Friday, February 8, 2013

Photoshop: Dealing with Noise the Right Way

Those of you following this blog have probably realized by my texts that I am not afraid of noise in photography. As I say, if all other things are great (composition being the most important), a little or even a lot of noise will not harm your photo.

Having said that, in some cases noise can be distracting and needs to be kept in check. Notice: in some cases. There are plenty of occasions where noise is not only acceptable but even useful, either for artistic purposes (for example to give an aura of tradition) or even for technical reasons as well.

Did that latter sound impossible? Consider the following image. I have artificially created banding, by using the posterization option, but something a bit like this image can occur without any aggressive post-processing, as a result of saturation and/or contrast adjustment. Now, click on the image to enlarge it and look carefully at the version without noise (top) and at the version with noise (bottom). Which one seems better and more natural?

5% noise was added to the bottom image.

The difference is easier to see in a non-real-world example. Consider the following image, showing a gradient. The bottom version is exactly the same with the top, only with 3% noise added.

Banding has all but disappeared once noise is introduced

It becomes apparent, that noise is not your enemy by definition. It all depends. But even in those cases where noise is intrusive and somewhat distracting, using a blanket solution (that is, a program or plugin removing noise all over the image) results in substandard results. Noise, when it has to be dealt with, has to be dealt with the right way, that is, selectively. Look what happens if you don't do that:

Using an all-out noise removal solution on the entire image, simply results in loss of detail and the appearance is too soft, perhaps even no longer natural

First of all, you must recognize which parts are noisy. Both in terms of channel, and in terms of shadow/highlights. Different cameras have different noise characteristics. On that specific image (as often is the case) noise was more visible on the shadows. I examined the channels, and the green channel is the one that gave me the better contrast. So, I duplicated it, increased contrast on it even further, and then loaded it as a selection. I duplicated the layer and clicked the "new mask" icon, to create a mask from the selection. Then inverted the mask (CTRL + I). Now, whatever we do on the duplicate layer, will have an effect only on the shadows. I ran the noise removal plugin, and here is the result using this selective method:

A cleaner than the original, yet much more natural result

NOTE: If you want to be 100% accurate regarding choosing shadows and highlights, as described above, you should convert the image to Lab mode (Image>Mode>Lab) and then work on a duplicate of the Lighting channel.

No comments:

Post a Comment