Sharpening

Digital camera sensors and lenses always blur an image to some degree. Even the built in anti-aliasing filters of the cameras soften the image. All this has to be corrected in post-processing or in-camera processing. I click raw. For me this correction is always done in post-processing.

Sharpening remains a particularly confusing topic among photographers, especially given the large number of post-processing options available. Some programs have too many settings whereas others do not give any control. If you are trying to use the best sharpening settings, start with the basics. Try to click at the optimum ISO for your camera, which is usually at around 100 ISO. Meter well and have a nicely exposed image to start with. This lowers the possible levels of noise and other artifacts.

Sharpening is one of the last steps that I do when I post-process. My recommend steps in post-processing are given here – Post Processing RAW. Correcting chromatic aberrations makes the images look sharp. A high contrast image also looks sharper than a low contrast image.

Also, before I indulge in sharpening, remember that it is impossible to restore detail that was not captured in the first place. Nothing new is created by this tweak. Sharpening does not sharpen images that are blurred due to moving subjects, low depth of field, hand shake and out of focus. Sharpening is nothing more than increasing the local contrast of details you already captured, more so of the adjoining areas or edges.

 

Generalized Sharpening

Most image editing softwares have something called an “unsharp mask/tool,” This is a misnomer since it actually sharpens the image.

Most of the sharpening settings within image-editing softwares are reasonably standardized. The sharpening tool usually has the following settings –

Radius: Controls the size of the edges you wish to enhance, where a smaller radius enhances smaller-scale detail. The key is to have this radius small enough that it is near the limit of what our eyes can resolve (at the expected viewing distance), but is also large enough that it visibly improves sharpness, while keeping a tab on the changes in the smallest details. Sometimes it is also listed as percentage which can get confusing.

Amount (Intensity or Factor): Controls the overall strength of the sharpening effect, and is usually listed as a percentage.

Threshold (Masking): Controls the minimum brightness change that will be sharpened. This can be used to sharpen more pronounced edges, while leaving more subtle edges untouched. It’s especially useful to avoid sharpening noise.

Detail: Controls the relative sharpening of fine versus coarse detail (within a given radius value), in addition to affecting the overall strength of sharpening. Higher values emphasize fine detail, but also increase the overall sharpening effect. You will therefore likely need to adjust this setting in conjunction with the amount/percent setting.

 

Apart from the Sharpening Tool there are some other tools as well that can be useful for sharpening in difficult conditions. One such tool is a High-Pass filter than can be used as a sharpening tool where only some parts of the images are to be sharpened. Once you get comfortable with the Sharpening Tool, start experimenting and learning this too while using it in a duplicate new layer and then blending it back to the original.

 

Sharpening

(The image on the left is without any sharpening. The one in the middle has some amount of sharpening. The image on the right is an example of going overboard with excessive sharpening)

Avoid excessive sharpening. It is very easy easy to overdo it. Excess sharpening looks like halos. For good control, view the image at 100% and after achieving the required sharpening, reduce the values a little. Another good way is to work with specific color channels rather than RGB if your preferred software has this option.

So, which sharpening settings are ideal?

  • Set the radius as small as possible (1 pixel maybe). The radius should be set after taking into consideration the smallest object that has to be clearly visible. If the smallest object that you want to be seen is a textureless 10 pixel wide element, then feel free to go upto 8 pixels in radius setting.
  • Set the detail as high as possible (100)
  • Set the threshold or masking as low as possible (0 or 1). It depends on your image editor.
  • Then, adjust the sharpness value (amount or intensity) so that the noise in low-detail areas is tolerable.
  • Do not use the ‘clarity filter’ for sharpening.
  • If your image is noisy to begin with, increase the radius a little and reduce the threshold.
  • If your image has areas which are out of focus and noisy, experiment with the ‘high-pass’ filter method (I’ll be writing about it sometime soon).

(Some of the image editing programs have the scales revered, so preview each change and learn)

Pay attention to the areas without detail more than the areas with detail for generalized sharpening since they are prone to noise.

This type of sharpening uses some smart algorithms at times to have better control over the effect. This is called deconvolution sharpening. Though better than the generalized sharpening, this tool is not available with all the popular post-processing or image editing programs. If your preferred program has it, then use deconvolution sharpening instead of general sharpening in the same manner as above.

 

Creative Sharpening

The most important sharpening step is local or creative sharpening. Here, you are sharpening only areas that already have high levels of detail.

Say, for example, that you are photographing a portrait. After you apply a light generalized sharpening across the entire image, you are ready to target more specific details. You might want to sharpen a model’s eyes or lips, for example, without also sharpening any flaws on the skin.

Sometimes, I use Smart Sharpen with photoshop. These are the steps that I take –

  • Create a new layer by duplicating.
  • Apply Smart Sharpen to the top layer at the strongest setting that I feel.
  • Add a white mask to the top layer.
  • Decrease the amount of sharpening in low-detail regions by painting black onto the mask.
  • Merge the layers and you are done.

(Most of my post-processing and image editing gets done on ‘darktable’ and ‘gimp’ with ‘ufraw’.)

 

Output Sharpening

Output sharpening is simply the additional sharpening that is required to counteract effects of different ways of output.

Consider a canvas, matte print, or textured paper. If you don’t add any additional sharpening, the photograph will be blurry simply because of the material used!  Even on consumer grade glossy photo-paper, sharpening is required because the printing process is a bit fuzzy and doing this helps increase the perception of sharpness. The way the ink reacts with matte and glossy paper is not the same so sharpening for matte is a bit more aggressive. When you’re sending a file to get printed, that’s what you’d use, even if it doesn’t look right at 1:1 on screen. Output sharpening for “Print”, when seen on computer monitors, looks nasty, like you’ve gone too far. Don’t panic. It’s normal.

Even photographs intended for the web need some output sharpening. Every time that you downscale an image,  you may lose detail. The best way to counteract this is to add sharpening at the end of any size changes. The images meant for viewing on monitors do good with far less sharpening than is required for print.

 

Speeding Truck

(Speeding Truck – Nikon D200 with Nikkor 18-55mm lens, f/14, 1/20 sec, ISO 100. No amount of sharpening will make the truck appear sharp. 😉 )

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