a5 June 2012

Tutorial: Sharpening - Part 3: Sharpening HDR photos

I don't often do selective coloring but I wanted to try this technique. With the chain hidden in the grass it was really time consuming and difficult task but as you can see I succeeded :) The original photo was HDR from 3 exposures taken in Masuria, Poland.
It's the third part of my tutorial about sharpening. Previously I wrote about unsharp mask and high-pass filter sharpening. Today I won't describe another method of sharpening (I plan to write about selective sharpening but a bit later). I will instead describe how to approach sharpening of HDR photos.
Why such a topic? I've heard and read many times users complaining on various forums that output from their HDR software isn't that sharp as preview or that output photo needs additional sharpening before considering a photo as finished.

The reason for this is that to asses sharpness of a photo we would need to view it at 100% magnification about what many users don't know or forget. Why? If the magnification is smaller then pixels in the image don't directly correspond (i.e. 1 to 1) to the pixels of our monitor what makes some of them interpolated. It is why in general it is recommended to do sharpening at 100% magnification. Depending on the interpolation algorithms used by the photo editing software you can also sometimes do sharpening at other magnifications, eg. in Photoshop you can use 100%, 50%, 25% to get believable results but not for instance 53,11% (you can do a test to see how differently the image looks at 50% and 53,11% despite very small difference in zoom).

Ok, so here is the procedure which should be used when doing HDRs:
  1. If you're developing your RAWs (eg. in Lightroom) prior to loading them to HDR software (as always I use Photomatix as an example) you shouldn't apply any sharpening prior to merging to HDR or tone mapping. The reason for this is that sharpening algorithms are about increasing local contrast - the same what local tone mapping operators like Details Enhancer are. So you might get slightly different results.
  2. Tone map your image as always. To assess sharpening of your image you can use loupe in Photomatix Pro. Just click on the image region and another window will appear. Note that for this to work you can't be in the selection replacement mode (i.e. make sure Selection Mode checkbox is unchecked). Take a look at this screenshot. I opened loupe window on a Jeep. Although preview image might look sharp enough loupe reveals that some further sharpening might be necessary.
  3. Process your image.
  4. (Optional) At this stage you might apply sharpening directly from Photomatix (at least when you use version 4.2 or newer). The small window which appears after processed image appears allows you to do that. Switch to sharpening tab and click on the "Show Options" button. The controls you will see are nothing else but controls controlling unsharp mask. You can also choose preset (Mild, Medium and Strong).
  5. Save tone mapped image. 
  6. If you skipped step 4 you can now open your image in Photoshop/Lightroom and apply sharpening there either by using High Pass Filter or Unsharp Mask. 
Important note: always check how areas of uniform color look in Photomatix at 100% (using loupe). It might be often the case that after applying tone mapping a lot of noise will appear there what will make sharpening in post processing more difficult. To reduce amount of noise you can try to decrease Detail Contrast setting or increase Smooth Highlights or Micro-smoothing.

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