I've been asked for some help on how to darken the sky in an image without leaving a telltale halo in the area between the sky and whatever the scene might be. It's actually a reasonably easy fix. If you compare today's finished image (the larger image on the left) with the original (smaller in the middle) you can see that the sky in the end image has more detail and is considerably darker than what we started with. BTW: Clicking on any of the images (including the Layer Panel image) will provide you with a larger view of what we're talking about. There's actually three sections to the correction of this image and it's just about typical of what could be done to any shot. Section one would be to get the shot setup to work on. It's the first four lines of the panel (starting from the bottom). The second is making the color saturation adjustments to "develop" the image from the starting RAW file. The third is finishing the with sharpening and adding a vignette. To follow the sequence, hit the "read more".
There is one thing that doesn't show up in the Layers Panel. That would be making the mask for the sky. The mask has to be quite detailed, as the tips of the trees present a fairly serious problem. The easiest method of making the mask is to go to the Channels Panel and find the channel with the best contrast. In the case of this image, that would be the blue channel. Make a copy of the blue channel by dragging the blue channel to the "make new channel" icon at the bottom of the panel. Go to Image/Adjustment/Levels (you can't use an Adjustments Layer for this step) and carefully move the sliders to get the best blacks in the foreground (the darker areas) and the maximum whites in the sky. Once you've made the best alpha channel you can, go back and work on the image itself. We'll use the alpha channel later.
Next, use a Threshold Adjustment Layer and it's sliders to find the deepest black, clearest white and optimum grey. This has been gone through in detail on the June 10, 2009 post. This takes care of the first four steps (from the bottom) of the Layers Panel.
The third step is to do whatever color saturation changes you feel "develop" the digital negative to it's full potential. That involves using a Hue/Saturation Adjustment Layer for each available color (red, yellow, green, cyan, blue, and magenta). In today's image the only Adjustment Layers necessary were red, yellow and green. Magenta added nothing and cyan and blue created halos at the tree tops. Both would have increased the "blueness" of the sky, but we're not looking for blue. The sky, in this image, needs to be grey, not blue.
Going on, rather than flattening the image, I prefer to created a merged copy using the Crtl, Atl, Shift, E shortcut to put a composite layer atop the layers stack. Then copy the layer (Ctrl J) and change the Blend Mode of the copy to Multiple. This will darken the entire image. Here's where we go back to that alpha channel we started with. Hold the Crtl key and click on the icon for the alpha channel. This makes the channel a selection. Release the key and the click (the selection will stay) and click on the Layer Mask icon at the bottom of the Layers Panel. This step will put a layer mask with the proper selection already on the mask. You should immediately see (in the case of today's image) trees and grass pop back to a normal density and the sky remain darkened. There won't be any fringing in the tops of the trees.
To finish the image another merged copy (Crtl, Alt, Shift, E) was made and a copy of the composite layer also made (Ctrl J). Turn off the visibility (the eyeball) of the upper most layer. With the layer one step down from the top selected go to Filter/Convert for Smart Filters and make the layer a Smart Object. (to be as flexible as possible I believe any filter should be applied to a Smart Object). With the Layer convert to a Smart Object, select Filter/Other/High Pass. This will give a grey image with sharp outlines. I have settled on a value of 10.5 as my "normal" setting. Change the Blend Mode to Overlay and you have a highly sharpened image (much sharper that can be achieved using USM). If halos appear along edges, try Soft Light rather than Overlay. If the image is too sharp, use the Opacity slider to reduce the effect of the High Pass filter.
Last step. Turn the visibility of the top layer back on. Select the square Marquee tool (M). Set the feather to a level about 1.25 times your image resolution (if the resolution is 150 PPI, then about 200 PPI should be good). Pull out a rectangle, leaving equal distance from each side (left, right, top and bottom). Hitting the "Delete" key will punch a hole in the image. Change the Blend Mode to Multiple and adjust the vignette using the Opacity slider. Done.
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