i was looking at one of my digital images and sharpening it. i have a sailboat mast (several actually) that are protruding up through a tree line into a cloudy sky. i was working at trying to minimize the white halo around the mast and suddenly the question came to me.

is this halo effect only an artifact of digital? what about when you sharpen in a dark room with film?

Sharpening is inherent to digital. It is not required or done with film.

Michael

The name unsharp mask came from the film sharpening process. It's quite tricky to do and I never mastered it in the darkroom.

From Wikipedia;

"In the photographic process, a large-format glass plate negative is contact-copied onto a low contrast film or plate to create a positive. However, the positive copy is made with the copy material in contact with the back of the original, rather than emulsion-to-emulsion, so it is blurred. After processing this blurred positive is replaced in contact with the back of the original negative. When light is passed through both negative and in-register positive (in an enlarger for example), the positive partially cancels some of the information in the negative."

http://en.wikipedia.org/wiki/Unsharp_masking

Mike,

Your reply is, of course, correct. But the point is that all digital images require some amount of sharpening, and preferably a sharpening workflow as espoused by Bruce Frazer. Few if any negatives or transparencies need sharpening in the chemical darkroom.

Michael

I think that the unsharp mask for analog images was used mainly for the preparation of half tone images for the printing press. The halftone process resulted in some loss of sharpness, which was corrected by the sharpening process. For printing with an enlarger, sharpening is not needed.

Bill

To expand on this point:

The primary purpose of sharpening (unsharp mask) is to compensate for the diffusion that occurs as various papers absorb ink in the printing process. Printing on newsprint requires more sharpening than is needed when printing on a glossy stock. In this regard, it is needed just as much for film as for digital.

Of course some digital cameras don't produce totally sharp images out of camera and thus benefit from some sharpening. But the same thing could have been done to slightly blurred film images if you have the technology. (Now you can scan the film and sharpen it.)

Anytime you are talking about papers absorbing ink, you are usually dealing with a halftone process, and the relative need for sharpening according to ink spread or the effects of the halftone process are intimately linked. As mentioned previously, sharpening is not ordinarily used when printing film with an enlarger onto photographic paper.

If you scan film, then sharpening is needed to compensate for loss of sharpness during the scanning process, even if the original negative was perfectly sharp.

It is useful to separate sharpening into two phases: capture and output. With scanned film and digital captures, the capture sharpening compensates for loss of sharpness introduced during capture. For output, differing amounts of sharpening are needed according to the media and resolution. Also, more sharpening is needed for halftone than contone output.

Of course, if the negative is not in focus or there is blurring from motion, one could apply an unsharp mask when printing on photographic paper with an enlarger. Or one could scan the negative and sharpen it with an unsharp mask. However, in the latter case the use of a deconvolution method of sharpening may give better results, especially in the case of motion blur.

Bill

Most of digital unsharpness is from the anti-alias filter which of course is absent on film.

This is more pronounced on certain cameras and can lead to varying opinions about IQ if one doesn't learn how to handle this.

HTH

Warren Allen

thanks. now i remember my brother's father in law talking about the printing aspect of this. he used to do printing/graphics for a paper company and that's what he was talking about, the fact that the ink would be absorbed and spread out - it's just like inkjet printing for that matter. i knew unsharp mask came from somewhere. funny how it's evolved.

For many years I used unsharp masking in the darkroom for both color transparency printing and B/W negative printing. For color printing from transparency film is is very useful for reducing the contrast of images to fit the reversal paper steep curve (e.g. Ilfochrome/Cibachrome). In my experience with landscape images this technique was essential for a majority of my images.

For B/W printing unsharp masks are very useful for improving print sharpness. If a B/W negative prints well on a medium grade 2 darkroom paper, and then is sandwiched with an diffuse unsharp mask of appropriate density it can be printed on a grade 3 or more paper. While retaining the same global contrast, the edge contrast is significantly increased and sharpness is noticeably improved. Howard Bond wrote about this technique in Photo Techniques magazine years ago. I use(d) it all the time with great success.

Yes, halos can be a problem with this darkroom technique. On some B/W images I spent a long time using chemical bleach mixtures to mitigate halos. Making unsharp masks and using them is not very difficult. If you work with medium or large format film alignment is not difficult either.

This is probably more than anyone cares to read. Just one more thought, I still have a darkroom but now I scan my medium format film and B/W negatives and use Photoshop. The amount of control with digital unsharp masking, local and global contrast control, curves, cloning, etc is a tremendous advancement in my opinion, but at least some of us have fond memories of the hands-on sensory experience of producing prints in the darkroom too.

i have another question - if you were printing in the dark room, and exposed the paper to the image in focus, then de-focused and exposed the paper again, what would happen?

if you expose the paper longer, it gets darker and shorter it stays more white correct? so is what i'm saying above like a blend mode of soft light or something?

This is how I understand unsharp masks are used for film, although I never went that far myself. While dabbling with Ilfochrome printing (ages ago), one printer did demonstrate to me how he used unsharp masks to control contrast. It made an astounding difference. But creating pin registered masks was way beyond my reach at that time.



Your response did indeed bring back fond memories of seeing prints appearing in total darkness, probably the only thing I miss. I agree that sharpening is one of many advantages of digital imaging.

You also touched upon an important fact about sharpening, which is basically achieved by increasing contrast. Once an experienced traditional darkroom printer asked me to explain digital sharpening to her. I tried to simplify it by saying that it is "nothing more than increasing contrast". She looked at me in disbelief, perhaps expecting some "magic" about digital.

I think people are confusing contrast masking (which also is not a sharp mask) with "unsharp masking" which is used to increase edge contrast.

At one time I had a custom color printing business so I am pretty familiar with the various masking techniques.

The goal of a contrast mask is to add density to the highlights and light mid-tones to reduce the overall contrast and bring it in line of what the paper can handle. A contrast mask from a large format transparency is simply a piece of b/w film that is contact printed from the transparency usually with a piece of matte acetate between them to blur the mask. The b/w film is then partially developed to the required density. It is then sandwiched with the transparency for printing.

A more advanced method is to first make a separate high contrast blurred mask (on litho film) to preserve the highlights and sandwich it with the transparency when making the contrast mask.

At one time there was a company that sold special light sensitive glass for quick contrast masking. This was similar to the lenses of "Transition" sunglasses. You simply placed the transparency on the glass and exposed it to light. It would darken a bit in the necessary places and then you'd print with the two combined. So it was easy to keep them in registration even with small format film. I think you could then clear it with heat. I never actually used this because I had pretty much stopped printing by the time it was introduced. I have no idea how well it worked in practice.

Another option was the Logetronic enlarger that was commonly used in printing aerial photos.

Thanks for the clarification. I stand corrected for misusing the term "unsharp mask" in my last post.