Photoshop: Smarter sharpening with the High Pass filter

In previous posts I have introduced the basics of sharpening in Photoshop and I also looked at a way to spot sharpen an image in Photoshop Elements using faux layer masks. In this post I want to show you the benefits of using high pass sharpening in Photoshop as an alternative to using the Unsharp mask (click to enlarge some images in this post).

The Unsharp mask has historically been the sharpening tool that most Photoshop users start out using. There are, however, different and better tools to use and one of these is the High Pass filter. One reason for this is that the Unsharp mask operates on the actual image pixels and it makes changes to those pixels. The sharpening process that makes use of the High Pass filter doesn’t operate on the original image so it does not destroy the image pixels – this is particularly useful if you’re using Photoshop CS2 or earlier which doesn’t offer the Smart Objects for Filters option for applying the Unsharp mask filter.

In addition, instead of sharpening the entire image as the Unsharp Mask does, using this High Pass filter process limits sharpening to the edges in the image which is where the most value can be obtained from sharpening the image.

Step 1

To see the process at work, open an image and duplicate the background layer of the image. If your image has multiple layers, add a new layer at the top of the layer stack, click it to select it and press Ctrl + Alt + Shift + E (Command + Option + Shift + E on the Mac) to fill the top layer with a flattened version of the image – without affecting the other layers.

Step 2

In the Layers palette set the blend mode of the new top layer to Overlay. This will let you see the sharpening results in place on the image in the next step.

Step 3

With the topmost layer still selected, choose Filter > Other > High Pass. This filter has one slider to adjust the Radius value. Drag the slider until you see a pretty much gray image in the preview window with the only detail being around the edges of objects in the image. If you can see color in the preview image then the radius is set too high. Typically a Radius value of well under 10 pixels should be sufficient – we used 2. Click Ok.

Step 4

The image is now sharpened – check the original against the sharpened version by clicking the Layer Visibility Icon for the top layer on and off to compare the result.

To finish the effect, adjust the Opacity of the top layer to 0 and then move it back up stopping when you have a good sharpening result. The ideal Opacity will depend on your personal preference.

If desired you can use blend modes other than Overlay, for example Soft Light and Hard Light can be equally as effective depending on the result that you are looking for.

Tip

When you are sharpening an image, adjust the image to the way you want it to look if you’re planning to display the image on the web. If you’re printing it you can (and should), sharpen more aggressively.

If you are using Photoshop CS3 or CS4, before you apply the High Pass filter to the top layer of the image, convert it to a Smart Object by selecting the layer and choose Filter > Convert for Smart Filters. Then apply the High Pass filter to the new smart object in the same way as I have outlined above. When you apply a filter to a Smart Object you can return later on to edit it – simply double click the filter name in the layer palette and the filter dialog opens allowing you to change the Radius value.

Tags: Photoshop, Sharpening

Post from: Digital Photography School - Photography Tips.

Photoshop: Smarter sharpening with the High Pass filter

How to Sharpen Photos: An Introduction

We’ve received quite a bit of feedback from Post production readers asking us to look at the issue of sharpening photos. Sharpening is one of those everyday tasks that most photos can benefit from. In this post I’ll explain what sharpening is, when you should perform it and how to do it. The information here, although it is explained using Photoshop, is relevant to all photo editing programs.

Please click images to enlarge in this tutorial - it will illustrate much better the points being made with images at full size.

Sharpening does as its name suggests and sharpens the image making it look crisper and making the edges in the image more distinct.

In the darkroom the process is achieved by taking one negative and a slightly blurred positive image, sandwiching these together and making a very quick exposure of this sandwich. Then the exposure is completed using the negative. The resulting image has sharper and crisper edges than it would have had if the blurry (unsharp) mask image had not been used. The typical sharpening tool used in Photoshop and other graphics programs is named after this traditional darkroom process and is called the Unsharp mask.

In a graphics editor the Unsharp mask works by creating small halos along the edges in the photo. These halos enhance the contrast between the edges and the surrounding pixels making the edges look more obvious and giving the image a crisper and sharper look.

Here’s how to sharpen an image using the Unsharp mask:

Step 1

Sharpening should be done at the end of the editing process so finish doing all your edits to the image before you sharpen it.

Now create a flattened version of the image either by flattening or merging all the layers or press Ctrl + Alt + Shift + E (Command + Option + Shift + E on the Mac) to create a flattened layer at the top of the image. The Unsharp mask works only on the current layer so you need to have the image on a single layer for it to do its work.

Step 2

Choose Filter > Sharpen > Unsharp Mask. Set the Radius to somewhere between .5 and 1. This sets the width of the halos which are applied along the edges in the image – the smaller the radius, the smaller the halo and 0.5 – 1 is ideal – this is not always a situation where the more is better!

Set the Threshold to around 10. The Threshold value determines how edges are found – the higher the value, the more different adjacent pixels must be to be considered an edge so less of the image will be sharpened. A small value means that smaller differences in pixel values are considered an edge so more of the image is sharpened. The risk with a small Threshold value is that it can add noise to the image by enhancing edges in places where you don’t want to see them.

The Amount setting controls how much contrast is added to the edges – a higher value means more contrast and a more obvious sharpening. Start by setting this value to around 150.

Step 3

Take a look at your image and adjust the sliders from this starting point until you see more detail in the edges in the image but not so much that you see unattractive halos around the edges.

Typically, if you have an image with a lot of very fine detail you can use a very small radius value (so the halos are small) and a correspondingly high Amount value (so that the halos can be seen to sharpen the image). On the other hand, if you have an image without a lot of fine detail can use a larger radius say, 1 – 1.5 or more (which gives larger halos), and a smaller Amount setting because the halos will be bigger and more visible anyway.

Adjust the Threshold value so you get sharpening in the areas you are interested in being crisper but not so that it results in unwanted noise in the image.

This image is nicely sharpened – you can see the crisper edges.

This image is over sharpened – notice the unsightly halos around the edges.

Tips

It is generally advisable to view the image at 100% when you are sharpening it so you can see the effect on the image. You can do this by sizing the image to 100% before launching the Unsharp mask tool. Alternately, use the 100% preview in the Unsharp mask dialog –click on the preview to see the unaltered image so you can compare it with the preview..

When you are sharpening for printing you can generally sharpen more heavily than you should do for onscreen viewing.

There are other sharpening tools available in Photoshop CS2 and later which do an even better job of sharpening than the Unsharp mask. I’ll look at these tools in a future post. For now, regardless of which graphics editor you use, you should have an Unsharp mask tool and it should work in a similar way to the Photoshop Unsharp mask shown here.

Tags: Photoshop, Sharpening