Post by PeterW on Jan 11, 2007 16:23:05 GMT -5
Hi Sid,
Hi,
Yes, I worked for a magazine publishing company for best part of 16 years, and though I was a journalist I visited our printers about once a month and often chatted with the darkroom people there. Like most technical journalists I’m naturally nosey about how other people do things. This, of course, was in the days before PS, Quark and computer page make-up.
As I recall, the unsharp masking technique in a wet darkroom wasn’t to use a positive with a negative, it was to use the original negative clamped in register with a slightly blurred copy negative known as the mask. Hence unsharp mask. The printer had to be very skilled and experienced to judge how much to blur the copy neg. With three-colour printing this had to be done three times, once for each colour, and three transparencies of exactly the same size made so the printer could sandwich these in register using a light box and a loupe for the final colour print.
It was a technique used only on very high quality work, usually very expensive limited edition art prints. It required highly skilled and experienced people, and on cheap prints the lack of skill often showed as colour fringing. It would have been completely pointless, as well as horribly time consuming, on a weekly or monthly publication before the days of web offset because letterpress needed a fairly coarse screen for the pictures which would have made sharpening pointless.
Then Photoshop followed relatively soon after web offset, and a wet darkroom and a blockmaker’s shop at the printers weren’t necessary because the printers worked from prints made in the publisher’s darkroom – until high-pixel digital cameras came along, then no-one needed a darkroom for publishing.
Does PS do the same thing? OK, here we go again. Yes, it does. Let’s see how much I remember from my PS tutorials.
When you click on Unsharp Mask PS makes a copy of each of the three colour channels, applies Gaussian blur to them and holds them in RAM. Then it compares these with unblurred copies of each channel, giving a ‘brightness’ number to each corresponding pixel in the blurred and unblurred copies of each channel. Then it subtracts one number from the other, and the higher the result the nearer a pixel is to an edge where the contrast changes.
Unsharp Mask has three sliders. If you move the Amount slider to the right the dark pixels get darker and the light pixels get lighter till eventually you get a horrible posterised effect with no mid tones at all.
The Radius slider controls how much Gaussian blur is applied to the blurred copies, and the Threshold slider controls how much difference there has to be between each pixel after subtraction of the ‘brightness’ numbers before any lightening or darkening happens.
When you click on OK, the blurred and unblurred copies of each channel are combined. And then incorporated in the colour image you see on the screen.
So basically, PS uses the same process as unsharp masking in a wet darkroom, but the wonder of PS and a computer is that they do all this in milliseconds. If you click the preview button you can see the result almost instantaneously.
OK, ‘nuff said. I’ve already waffled on for too long.
PeterW
Didn't I read in one of your postings that you had experience from publishing? I thought perhaps you might have been acquainted with the original (analogue) unsharp mask, exposing through a sandwich of negative and positive images to increase edge contrast on the printing plates.
Hi,
Yes, I worked for a magazine publishing company for best part of 16 years, and though I was a journalist I visited our printers about once a month and often chatted with the darkroom people there. Like most technical journalists I’m naturally nosey about how other people do things. This, of course, was in the days before PS, Quark and computer page make-up.
As I recall, the unsharp masking technique in a wet darkroom wasn’t to use a positive with a negative, it was to use the original negative clamped in register with a slightly blurred copy negative known as the mask. Hence unsharp mask. The printer had to be very skilled and experienced to judge how much to blur the copy neg. With three-colour printing this had to be done three times, once for each colour, and three transparencies of exactly the same size made so the printer could sandwich these in register using a light box and a loupe for the final colour print.
It was a technique used only on very high quality work, usually very expensive limited edition art prints. It required highly skilled and experienced people, and on cheap prints the lack of skill often showed as colour fringing. It would have been completely pointless, as well as horribly time consuming, on a weekly or monthly publication before the days of web offset because letterpress needed a fairly coarse screen for the pictures which would have made sharpening pointless.
Then Photoshop followed relatively soon after web offset, and a wet darkroom and a blockmaker’s shop at the printers weren’t necessary because the printers worked from prints made in the publisher’s darkroom – until high-pixel digital cameras came along, then no-one needed a darkroom for publishing.
Does PS do the same thing? OK, here we go again. Yes, it does. Let’s see how much I remember from my PS tutorials.
When you click on Unsharp Mask PS makes a copy of each of the three colour channels, applies Gaussian blur to them and holds them in RAM. Then it compares these with unblurred copies of each channel, giving a ‘brightness’ number to each corresponding pixel in the blurred and unblurred copies of each channel. Then it subtracts one number from the other, and the higher the result the nearer a pixel is to an edge where the contrast changes.
Unsharp Mask has three sliders. If you move the Amount slider to the right the dark pixels get darker and the light pixels get lighter till eventually you get a horrible posterised effect with no mid tones at all.
The Radius slider controls how much Gaussian blur is applied to the blurred copies, and the Threshold slider controls how much difference there has to be between each pixel after subtraction of the ‘brightness’ numbers before any lightening or darkening happens.
When you click on OK, the blurred and unblurred copies of each channel are combined. And then incorporated in the colour image you see on the screen.
So basically, PS uses the same process as unsharp masking in a wet darkroom, but the wonder of PS and a computer is that they do all this in milliseconds. If you click the preview button you can see the result almost instantaneously.
OK, ‘nuff said. I’ve already waffled on for too long.
PeterW
