Sharpening: Unsharp Mask vs High Pass Filter

October 7, 2009

One thing that all photographers really strive for are sharp photos – crisp edges that take your breath way. Obviously things like tripods and fast shutter speeds help give you sharp photos, but sometimes we all need a little help getting a sharper photo after-the-fact.

The most popular way of sharpening photos in Gimp is probably the Unsharp Mask, but today I’m going to show you a different option using this High Pass Filter – an option that’s built into Photoshop.

I don’t want to go into too much technical detail on the difference betwen the two (if you’re interested, there’s a great article here) – but the short of it is: the Unsharp Mask will look for lines of contrast differences in your photo and make the dark side lighter and the light side darker. The High Pass Filter will turn most of your image gray, only producing visible lines around what it determines to be the edges in your photo. You can then use a layer blend mode to make those edges sharper.

Let’s see them both in practice, with this image I took of the Bellagio Ceiling when I went to Vegas last year:

Bellagio ceiling

It’s not an amazing image, but maybe with a bit of sharpening we can make it better.

First let’s try to Unsharp Mask, which is found at Filters –> Enhance –> Unsharp Mask. You have three options:

  • Radius – Remember how I said that the Unsharp Mask looks at the edges in your photos and turns the darker side lighter and the lighter side darker? The Radius determines how wide of a space that darker and lighter part will be. The higher the number, the wider the space.
  • Amount – This is how much darker/lighter the areas around the edges will get – the higher the number, the darker the lights will get and the lighter the darks will get.
  • Threshold – This is basically how sensitive the mask is to edges – the lower the number, the more things it will think are edges. Using a higher number will leave more subtle edges untouched and only sharpen more pronounced edges

For some reason, and I can’t remember now why I got stuck on these numbers, I’ve always started off with the values 2, .85, and 4 respectively:

Unsharp Mask Options

Let’s compare a before and after of just one part of the photo to see the effect it had. The top image is before the Mask was applied, the second after:


After Unsharp Mask

While you can see a slight haloing effect, it’s not too bad. I could play with the settings more (perhaps decrease the Radius and Amount), but it’s hard to see the haloing in the photo as a whole (at least at the size I’ll post online), so I’d say this is good enough. Here’s the final image with the Unsharp Mask:

With the Unsharp Mask

Now let’s take a look at using the High Pass Filter. After you install the Plug-in, the filter will show up under Filters –> Generic –> High Pass Filter.

Here’s the default options for this filter:

High Pass Filter

Also, this is where you’ll see the biggest downside between this High Pass Filter and Photoshops – no preview. This is annoying because it’s hard to figure out the radius you want to use without being able to see the final product, so we’re going to have to do some guessing and checking here.

We’re going to stick with the Mode of Colour, because that closely duplicates the High Pass Filter in Photoshop… and quite honestly, it’s the only one I’ve used, and it’s worked, so I’m going to stick to it!

The goal at this point is to get our image so it’s all gray and only the edges stand out. Let’s see what our image looks like with the default settings:

High Pass with Default Settings

Uhhh, well. We got the all gray part down, but the edges don’t stand out too much yet. I hit undo and did some more guessing and checking until I got the radius to 25 and now it looks like this:

Radius set to 25

And now we’re done!

No. I’m kidding.

But we’re close! Next, I change the blend mode of the High Pass layer to overlay:

Set Blend Mode to Overlay

Let’s compare the one part of the photo again. Once again, unedited on top, using the High Pass Filter below:


After High Pass Filter

And here’s the final product:

After High Pass Filter

Not too shabby! I think in the case of this photo, the High Pass Filter technique worked better – but I’m not confident that it always will. I don’t have enough experience yet to really know which one will work better when, and I’ve read about professionals that use both and use Layer Masks to hide one and display the other just in certain parts of the photo, but honestly? I can’t be bothered with that! This isn’t how I make my money, and my professionalism isn’t on the line, or do I have clients breathing down my neck. I’m just a woman typing away in Notepad right now, and uploading my images to my flickr account. No offense to those of you who follow me on flickr, but I can’t be bothered with such techniques at the moment. So from now on, I’ll try one or the other and just leave it at that!

By the way, if you try using the High Pass Filter technique, you can play around with the Opacity of the layer and also try switching it to Hard Light (for more of an effect) or Soft Less (for less).

Stay tuned later this week, when I show you how to use the High Pass Filter for the opposite effect – smoothing skin!

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