Photography News | Issue 47 | photographynews.co.uk Technique 47 Camera School PART 12 Here we lift the lid on all things camera related, showing how to get better results from your CSC or DSLR, and providing all the info you don’t find in the manual. So, stick with us and you’ll soon be wielding your camera like a pro. This month, how to use noise reduction in camera and in software to improve high ISO shots
Types of noise High ISO noise is random in nature, whereas noise caused by very long exposures has a fixed pattern. Matters are complicated with high ISOnoise coming in two types; luminance (grain) and chrominance (blotches of colour). In high ISOnoise reduction, an algorithm tries to detect the noise, then reduces it without affecting the real details in the picture. This is difficult, because the algorithm has no sure way of discerning between fine textures in the original and any grain caused by interference. For example, with luminance noise showing as pixels of a different brightness to those around them, it could easily be natural texture. In-camera noise reduction All DSLRs and CSCs have a High ISO Noise Reduction setting, which can be switched on from main shooting menu, or via a quick menu. The first question to ask is when do you need to turn it on? No matter how efficient it is, and how
lightly applied (see above) High ISO NR will degrade detail in an image, so it’s not worth applying it until absolutely necessary. When that is depends on what you personally find acceptable in terms of digital noise; some may think pictures too grainy at ISO 400, while others might go to 1600 or 3200 before feeling it’s too heavy. This also depends on how well the camera itself handles noise. It also depends on the subject. Noise will be more noticeable in smooth areas like clouds than rough textures like rocks. It’s also more visible in shadows areas. So if you’re shooting a high-key, textured view it’s not so bad; a low- light minimalist landscape on the other hand may be more of a problem at the same ISO. How much NR to use A camera’s High ISO NR function can usually be set to off, low, medium or high, which relate to the amount applied. HighNR can lose fine detail; Low may leave more grain than desirable. The only way to knowwhich to pick
is to practise. If you take a series of shots at different settings then assess the results, you’ll be able to see how much detail is sacrificed. It may be better set to off or low, where it’s mainly colour noise that’s suppressed – a grainy but detailed image is preferable to a muddy looking smooth one. Like any other processing, High ISO NR doesn’t affect Raw files, only JPEGs. From that perspective, shooting in Raw+JPEG mode can be a good option, as you’ll have a safety net. Multiple shot NR Some cameras feature a multi-shot NR mode, which uses the same principle as image averaging (see below). Here, several images are taken in quick succession, compared and the noise removed, usually with less loss of detail than high ISO NR in-camera. The drawbacks are that images need to be shot from the same position, and anything that moves between frames will be blurred, so it’s really just for shooting static subjects on a tripod.
Words & pictures by Kingsley Singleton
When you decrease or increase ISO you adjust the ‘gain’ on the sensor, effectively meaning you need more or less light to get a decent exposure. This is an incredibly handy feature, letting you adapt to different lighting situations while still using the shutter speed or aperture you want. If ISO didn’t exist, you’d almost always need to use long exposures in dim light, or very fast exposures in bright light. So, it’s a very handy tool, but it comes at a cost; digital noise. This is interference caused by amplifying the signal, and presents itself as grain and coloured speckles on the picture. Modern cameras are good at dealing with noise, and with each generation higher ISOs can be used with less interference. These days, digital noise may only be apparent when shooting at higher ISO settings like 800, 1600. However, it can also become more noticeable when pictures are lightened in software.
Using Noise Reduction in software
The main Color slider is the overall setting, and is set to 25 by default. At very low settings youmay see colour noise creep in; very high and the picture will lose overall saturation. Detail is used to stop the image softening, particularly around edges, but at the expense of a little more colour noise in those areas which looks like colours are bleeding; at very high settings edges can look speckled. Smoothness reduces large blotches and is useful if dealing with low quality JPEGs. It’s also possible to remove noise manually using a technique called image averaging. Therein, multiple shots are taken from the same position, stacked as layers, and turned into a Photoshop Smart Object. The random noise is then compared from each, making it easier to identify and remove. Finally, another way to reduce noise is simply to downsize; a picture that looks noisy at its full resolution will look a lot less grainy at 50% size (and sharper, too); this isn’t much use if you want to make big prints, but for images to be used online it’s very helpful.
In software, high ISO noise can be reduced in a more controllable way than in camera. That doesn’t mean results will necessarily be better, as the algorithms at work are catch-all, rather than engineered for specific cameras. It also takes longer. Lightroom and Photoshop use similar sliders for noise reduction, both under the Detail heading. Typically, you’ll need to view the picture at least 100%magnification to see changes (in Lightroom there is a loupe view, too). Controls are split between Luminance (grain), and Color (saturated blotches). The Luminance slider flattens the grain, blurring it but retaining edge detail; low settings show little reduction in noise, while high ones can soften the image too much. The Detail slider controls the threshold at which NR is applied; set low, NR is applied to the whole picture; higher, some noise is retained to improve detail. Contrast, when set high keeps contrast in the noise, but at the expense of a mottled look; low settings reduce it but can look washed out.
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