Fritz:

It sounds to me like you're trying to create a posterization - it's similar to the screen printing technique Andy Warhol used for his soup cans and Marilyn Monroe prints he did during the sixties. A posterization is a one-tone image without grey or mid-tones. The reason for this is it is difficult to print a small halftone dot in most screen printing processes. It can be done - it's just pretty expensive. The cost of that really fine mesh can be prohibitive.

Do the levels adjustment previously suggested, then change the mode to greyscale then choose Image>Adjust>Posterize. Use 2 or 3 levels. Color separations could be created by making new spot color channels of the selected tones expanded a couple of pixels to allow for trapping where color meets color. (this depends on the resolution of your original)

I teach screen printing and graphics software at a school, so don't hesitate to ask if you have any problems.

Al

On 9/28/04 1:56 PM, in article
, "fritz"
wrote:

Thanks Odysseus, Tacit, that is some very helpful information. I did not know that you can use halftones for screen printing. I will use your suggestions and post a follow up to let you know how it went.
Thanks again,
fritz

Odysseus wrote in message
news:...

In article ,
(fritz) wrote:

Hi, I am trying to use Adobe Photoshop to convert a color .jpg to a black and white, one-tone image for the purposes of screen printing. In other words, there can be no shades of color, only black and white.

As Tacit suggested, you should talk to the screen-printer about halftoning. But the procedure is basically: convert to greyscale, adjust levels (increasing the gamma usually helps distinguish
greys-that-used-to-be-colour from each other) or curves to taste; then convert to a bitmap.

There are quite a few options for this latter step, if required -- that is, if the printer won't accept a greyscale file -- the simplest is just to use the 50% threshold, but you may not like the results ... a little experimentation with levels may let you find a good point for the threshold (studying the histogram can help). You can also convert to a halftone in the Bitmap dialog; in the absence of instructions from the silkscreeners I'd use a 65-lpi elliptical dot rendered at 1200 ppi (at least). The nicest-looking results will come from dithering, but they can be very hard to reproduce; part of the trick involves choosing a resolution low enough that the tiny speckly dots will hold up through the remainder of the printing process, but high enough not to look pixelated.

Fritz.Swartzbaugh

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Converting a photo to an image suitable for screen printing