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Unlikely Friends: Grain, Noise, and Sharpening

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This post is part of a series called How to Sharpen Digital Images.
Quick, Best, Selective: 3 Image Sharpening Techniques for Every Situation
Photoshop in 60 Seconds: How to Sharpen Images With Photoshop Actions

At the start of this series, we set out to demystify sharpening. It’s very poorly understood by almost all photographers. By now, you should no longer be one of the confused majority. We started by looking at what sharpening is, then we considered some of the problems around sharpening, and finally, we covered how to use the tools available in Adobe Photoshop.

In this tutorial we’re going to get advanced look at one of the most misunderstood areas of sharpness: noise.

Noise and Sharpness

The problem with noise is that, when it comes to sharpening, it can both ruin and improve your images. Understanding when it does one and when it does the other is the key to getting the best possible sharpening results.

The conventional wisdom is that visible noise is always an issue. This isn’t the case. Too much noise, or noise that’s too sharp, will lead to unpleasant results. If you have noise in the shadows and you badly sharpen the image, it will look awful. As we learned last time, if you’re careful with how you apply Smart Sharpen and use the Reduce Noise slider, though, it won’t be a problem.

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The inset box shows the shadow noise in a long exposure, night time shot I took of a dancer. While it's fine as it stands, sharpening this image too much more would create an unpleasant look.

If you're careful, the right amount of noise can actually be a big plus. This is because noise can create the appearance of higher resolution and increase acutance. These are the two factors that determine an image’s apparent sharpness. So noise makes images look sharper. Confused yet?

Part of the problem is that desirable and undesirable noise are often conflated. The shadow noise most often seen on digital images is ugly. It also only affects small areas of the image so it’s presence is especially noticeable. Film noise, on the other hand, is much more uniform. We call this noise grain (because film really is made of little grain-like bits of silver), and to most people is one of the best features about film photography.

Even digital images that are perfectly exposed and have no apparent noise, a little bit of grain can help 'sell' the picture, especially with prints. Viewers find an image that is too smooth and too perfect a little disturbing, if even only subconsciously. Many years of film photography have set up the cultural expectation that photographs have some grain.

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A small amount of noise has been applied to this image to increase its apparent sharpness.

The main takeaway is that too much localised noise that’s too visible is unpleasant but that, through the amazing ways our brains handle vision, a small amount of noise across an entire image increases apparent sharpness and creates a more pleasing photograph.

If you’re shooting with a digital camera, the only noise that you’re going to automatically get is digital noise. If you want some of the more pleasing effects of noise, you’ll have to add it manually. 

There’s a few reasons to do this. The first, as I described above, is to increase an image’s apparent sharpness. The second is to emulate the the film look. Finally, if an image has digital noise that you can’t get rid of without destroying the look of the image, adding noise can compensate for and disguise it.

How to Add Noise

Adding noise to an image should always be done after you’ve completed your final sharpening. It should also be applied, like sharpening, with the image at its output size. You need to add different amounts of noise depending on the size of the finished image.

The first thing you need to do is create a layer filled with neutral grey. I find the easiest way to do it is create a new layer (Command-Shift-N) and, in the new layer dialogue box, change the Layer Blend Mode to Soft Light and then click Fill With Soft-Light-Neutral Colour. Once it’s created, change the blend mode back to Normal so you can see what you’re doing.

To add the noise, go to Filter > Noise > Add Noise…. The most important thing is to make sure the Distribution is Gaussian and that Monochromatic is checked. Uniform colour noise looks ridiculous. 

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The Add Noise filter.

How much noise you add depends on your purpose. If you want to increase sharpness or cover digital noise artefacts you want a barely visible texture: an Amount of between 1% and 2% works well. If you’re trying to emulate film grain for aesthetic reasons, low-opacity layers with an Amount up to around 20% can still produce great results.

Noise on it’s own can be quite harsh so the next step is to blur it slightly. Select the noise layer and go to Filter > Blur > Gaussian Blur… We’re not trying to completely remove the noise so a Radius of 0.3px or 0.4px is appropriate. 

To apply the noise layer to the image, change the blend mode to Overlay, Soft Light, Vivid Light or Linear Light. Different blend modes will have slightly different effects so play around to see what works best for your image. Most of the time, I use Overlay. If the noise effect is too intense, lower the Opacity of the layer.

The image on the left has no noise added. The image on the right has two layers, one to increase sharpness and obscure shadow noise, the other to emulate film. Which do you think is more appealing?

If you’re trying to emulate film grain, multiple noise layers, each with different Amount works best. Repeat the process above until you’re happy.

Wrapping Up

The relationship between noise and sharpness is complicated and contradictory. If there’s too much noise in the wrong places, it looks terrible. But the right amount of noise subtly applied across the whole image can make it appear sharper, or even look like an old film image. Digital images shot at a really low ISO in a studio often benefit from added noise; if you don’t add it, they look almost too perfect and fake.

This tutorial is the last one in this short series on sharpening but it’s far from the last time we’ll cover the subject on Tuts+. If there’s any aspect of it you’d like us to dive deeper into, let us know.

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