How to reduce noise but preserve image quality
Short days and long nights make winter a perfect time for experimenting with low-light photography, but the high ISOs required will often produce noisy, gritty pictures. By using a tripod you can reduce the ISO setting, which will help with some noise issues, but in areas of even tone and shadow the noise may still sneak in. When it comes to enhancing the image with Photoshop effects – altering levels, increasing saturation and adding sharpness – that noise will become even more prevalent.
There are two basic types of noise you need to tackle in your low-light photography. The first is Chrominance noise, which introduces itself with higher ISO shots and can be recognised by its coloured speckling in shadowed or even-toned areas.
The second is Luminance noise, which is trickier to remove and can be seen in the form of random variations of brightness between pixels. Reducing this can result in a loss of overall image detail, so in this tutorial we’re going to look at techniques to reduce both types of noise while preserving quality.
01 Initial enhancement
Open a raw format low-light photo of your own in Adobe Camera Raw. Before you start to reduce the image noise, make some initial adjustments by setting Exposure to +1.25, Recovery to 25, Fill Light to 10, Blacks to 0, Brightness to 50 and Contrast to 0. You’ll see the amount of noise within the image increase.
02 Preparing for noise reduction
With the basic raw enhancements complete, you now need to look at noise. To do this, click on the Details tab. Make sure that all the sharpening and noise sliders are set to 0 and that the Preview window is at 100%. Initially you should see a fair amount of colour noise appearing in the shadow and sky areas.
03 Tackling chrominance noise
The first area of noise to tackle is the chrominance variety. You can see this in the shadows along the tree line – it looks like speckled colour. Increase the Colour slider to 16 and you’ll see the colour noise disappear. Use the Hand tool (H) to move around the image and check that the noise has been reduced in both the sky and shadow areas.
04 Reduce luminance noise
The second noise type comes from random variation of brightness between pixels and is trickier to remove. If you move the setting to high, you’ll have an image that looks painted rather than photographed. Increase the Luminance slider to 20 to start reducing noise; further reduction will need to be done later using Photoshop’s noise filters.
05 The Reduce Noise filter
Now click OK to edit the image in Photoshop. Go to Filter>Noise>ReduceNoise. Set all values to 0 and make sure Preview is set to 100%. Before adjusting the strength, try the Colour Noise slider. This should have no effect as we tackled this at the RAW stage. If it does, just increase it slightly to remove any last remnants of colour noise.
06 Reducing luminance noise further
Even with the colour noise removed, you’ll see that noise is still present. This is luminance noise and is trickier to remove. Increase the Strength slider to a maximum of 10; you’ll see the noise reduce, but details within the image will be smoothed. Reduce the slider to 7, which will reduce the noise but still preserve the main detail.
07 Preserve image detail
Although we have reduced the strength of the noise reduction, there is still a major problem: the detail within the image has been smoothed and finer detail has been lost. Regain this detail by increasing the Preserve Detail slider setting to 25%. Once you have a good balance between noise and detail, click OK.
08 Sharpen up
As with any enhancement, sharpening should come after you have made your noise reduction. Go to Enhance>AdjustSharpness. With an image that has contained a lot of noise, the amount of sharpening that can be applied will be considerably less than a low-noise image, so apply sharpening in moderation to avoid reintroducing noise.
Expert tip: Photoshop Noise filter
Noise reduction in Photoshop gives you much more control than Elements because of its ability to tackle noise in the separate colour channels.
Once in the Noise dialog box you can use the sliders in the Basic window to affect the image in much the same way as in Elements, but with the added feature of being able to sharpen detail as you apply the filter.
However, many images are affected by noise of different degrees in different colour channels. In other words, much of the noise can often appear in just one of the red, green or blue colour channels.
Click on the Advanced tab and you can adjust the influence of the filter per channel, rather than over the whole image.
By using this technique you can dramatically improve the actual noise reduction, as it’s closer tuned to the affected areas, resulting in better detail and less smoothing in the final picture.
This entry was posted on Wednesday, November 14th, 2012 at 2:00 pm and is filed under Tutorials. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a comment. Pinging is currently not allowed.