Photography News | Issue 34 | absolutephoto.com
Photo school Camera class Everyone has to start somewhere, even pros, so every issue we’re looking at the core skills every beginner needs. This month, how to control digital noise, both in camera and in software In association with
Words & pictures by Kingsley Singleton
It might seem as though digital ‘noise’ in pictures, caused by increasing the ISO sensitivity, is a new problem for photographers to deal with, but anyone who ever shot on ‘fast’ film knows that there’s always been a trade-off between increasing the camera’s sensitivity to light and losing picture quality. When increasing the ISO, you’re basically amplifying a poor signal, so just as when you turn up an FM
radio and the hiss gets louder, so when you amplify the light received by the sensor, the noise grows. But there’s another kind of digital noise unrelated to ISO. This is called long-exposure (LE) noise and it appears as bright specks, and red, green or blue ‘hot’ pixels as well as a glow or ‘fogging’ usually at the edges of the image. LE noise is caused by the sensor being switched on (charged) for extended periods,
but it may only be noticeable when shooting for longer than 30secs, and usually only when you go into the minutes rather seconds. In-camera noise reduction Both high ISO noise and LE noise can be dealt with in camera; but both have drawbacks. Tackling high ISO noise reduction (NR) first, you’ll find it in the shooting menu, or possibly as part of a ‘quick’ menu. The function can usually be set to high, medium and low settings, which relate to the amount of NR applied. Too much NR can make the picture look soft and blurry, with a loss of fine detail, while not enough will leavemore grain thanyoumight like. High ISO NR is also only applied to JPEGs, so if you’re shooting Raw, it won’t make a difference. The only way to know what settings to use is practise; shoot a series of test images with the High ISO NR function switched on and off, at a variety of ISO settings, then assess the results on your computer. Compare how much detail is sacrificed in the pursuit of flattening out the grain, and you may find that it’s better left off, or set to Low, as a grainy but sharp image is often preferable to a muddy looking smooth one. It all depends on how you want the picture to look. Long-exposure noise reduction Long-exposure NR is found on all DSLRs and CSCs, and can be
switched on or off. The point at which it kicks in depends on the camera – on a Nikon D800 for instance, if the Long Exposure NR function is employed it’s only applied
to exposures over 1sec. On some cameras it may kick in at 2secs, 4secs, or longer. Long-exposure NR works by taking a second, ‘dark’ frame after the first, but of equal timing. On this dark frame, the shutter doesn’t open, sono light is recorded, only the noise characteristics of the sensor. This dark frame is compared to the first and the corresponding noise is removed (a process known as dark- frame subtraction). It is applied to Raw files as well as JPEGs, and the only drawback is that your exposure time is effectively doubled, with the dark frame taking the same time as the first. This, of course, means you can’t shoot while it’s working. Like High ISO NR, the effectiveness of long-exposure NR depends on your camera model, and the best way to see what you need is to try it. Shoot some test images with the function on and off, at different shutter speeds; you’ll probably find that it appears to do little up to 30secs or 1min, so can be left off, and only used for very long exposures. Next month: How to use your camera’s interval timer
High ISONR (Low)
High ISONR (High)
Noise reduction Above left is a close up of the image after Long Exposure Noise Reduction has been applied; on the right, no NR has taken place, so there’s lots of hot pixels and bright speckling.
Softwareskills Howto reduce digital noise in Lightroom
In-camera Noise Reduction (NR) reduces digital ‘grain’ and colour noise, but it’s only applied to JPEGs, not Raws. Therefore, noise must be manually controlled when processing Raw files. NR is actually more controllable in software and using it becomes more important if you’ve made heavy changes to the exposure, such as lightening areas. In Lightroom, NR is added by default from within the Detail tab, but this only applies to Color noise (the kind that causes red, green and blue blotches), and not the Luminance noise, which is the texture or grain. The following steps will show how to remove noise in Lightroomusing the Detail tab, but if it’s long-exposure noise you’re dealing with, it’s also worth noting that ‘hot pixels’ can be removed with careful use of the Spot Removal Tool. It’s also possible to hide noise by setting the Blacks slider to a low level, and you can lessen the impact of noise by reducing image size – not ideal, but effective if your print size isn’t large. Nextmonth: Howto create timelapse effects inPhotoshop
Step 1: Get a good look at the noise. In Lightroom’s Develop module, click the Detail tab to find the Noise Reduction options. If you see an exclamation mark (‘!’) at the top of the palette, it means you’re not looking at the picture with a 1:1 (or 100%) view, which is required to see the effect that any Noise Reduction you’re adding will have. So, either zoom to 1:1, or at the top of the Detail tab click on the arrow which opens a 1:1 loupe view. You can then use the crosshairs to pinpoint specific parts of the picture. Concentrate on shadow areas in particular where the noise will be more pronounced and when you’ve got a good view you can start to remove the noise from the image.
Step 2: Control the luminance noise. Move the Luminance slider right until the grain flattens out to the extent you want. You can hold the Alt key for a grayscale preview, which makes judging the effect easier. The Luminance Detail slider is used to retain detail and compensate for the flattening effect; a high setting will firm up the details, but at the expense of smoothness, while a low setting will soften detail and compromise sharpness. Applying lots of NR to high ISO images can also leave images looking washed out, which the Contrast slider compensates; high values retain contrast but show more noise, while lower settings will look smoother but can make the picture look flatter overall.
Step 3: Control the colour noise. For removing colour noise, the Color slider controls the general amount of reduction and is set to 25 by default. Take it left and you’ll see blotchy colour information return particularly in the shadow areas. Conversely, higher settings can make the picture look a bit washed out, so don’t push it too far. Much like its counterpart for luminance noise, the Detail slider can be used to stop the image softening, particularly around edges; but this is at the expense of a little more colour noise in those areas. The Smoothness control can help reduce any further stubborn blotches and is useful if you’re dealing with JPEGs that have been saved at low quality.
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