… well, putting that title in Google gives me more than 90,000 hits, so it mustn’t be that secret. However, a wise use of the USM (Unsharp Mask’s nickname) can really help making most of your pictures.
Obviously, no amount of digital trickery will be able to bring a blurred photo back to sharpness. The principle at work with USM is to locally enhance edge contrast which makes an image look sharper.
So, say we have a dark gray edged with light gray area. The USM will enforce the dark area close to the limit and lighten the light one (if you sill follow), thus giving a contrast push and enhance the impression of sharpness. This is an over-the-top example to show the principle:
In the dark ages…
Or more precisely in the darkroom age, USM was achieved by getting a blurred positive (out of focus exposing of the negative on film slide) and combining it with the original negative for printing.
Because of this “blur” component, USM is called Unsharp even though its final goal is to make the image look sharper.
In the digital age…
Nowadays, the principle remains, but it is achieved digitally. So when you open your USM dialog in your favorite photo application, you get 3 parameters to play with:
- amount: how much of the unsharped image will be used in the final version.
- radius: what the radius of the blur should be.
- treshold: at what level an edge is considered an edge (and worth unsharping). This is mainly used to avoid sharpening noise in high iso images (and degrading the image quality with USM), but you could also use it to avoid sharpening skin defaults for example.
How to use it
It is not possible to define general work-everywhere settings, so get ready to experiment. Remember to look at your image at 100% zoom level when applying USM otherwise you won’t be able to really judge its effect.
Once applied (and image saved), the USM cannot be undone. Adding blur will not remove its effect. So generally speaking, a high quality image like the ones we are learning to produce should not be saved / archived with USM applied.
Actually (and just because things are always a bit more complicated), there are two different usages you with with USM: local contrast enhancement and micro details boost. The first one can be applied on your high quality image (like we did before), while the second one depends on the ouput format (web, print) so you don’t want to apply it on the high-quality image you archive.
Local Contrast Enhancement
In this case, you boost the general contrast of an image. You use a large radius (something like 30) and little value (from 0.10 to 0.50). The effect is quite radical and image saturation may need to be toned down a bit afterwards (10-20 points).
Another over-the-top example – but you can see that the contrast boost… The values used were radius:30, value:0.70 and treshold:0.
If you over-do this step (apart from bad taste), you will start seeing light halos in dark areas. The undo button is your friend in this case.
Micro Detail Enhancement
This is the part you don’t want to add to your high quality file. Typically, when you scale down an image for screen view, you start with your 2000 x 3000 (or more) pixel image to shrink it to 800×600 (or less). The result is a bit blurry and image lacks “pop” – which we can “retrieve” with USM. So this Micro Detail Enhancement USM part depends on what image you start with and the final destination of your image – we will discuss the screen version here, the print version will come later.
So after resizing you want to apply an USM with small radius value (0.1-1), amount has to be decided case by case (between 0.2 and 1 .0, maybe more) and treshold depending on the image too.
Be careful not to overdo it. In this case the image “hurts the eyes” – it is hard to describe but you see it when it happens. The problem is that the older the screen the more blurry the image: so what looks good on your old screen will scream “too much” on a more recent one. So in a much as the effects nicely boosts the image, better have a little less than a little too much.
So here is again my favorite autumn view strait after resize:
The same with an USM of 1.0 / 0.50 / 0 – what I consider a good version:
Finally with an USM of 1.0 / 1.0 / 0 – too much:
Final word: good taste and undo
Like with so many things, trial and error is the best way to learn – undo is your friend. Don’t hesitate to save several copies (or make several layers) with different USM settings and compare. Don’t hesitate to leave the picture unattended for 2 days and get back to it for a fresh look. Practice, practice and practice…