Noise Reduction & Sharpening

In previous Photoboomers issues we have discussed some of the final steps in post processing before printing, including handling unwanted spots and halos as well as improving color rendition. These articles can be found under the Photoboomers tab on my home page.

This month we are discussing noise reduction and sharpening as a final step before printing. There are reasons why you wait until the end before taking these steps. The primary reason is that these actions are destructive…once done, you will not be able to recover the information from the file that you had before the actions were taken unless you revert to a previous layer in your post processing. Also, you may be doing a number of steps such as cloning, healing or other stamping whose results are best judged prior to noise reduction or sharpening. An exception to this rule might be when you have a marginal image (one you would only print if the sharpening or noise reduction met its goal) where you want to make sure all your other post processing is worth it. In this case, you can do a test sharpening or noise reduction immediately to judge the results, then revert to the previous step if you determine it is  a file worth processing.

When you enlarge or shrink a file, pixels get interpolated. That is, new pixels must be added or subtracted from the file and these pixels have average values of the adjacent pixels, not the same value of the pixels on either side of them. If you were to sharpen a file prior to enlargement or shrinkage, these pixels would not end up the same. Multiple sharpening or noise reduction steps would create increasingly inaccurate results. Noise reduction, because it already reduces the variation in adjacent pixels is not affected as greatly in this way as is sharpening. So we generally use sharpening, especially, and noise reduction much of the time, after changing the file to its correct size. If you are sending a file to a print lab that is not in its final dimensions, you should communicate with the lab to determine whether they should be the one doing the sharpening (not noise reduction so much). 

The first test file I am using is one of those cases where the quality is marginal at best, but it will serve us well as an example. It was taken at ISO 3200 from a moving boat with a fairly long lens (about 85mm) and a relatively slow shutter speed of 1/60 second. The result was a very noisy and soft image.

Here is the original image, with a section indicated with the white outline. This is an image that I have not worked on because I decided it had too much noise and softness to deal with. We will use it, however, to look at noise reduction with some of the more common tools available.

These are the sections that have been developed using various software tools. Note that the major issue here is noise. In reducing noise, we may also be decreasing sharpness. These tools generally allow us to make adjustments to preserve or increase sharpness at the same time, so I have used my judgment in how much noise to reduce while improving sharpness. My comments are below each image. You be the judge of what you like. 

Use your judgment on whether to use the Lightroom tool on your RAW file, considering other changes you might make to the tiff or psd file you are processing. If you aren't going to be doing much cloning or enlarging, it might be a best choice. If you do expect to do considerable enlarging, use one of the other tools and view the image at a normal viewing size and distance on your computer screen to determine how much noise reduction needs to be done.

Noise is not usually the issue in a well exposed file shot at a lower ISO, say 400 or less. In this case, we will rely on a sharpening tool to do the work for us. The original file was not post processed with other steps, but you can still judge the results can still be judged from these examples. I selected the raven from this file as it has somewhat blurry edges and details. 

All of the above results show more haloing than would be present on a print, because conversion to a jpg for web effectively reduces the color options, so on some high contrast areas darks get darker and lights get lighter.

The best sharpening detail was achieved with On1 Sharpen and Topaz Sharpen. These two programs brought out more details in the subject. Note that they also created more of a halo in the high contrast border areas. If you sharpen this amount and have considerable high contrast areas (think of tree branches against the sky) you will get halos. This can create a lot of work for you if halos are not your thing. Those halos are always accentuated when you reduce the number of colors in your image (going from tiff to jpg), so even more work might be needed before you post on the web.

In some cases, it works best to selectively reduce noise or add sharpening. For example, I might not mind some noise in the highly textured sandstone landscapes of the southwest, but I don't like noise so much in a sunset sky above the sandstone. In that case, you may either make a sky selection in ACR and reduce noise just reduce noise on that selection (or its inverse) or I may select one of the other tools to reduce noise on a new layer, then mask the area where I don't want the noise reduction.

Be particularly careful about adding sharpness to any area that is quite smooth (clouds, water, a wall) because the sharpening tools can add artifacts that you don't want to discover after printing. In this case, it is also advisable to sharpen selectively or use masks to avoid sharpening certain areas.

If it is impractical for you to reduce noise or sharpen separately on every size print you might make from a file, you might want to try working with the largest file first and reduce sizes rather than working on a smaller file and increasing sizes. 

Please send any comments to my email address: rs (at) wanderluxphotograph.com