The importance of camera noise

On printing a photo and seeing unwanted banding, it is tempting to blur the banded areas (because this appears to fix such issues when viewed on a monitor screen). Perhaps counterproductively, the solution is to add a little noise.

 

Recent versions of Lightroom/Camera RAW have excellent noise reduction algorithms. You can typically remove all noise in digital photos. Don’t do this: you should always leave at least some of the original noise. Here’s why.

The effect of noiseless high dynamic range on final output

Cameras see via three channels: red, green and blue. (as an aside, this is slightly different from our eyes: we also see brightness via a separate b/w  channel).

Pure RGB gradient

The image here is a good test of such a camera: it shows pure red, green and blue fading to white via perfectly continuous  gradients.

Assume we have taken a shot of this gradient, removed all noise in post processing, and now want to create a print. A printer doesn’t have the same dynamic range as as a typical out-of-camera digital file, and this is what results:

Banding caused by printing noiseless, high range gradients
Banding caused by printing noiseless, high range gradients

We see banding in all three color channels. Yuck!

The beneficial effect of noise on final output

Now let’s assume you have  taken the same shot again, but this time performed no noise reduction.

Pure gradients with added noise (uniform, 5%)
Pure gradients with added noise (uniform, 5%)

The image of the gradient will now look something like this (I have added 5% noise to the image).

Close up of noiseless (left) va noisy (right) gradients
Close up of noiseless (left) va noisy (right) gradients

If we pixel peep the digital image, the noiseless (left) vs noisy image (right) are as shown.

But look what happens when we come to print…

Effect of printing the noisy gradients
Effect of printing the noisy gradients

The noise actually prevents banding significantly. We end up with a better final output!

Conclusion

Most photographs have no pure color areas. Even seemingly solid colors are actually a graduation of closely matched colors, and approximate color gradients. As we have seen, printers are not as good as digital files in showing these graduations, and banding will occur unless you allow for a little noise.

Clearly, high amounts of noise are undeseriable, but if your photo has only moderate levels of noise, don’t remove the noise altogether in Lightroom just because you can. Removing all noise, especially in subjects such as skies and well lit skin, can cause banding in the final print

On printing a photo and seeing unwanted banding, it is tempting to blur the banded areas (because this appears to fix such issues when viewed on a monitor screen. Printers actually work differently, and blurring is the worst thing to do in this situation (you make banding more likely because you make the color gradients more perfect). Perhaps counterproductively, the solution is to add a little noise. For example, if you have a photo that prints with small areas of blown, banded highlights, one way of reducing the effect is to add a little Gaussian noise (Photoshop: Filter > Noise > Add Noise…) in the blown areas.

Notes

  1. I created the gradient examples in Photoshop. The noise was added via Filter > Noise.
  2. I simulated the reduction of dynamic range between a RAW and printed photograph in Photoshop by changing the image mode from 8 bit RGB to 128 level indexed (with forced pure back and white).
  3. Although printers generally try to reduce banding via stippling, they are bad at this when the colors are very close to each other anyway (such as in a gradient). Adding a low level of uniform noise (or more usually, simply leaving some of the sensor ISO noise in the file) gives more variation and prevents banding further.
  4. This article is not an excuse for picking a noisy photo over a less noisy one. What I am suggesting is that whatever your output, you should never reduce the noise levels to zero during post processing: always leave some sensor noise in the photograph.
  5. Many stock photographs are post processed to remove all noise. Banding may occur when you print them. If this happens, try adding 1-3% uniform noise across the entire photo. You will then see the banding disappear. Better still, use your own photographs and do it all properly 🙂
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