendition of the famous Alberto Korda photo of Che Guevara. The original was shot on 35mm and continuous tone -i.e. with shades of grey:

This graphic rendition is relatively easy. Everything lighter than a middle grey becomes white, everything darker than the mid point goes black. Just moving the levels sliders around will achieve something pretty similar if you're lucky but there’s a better and more controllable way.

Workflow in Photoshop.

Choose a bold, graphic image that you think will work well and be recognisable at small size since the first test plates at least are likely to be small. Here, as so often(!) I’ve chosen a picture of Algernon.

Open the image in Photoshop and convert it to black and white. Use Image>Black and White to do this as you can tweak the colour conversion to control how the contrast goes. Here I’ve boosted the yellows but reduced the reds so the difference between his lighter and darker stripes stands out more.

tweaking the colour responses in the B&W converter to get good contrast.

You can also selectively lighten or darken areas of the image to help them go towards black or white as desired. Here I’ve used the burn tool to deepen the shadow under his chin and his pink nose, (cute though it is) needs to be darkened so it goes black not white.

Now choose Image > Adjustments > Threshold

The image goes full ‘all or nothing’ black and white. The slider controls the threshold value- the point where the tones are split. By default it’s in the middle of the range but pulling it left or right allows fine tweaking of how the image reads. 

Threshold level 103.  Too light...

Moved up to 128.  Much better.

- but further up to 149 is too dark.

The key is to find the best point to give a recognisable image, in this case a good stripy face and not a black or white blob.  If you can't get the detail you need everywhere, cancel Threshold, go back to the continuous tone image and dodge or burn the problem areas to bring them in range. 

Now all that remains is to Invert (cmd-I) the tones to make a negative and print out onto inkjet film. I use a pretty crude laser printer and cheap photocopier acetate as it gives a good dense black.

I'm not Algy - I'm his evil twin...

Note that you DON'T need to flip the image as it will be exposed 'emulsion-to-emulsion' and this will automatically produce the reversed printing plate you need.

Another thing I've found useful is to have a thick black border around the final negative when you print it. - Just choose Image>Canvas size and increase both width and height by 10mm or so.  This means there will be no unwanted edge lines near the image which would need trimming off before printing.

Dot screens for half -tones

It’s possible to get more tones (shades of grey) with letterpress by using dot screens. The image is still only made of black ink and white paper but a fine pattern of dots gives the impression of more intermediate tones. Here's a plate for a newspaper photograph I have in my collection:

Newspaper photo printing block. The original is about 5in x 7in

It's a metal plate mounted on a wood block (see the article on letterpress printing for discussion of the depth of the block). Close examination of the surface shows it's got a dot screen:

At a normal viewing distance, the dots are almost invisible but the effect is of more continuous tones. This method is used in more or less complex forms in most commercial printing. 

Cheap, soft paper like newsprint absorbs a lot of ink so a coarse screen is needed: around 85 lines per inch is common and the screen isn't too hard to see.  High-end art books may use high quality coated stock which allows a much finer (200 lpi or more) screen which cannot be seen with the naked eye. This is, of course similar to the way inkjet print resolutions work: posher papers can exploit finer detail while 'plain' paper cannot resolve as many dots per inch.

Personally, I don't like the dotscreen 'look' and prefer to use other processes (like the darkroom!) for mid tones in prints. However, if you do, here's how to do it in Photoshop:

Open your image and if it's not already black and white, convert it:

You'll need to boost the contrast quite a bit higher than you would normally. Something like this:

Next choose Filter>Pixelate> Colour Halftone.

The Max. Radius number will control the coarseness of the screen. Small numbers give a fine screen, larger ones make the dots bigger and more obvious. Here it's set to a fine 5 radius.
Set all the channels to 45 degrees for the classic dot screen effect (or vary it if you like of course):

For comparison here are some different dot radii. - Not easy to see on a web page  so I've exaggerated the effect by using a wide range of numbers.

Dot Radius = 5

Dot Radius = 20

Dot Radius = 30

On the finer screens, the dot pattern is still there and effectively made from  pure black circles of different sizes. Here's a closeup of the 5 radius:

Detail:  Dot Radius = 5

The image on your computer screen is of course made up of another grid of dots- the screen pixels so print out test samples on paper to see just how it looks. Once you're happy, don't forget to invert the tones to make a neg and load up the transparency material..