Quickie post – for a cyanotype curve (to create a digital negative);
I start with a worked out monochrome image which has been tonally corrected – thus;
Then I go into the curves dialogue in photoshop – this screen shot is gimp, but the technique is identical in either program – in the curves I select the freehand curve style and draw a curve that crosses the normal curve, this inverts the image.
You’ll notice that I started it part way down – this allows me to reduce the tonal range down to only a fraction of what it was – this is due to the fact that cyanotype is very high contrast and thus will return the dynamic range of the flat negative.
In the curves dialog you can save a curve, thus making it easier to do subsequent images – It’s easiest after freehand drawing the curve across to switch back to the smooth option to tweak it with points.
You still have to do a bunch of test prints to get the curve exactly right, and you do need slightly different curves for different sorts of images (high key, low key, etc).
I use transparency film in an inkjet printer to print it out in an appropriate coloured ink (the one with the best resulting tonal range), but the negative method should transfer to any decent laser printer/copier with the caveat that you’ll be able to see where the toner dots were in the final print.