I like negatives that are sometimes too contrasty or grainy. A number of years ago I saw a printer who was using a little bit of enlarger diffusion in the darkroom. When you're enlarging, remember, your bright areas become the dark areas and vice-versa - enlarger diffusion is when you put a soft-focus filter under your enlarging lens. So, the image that is cast on the printing paper is slightly flared at its brightest points, which are the parts of the scene that will be rendered black. This is a fun trick that gives a very mysterious and intense look as well as polishing away a bit of grain if there's too much in the image.
(an image printed on fine-art paper using enlarger diffusion)
When I do enlarger diffusion in the darkroom I hold a piece of anti-newton glass under my enlarging lens for 1/2 of the total exposure time of the enlargement. So if it's a 4 second exposure, I hold it there for 2 seconds. I usually dial in a bit of extra contrast (I use variable contrast paper) because I've found that flaring the blacks tends to soften the image a bit much, sometimes. Your mileage may vary.
What, to me, is so cool about photoshop is that not only can you create processes that have never existed in photography (e.g.: "healing brush") you can create duplicates of real-world processes using exactly the logic that works with real light.
So, to do enlarger diffusion in photoshop we want to create a softened "flared" overlay of our existing print. We want it to make our image blacker where there's more "exposure" in our "negative" - i.e.: the blacks. Then we want to expand the contrast a bit.
First step, we have an image that's "correctly exposed" (which, in photoshop land means we've already adjusted its levels to where we want them)
(click on any of the images for a full-sized version)
Then we take the image and make a duplicate layer that will be our "flare overlay" layer.
Here you can see in the "layers" window that there's a duplicate copy of the background layer. That's our "diffusion" layer. This is already a black-and-white image so we don't need to desaturate it. If you're working with a color image, you need to desaturate the "diffusion" layer. Then right-click on the "diffusion" layer in the "layers" window and bring up the "blending options" menu. Here I changed 2 things:
Now, let's make our "diffusion" layer actually diffuse! Go to Filters->Gaussian blur and pick an amount of blurring that looks right for the image. The amount you choose is going to depend on the size of the image - if the image is large, diffuse it a lot (see the thumbnail above) A great deal of blurring will make the whole image look mysterious and murky, whereas a small amount of blurring will produce a slight deepening of dark edges.
One thing that's nice is that the effect will preview on the image in the open window, so you can get an approximate idea of how the effect will work. What you're doing with this blurring is determining how far the blacks will "flare" into the rest of the image. Too much and it'll get murky. You can be very subtle with this technique and impart a kind of elfin silvery glow to the image, or you can just whack the heck out of it and go all gothic. Play around and see what works for you!
To complete our simulation of a real darkroom process, we adjust the contrast of the image. You'll notice that the histogram in "Levels" shows that the image is going to lose some of the tonal smoothness, but because the blacks flared out into the highlights, the peak values have been pulled down a little bit. I usually pull the lightness up a bit and then flatten and save the image!
And we're done!!
The effect can also be used on color images with varying results. It tends to make the image a bit darker and it seems to leach some of the color out of it. I can think of cases (winter scenes?) where that might be very very cool, though.
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