ORIGINALLY POSTED BY ALLY FOR THETAZZONE/TAZFORUM HERE
The High Pass Filter in Photoshop seems to have been around forever, and you can find it located under Filter>Other>High Pass. Think of it as one of The Others and listen to the Twlight Zone music start playing ……
First of all lets take a few seconds to see how the sharpening process works in image editing software. All sharpening methods are edge detection processes, wherever an edge is found it is exaggerated by making the darks, darker and the lights, lighter. The trick is to get this just right so as not to produce haloing effects which are the result of over sharpening.
Another thing to consider when sharpening, is not just sharpening but perhaps throwing what you don’t want sharp out of focus. Say for instance that you want to use a radius of 10 on an image, why not use a radius of 5 to sharpen and gaussian blur/lens blur with a radius of 5 to throw the rest of the image out of focus. sharpening isn’t always about sharpening.
Then we can simply selectively sharpen something in our image, a person, an animal, eyes etc., to draw more attention to that particular part of an image and add more impact.
What we’re going to look at today is sharpening with the High Pass Filter. There are a couple of other sharpening processes available in PS, the USM (Unsharp Mask) has been around for quite a while now, and the new Smart Sharpen Filter appeared in CS2 and is terrific. I used to have an old technique for adjusting motion blur with the Emboss Filter, which is yet another way to sharpen in PS. But lets concentrate on one filter at a time.
Sharpening is usually the very last thing that you do to an image, so go ahead an do what needs to be done with your chosen image, flatten your image, and then duplicate the layer by dragging it to the new layer icon at the bottom of the layers palette, or press ctrl J on the keyboard, cmd J for the mac. If you are working with CS3 you can turn the duplicate layer into a smart filter, which will then make the high pass filter editable.
This is my start image, a horrible bird shot
So you’ve got your duplicate layer, make sure that it is selected by clicking on it then I want you to change the blend mode to overlay (ignore the fact that the image will look awful at this stage), I also want you to make sure that the image is at 100% and go to Filter>Other>High Pass. An easy way to figure out the radius you will need to use to sharpen is the actual size of your image. My bird shot is less than 1m, but the first image will show that I used a radius of 1. If you have a 10m image, set the radius to 10. You can adjust this with the opacity slider later if you feel that it is a little too much. 5m file use 5, 20m file use 20 etc.,
What you are looking for is to have a similar look in your preview window as I have in the above image. The next image shows you what you don’t want the preview window to look like. You’ll notice I’ve used a radius of 5.
That is the basis of High Pass sharpening really. You can adjust the effect by taking the opacity slider down in the layer palette or if you feel that it isn’t enough you can trash the layer and start again, or you can go to Filter and at the very top of that box will be the name of your last filter, in this case High Pass, just click on that or press ctrl + F to repeat and then use the opacity slider to take the effect down. What you don’t want to see appearing on the image, is haloing. This is where the image starts to get a glow going on … which is a result of over sharpening as seen in the image below.
If you want some selective sharpening rather than the entire image to be sharp – go ahead and do the sharpening with High Pass then add a layer mask to that layer by clicking on the mask icon in the layers palette. Make the mask active by clicking on it, then press ctrl i on your keyboard to invert it (should be black). Grab your brush and make sure the f/ground colour is to white, then simply paint back in the area/s that you want to be sharp. In the image below I’ve sharpened the birds head, although I’m not sure that it will be very noticeable with this size image.
You can also try the blend mode at soft light or leave it at normal, just play around a little bit because different blend modes will produce different effects and they might just be useful for the image you are working on.
If you want to know what your image will look like on the web (computer monitors) view your image at 100%, if you want to get an idea of what it will look like printed, view it at 50%.