Good Morning from Cornwall (where the weather is fine and bright with great skies, ideal for photography, but I am stuck in the Office).
This is a general question about digital noise. Now that I have been working with my Hasselblad 3FR files in Lightroom for a while, I am puzzled by why, exactly, some areas of the image seem to be more affected by noise than others. In general, I can understand that noise should be apparent in under-exposed areas of the image, and not visible in the correctly exposed areas. But my pictures do not always seem to follow this logic.
Colour Noise – (the magenta and cyan speckles). This does seem to be worst in the shadowed areas of the image, particularly where the contrast is very low. But sometimes it is very noticeable where I would not expect it, for example on the shaded side of someone’s face, which is no darker than the side of a cream boat – the boat being noise-free, but the face badly affected. And particularly in skies, which are not exactly under-exposed, but seem to always be the worst area of the picture for colour noise. But correctly-lit dark objects (like a black-painted wooden shed) show no noise, even though they print at more or less the same luminance as the shadows, which are noisy.
Luminance Noise – (non-coloured speckles more like film grain). I very rarely seem to get this in the 3FR files, but when I do see it is almost always in a white area (notice-board, white boat) which is not in full shadow but semi-shade. Or again, in areas of very low contrast which are not in full shadow. What causes this, and how does the mechanism differ from colour noise?
Essentially, I would like to improve my technique so that I did not need to use any noise removal at all, although I appreciate that this is probably impossible. At present I need to use very little – in LR I usually need 0 luminance removal and between 5 to 15 for colour noise. For my ISO 50 and 100 frames this amount will completely smooth them, except for the skies which are always more tricky. In LR there there does not seem to be any way of selectively applying NR only to certain parts of the image, so far as I can see. Whereas using masking, it is possible to restrict the effects of sharpening. What would be nice would be to apply NR only to those parts of the picture which require it.
Having only worked with digital files for the past six months (previously I shot film and scanned it) I realise how little I understand this whole topic of digital noise. I can never entirely predict exactly where the noise will be, and because one can only see the effects of NR at 100%, it is completely possible to miss the parts of the image that are noisy when you can only see a tiny section of it on the screen at any one time. Or you spot the colour noise and deal with that, not realising that a completely different part of the frame is actually affected more by luminance noise.
Incidentally, as I increase the ISO to 200, 400 and 800, not only does the noise increase as I would expect, but DR shrinks also. So the higher the ISO, the sooner the highlights are clipped, which did throw me a bit at first, as you need to be more careful with your metering at ISO 400 and up.
John
