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Post-production: Sharpening your image using the High Pass filter

From our digital-only Photography Magazine, we look at how to sharpen images using the High Pass filter

Post-production: Sharpening your image using the High Pass filter
Girl on The Mandalay Express, Myanmar. Image: Tristan Bejawn

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A lens that renders the sharpest possible image can cost thousands, but when travelling, you’ll most likely be shooting on something versatile and light, and not necessarily super sharp. This can result in images that lack the distinct clarity capable of drawing in the eye and refusing to let go. It is possible, however, to correct these soft edges with a powerful and versatile sharpening tool, the High Pass filter.

Why the High Pass filter?

The High Pass filter is an edge-detection tool on Photoshop. It’s used to create a duplicate layer that only includes the edges of an image. When this layer blends into the original picture, it results in the dark side of the edges becoming darker and the light sides becoming lighter; this increases the contrast between the two edges, and our brain interprets this as sharpness. The Photoshop version we’ll use in this demonstration is CC, but the same method applies for earlier versions too.

The image

The photo I’m using in this example was taken in Myanmar. I was able to capture a portrait of a girl lost in thought while on a local train in Yangon. Though the eyes are in focus, they are a little soft, which takes the impact out of the image. I chose to use the High Pass filter to bring out that clarity — but only within the eyes. I used a tool called ‘layer masks’ to make sure that only the eyes were affected. This selective sharpening ensured I didn’t get any unwanted artifacts on surrounding areas of the image.

The process

In this demonstration, we’ll be creating a duplicate layer, applying our sharpening filter and then masking out everything we don’t want sharpened. We’ll use a separate layer to the original image, which means we can go in later and change the level of sharpness at any time during the edit. This is great practice for all types of adjustments, so it’s useful to get to grips with it.

Step 1 – Duplicate your background layer

Once you’ve opened the image you want to sharpen, create a duplicate layer by right clicking on the background layer and selecting ‘Duplicate Layer’ (CTRL + J or CMD + J). Rename the new layer ‘Sharpen’.

Step 2 – Apply the High Pass filter to the newly duplicated layer

With the ‘Sharpen’ layer selected, go to the top menu and select: ‘Filter > Other > High Pass’. This will bring up a dialogue box. The radius is important; we want to see enough of the edge to allow the sharpening to take place, but not too much that it ruins the image. For this example, I’ve used 1.6 pixels, as this gives a good amount of detail to the area that I want sharpened, which is specifically the eyes. Press ‘OK’ to apply the filter — your image will look totally grey; do not panic, this is normal!

Step 3 – Blend the new layer into the original image

With the ‘Sharpen’ layer still selected, go to the drop down menu above the layers panel that reads ‘Normal’, and select the blend mode ‘Overlay’ or ‘Hard Light’. You can play around with this until you find something that works best for you. Once blended, your image is effectively sharpened.

Step 4 – Create a layer mask to target the sharpening

As it stands, the whole image has been sharpened. To make our editing more exact, we’ll mask out the areas that we don’t want to be affected. To do this, create a layer mask by holding the ‘alt’ key and clicking on the ‘Add Layer Mask’ button at the bottom of the layers panel (pressingalt’ creates an inverted mask, which is what we need, meaning it’s filled in black rather than white as usual).

Next, select the layer mask by clicking on it, and use the brush tool (selected by pressing the ‘B’ key) to paint on the areas where we want to bring back the sharpening. You’ll need to paint in white — layer masks work such 100% white areas create 100% opacity, and areas in 100% black create 0% opacity, meaning that the black area is ‘masked’ and none of the sharpening layer will be visible. Note that there must be a white border on the black frame of the layer mask to ensure it’s selected before painting.

The final step is simply to paint directly onto the main image in the areas you want sharpened, altering the opacity of the white brush to vary the level of sharpening as desired— if you go too far, just brush over it with black. Use a low opacity brush and gradually build up until you get the effect you want. The result is that only the selected areas are sharpened — in this case, around the eyes.

Conclusion

You’re done — having sharpened your image in a non-destructive and flexible way, you’ve also avoided unnecessarily modifying the surrounding areas, and you’ve learned how to use layer masks. A word of warning however: sharpness is a great way to add something a little extra to your image, but overdo it, and you may do more harm than good.

 

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Published in issue 7 of National Geographic Traveller (UK) Photography Magazine