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Photography

The 7 Hidden Dangers of Image Sharpening

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This post is part of a series called How to Sharpen Digital Images.
What Is Image Sharpening?
Selective Sharpening Using High Pass in Adobe Photoshop

In the previous tutorial, What Is Sharpening?, we looked past the jargon to figure out what sharpening actually is. To quickly recap, it’s a technique to increase edge contrast (acutance) and thus apparent image sharpness. Sharpening is an important tool for photographers, however, it’s not without its problems. In this tutorial we look at what those problems are and how to avoid or deal with them.

sharp on screen
This image is nicely sharpened for screen, but for print it would be too soft.

1. Viewing Conditions are Out of Your Control

Back in the days of darkrooms and developing fluid, any changes you made happened to a physical image. You could see how any effects you were using turned out in your test strips. It was all right there in front of your eyes. If you are creating an image for sharing online you can, at best, cross your fingers and hope that people viewing it will have a decent enough screen.

However, there is an almost certain chance that people online are viewing your image in a less than ideal way. The size and capabilties of their viewing device, bad environmental lighting, even how close the device is to the eyes: these things are completely out of your control, and they all impact how people perceive sharpness.

The best way to deal with this problem is to accept that there's no way to create a single image that will suit everyone. Instead, think about your ideal viewers, the main ways they might be looking at your images, and create versions of your images to suit these needs. For example, Facebook, Instagram and Twitter each take a particular size of image, and most people look at these pictures with smartphones. You can optimize for this. For your own website, a web design that uses responsive images will let you deliver multiple sizes of image to viewers with different screen resolutions.

2. Screens are Sharper Than Print

If you’re creating an image for print there’s no sure way to preview things accurately on your screen.

Screens are one of the sharpest mediums for displaying images, and they're gaining resolution with each new generation. An image that is well-sharpened for a screen will be under-sharpened for a print. Things get even more complicated because different kinds of paper (and more exotic printing materials) have different sharpening needs. If you’re printing at home, you can do a few test prints, but if you’re working with a printhouse then a few tests would be prohibitively expensive.

Sharpening for print is covered by Chamira Young in her course From Pixel Perfect to Print Ready, and we’ll dive into it again in a later tutorial. Dawn Oosterhoff did some very interesting tests in How to Get The Best Results from Online Photo Printers.

3. Not All Images Benefit From Sharpening

Sharpening isn’t a magic bullet that suits every image. The most popular advice is that some sharpening improves most images but even that vague truism leaves room for images where it’s not going to be of benefit. 

not all images benefit
Sharpening details in smooth textures, such as snow, won't always benefit your images. I feel this shot is stronger because you can't pick out every ripple.

Sharpening pulls attention to details but some images work because of their lack of details. A beautiful snowy scene or long-exposure image of water doesn’t look better if every ripple is crystal clear—instead, they look best when the snow or water is smooth. Even a small amount of sharpening can detract from such an image.

4. Every Image Has Different Sharpening Needs

While many images do look better with a little overall sharpening, local sharpening to draw attention to specific details—such as the eyes of a model—is often a more balanced and appropriate post-processing tool. This means sharpening is a choice that needs special consideration with every image.

local choice
Local sharpening is just one of many different retouching tools available.

With any sharpening technique there is a real risk you’ll go to far. Over-sharpening is far worse than under-sharpening. An under-sharpened image isn’t the strongest image possible, but an over-sharpened image can be a visual disaster.

If you plan to use local sharpening in your workflow, it’s something you need to practice and perfect with time. There are no one-size fits all rules.

5. Sharpening Can Add Too Much Contrast

Sharpening is all about introducing edge contrast to boost acutance and apparent sharpness. If you’re working with an already contrasty image, sharpening can push things over the edge. Some high-contrast images work great but too much contrast—just like too much sharpening—can take an awesome image and make it look like an LSD powered finger-painting. 

t contrast
The T on the right started out the same colour as the one on the left, however, adding sharpness has increased the contrast across the whole letter.

Inherently low-contrast images can also be hampered by sharpening. If you’re not careful, adding too much apparent sharpness will undermine the effect you’re going for.

6. Sharpening Shadows and Highlights Can Turn Out Badly

Sharpening, noise and colour are all areas we’ll explore later in the series. For now, it’s important to understand that they can interact in both positive and negative ways.

highpass shadow noise
In this image, I've used a highpass filter to detect edges. You can see the noise on the dark background. This is the sort of detail you don't want to draw attention to.

Shadows, in particular, often have noise that you don’t want to sharpen and pull attention to. Similarly, highlights often look best when they smoothly run out to white—sharpening little colour variations will make it look like there is dust on your photo. The midtones of your image is where you get the most flexibility, and benefit, from sharpening your image.

With the Unsharp Mask technique I covered last time you don’t have enough control to selectively sharpen different tonal ranges, however, in the next tutorial we’ll cover a method for doing just that.

7. Sharpening Can Create Unwanted Saturation

Adding contrast to an image also increases the apparent saturation. The same is true with sharpening. This is especially a problem because of where sharpening falls in the post-processing workflow: at the end.

subtle toned
This image has a subtle blue-gold toning applied. Any extra saturation would make it appear garish.

If you are manually sharpening your images, as long as you pay attention, this isn’t a major issue. If you see that your sharpened image is a bit too saturated you can add a HSL Adjustment layer and pull it back. If, however, you’re relying on an automated sharpening method (such as Lightroom’s export sharpening defaults) this can be problematic; the final version of the image won’t accurately reflect your editing choices. It’ll be more saturated than you want. For images with subtle tones, such as toned black and white images, even a small bit of extra saturation can create a totally different look.

The simple solution is to sharpen all your images, or at least all the important ones, by hand and be aware that over-saturation is something you need to watch out for.

Towards Hand Sharpening

I’m a big proponent of sharpening images… when they need it. Most of the time they will, but you need to be aware that for some of your photos, sharpening can introduce some problems. You also need to understand that different situations may call for different sharpening techniques.

In the next tutorials, we’ll get into the practical side of things and look at different techniques for sharpening your images. We'll cover the best sharpening techniques to use, when to use them, how to apply them conservatively, and, most importantly, how to use sharpening to direct your viewers' attention.

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