Sharpening with the Unsharp Mask filter
The unsharp mask filter in Photoshop can be used to save certain blurred pictures. Sharpening a photo in Photoshop will not save any shot that is very blurred, nor will it save photos where the point of focus is in the wrong place, but it may help with a picture that is a little bit 'soft' due to the use of a slower than ideal shutter speed.
In Photoshop, under the filters menu, there is a submenu called 'Sharpen' where you will find several choices.
The bottom two choices, 'Unsharp Mask' and 'Smart Sharpen' open boxes full of faders that you can twiddle.
The other choices are presets that will give you a quick fix in a hurry but do not have any adjustments, so we will ignore them.
At this point I have to confess that I am still using Photoshop CS2 so there might be new filters in CS3 that I am not aware of, in fact I have only just discovered the 'Smart Sharpen' option while preparing for this article.
I will leave the 'Smart Sharpen' option until later and start with my tried and tested favorite the 'Unsharp Mask'.
There are three sliders in the 'Unsharp Mask' window, the top one 'Amount' does what it says on the can, it adjusts the amount of sharpening.
The 'Radius' slider adjusts the width of the halos that are created. Basically the way sharpening works is that it finds the edges in your picture by looking for areas of high contrast and increases the apparent sharpness by adding dark and light halos. If you look at the edge of the apple in the picture on the left you can see a dark line, rather like a pencil line along the edge.
The 'Threshold' slider allows you to specify how much contrast difference there has to be before that area is sharpened. Set this fader too low and the picture will appear grainy as there will be a sharpening halo around almost every pixel. Set the value too high and only clearly defined edges will be sharpened which can make the blurred bits look even more blurred.
This filter needs to be used very carefully to achieve the best effect. Too much sharpening can make the picture look false. On the other hand, if you look for every single little tell-tale detail and every little line, you end up not using the filter at all.
How Much Sharpening? What Settings to Use?
The final adjustment of the sliders is down to you and will vary from picture to picture, but here are some guidelines to get you started. The amount of sharpening you can get away with, especially the 'Radius' settings, depend on the resolution of the image. If it is an image destined for viewing on the screen the resolution of the image should be 72 pixels per inch. (some say 96ppi) If the image is destined for print the resolution should be 300ppi or as near as you can get it to that.
If you are planning to change the size of an image, by this I mean the file size, make sure you do the sharpening after the sizing, especially with JPEGS. Sharpening should be the last thing you do in Photoshop.
For an image that is 72ppi you should use a radius setting of about 0.4 or 0.5 pixels. Use less than this and you won't see much happening, use more and the halos will start to look too obvious. For a print image, 300ppi, start with a setting between 1 pixel and 1.7 pixels.
I prefer to use as small a 'Radius' setting as possible and push the 'Sharpen' slider quite high.
I read somewhere a while ago that the 'Threshold' slider should be set at about 3 levels. But I find that I use it anywhere between 0 and 20 or so. This is one you have to play with yourself as it really does depend on the type of picture.
Here's a picture ripe for sharpening, although I used a tripod it was taken at a very slow shutter speed in available light. The depth of field is not great so we are never going to get the whole photo in focus but at least the middle apple should be sharp. I have made the pictures nice and large so you can see the effects better.
As we are using this on the web, the radius setting should be about 0.4 pixels. So set that first and then set the 'Threshold' slider to zero for now. Move the 'Sharpen' slider from side to side to see what it will do. Normally you need to move it past 80 to see anything happening at all. If you get all the way to 500 and the picture is still not sharp enough then you need to set the 'Radius' higher.
Below you can see the settings I chose and the result. Overall it is probably the best compromise. There is a white halo around the stalk which is a bit more obvious than I would like and the black line between the apples is a bit bold but, as I said before, if you look for every bit of evidence you would end up not using the filter at all. The low 'Threshold' setting has sharpened up the skin of the apple which is where we really wanted to see the improvement. The stalk of the middle apple is now as sharp as you could want.
Sharpness always used to be the ultimate goal in my pictures but nowadays I can see that you can sometimes have too much sharpness and soft focus has it's place too.
Here's a version that has been sharpened way too much, just so you can see all the bad things to watch out for. You can clearly see the black and white halos around all the edges. This is what we need to minimize. So, use the sharpening filter with great care, better still, get the focus right in the camera.
Finally a quick look below at the 'Smart Sharpen' filter. This has many more knobs and dials to play with and so is a bit confusing at first. The left tab is much the same as we saw before but we now have a drop down menu with three different types of blur to choose from. 'Gaussian Blur' is what we have been fixing with the other filter so 'Lens Blur' in this case or 'Motion Blur' when appropriate would be a better choice for photographers. The shadow and highlight tabs give you a chance to minimize the halos and, as you can see from the preview, they can be greatly reduced. The halo around the stalk is gone and the black line between the two apples is almost gone.
This filter then, used carefully, can give you a more subtle effect. Look at all three pictures carefully, the second picture is obviously sharper but with a few tell-tale signs of sharpening, the bottom picture is more subtle but does not appear quite so sharp.
In the end, the choice is up to you. You will probably find that you over sharpen everything at first, so make sure you work on a copy file and keep the original safely out of harm's way. I now keep all original files straight from the camera and only ever work on copies. That way, as my skills improve, I can go back to the original and try a new edit.
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