Expanding the dynamic range of a single RAW file

December 3, 2008 at 1:54 pm (Photography, Techniques)
Tags: acr, adobe, camera RAW, dynamic range, hdr, highlights, Photography, photoshop, RAW, tutorial

This tutorial will focus on a technique tailored to expand the visible dynamic range of a single RAW file (as opposed to creating a HDR file or masking a bracketed sequence).

Most RAW editors has several tools to do just that built-in, but often you can not achieve your desired end result without complicated Photoshop work and masking techniques. In this tutorial, I’ll share some of my secrets for expanding the dynamic range of a single RAW file.

Observe the following photograph:

'Say cheese' Single RAW file - developing for the shadows blow the highlights.

This image is from a Canon 40D RAW file developed with Adobe’s Camera RAW 5.2. As the original image was underexposed, out of necessity to keep the highlights in range, I’ve added a positive exposure compensation to brighten the shadows, in turn leading to a clipping of the highlights. Even with tricks like Curves, Fill Light and Highlight Recovery, I cannot both expose the shadows as I want them, and also get great highlights – at least not without an unintentional hit in contrast in the process. My opinion is that neither Fill Light or Highlight Recovery are perfect in ACR, as by affecting the left or right third of the histogram respectively, they are both too broad, and don’t go far enough. – And they don’t give you precise control over the tonalities you effect.

Let’s look at my technique for dealing with this problem. For this tutorial we will be using ACR 5.2 as our RAW developer on a PC, as well as Photoshop CS4. The beauty of this technique is that it offers a more or less automated, lets call it ‘intuitive’, way of expanding your dynamic range without delving into Curves and complicated masking for every problem shot.

1 ) Develop your RAW image in ACR only for the shadows (left side of histogram). That is, concentrate on getting good shadow separation, and let the highlights blow for now (that means do not even try to recover the highlights, don’t touch the Recovery slider, better to let them blow out).

2 ) When satisfied, hold Shift key, and click ‘Open Object’ in ACR to open the image in Photoshop as a Smart Object.

3 ) In Photoshop, right-click the SO layer, and select ‘New Smart Object via Copy’.

4 ) Double-click the thumbnail of your new SO layer, to bring up the ACR dialog again.

5 ) This time, develop for the highlights (right side of histogram), and ignore the shadows (left side of histogram). Typically for this technique, there’ll be around 2 stops of difference between the two versions. Essentially this is just a matter of dragging the Exposure slider to the left.

6 ) Click ‘Done’ when satisfied that all highlights are recovered.

7 ) In Photoshop, now hide the top layer, then go to the Channels Palette and Ctrl-click the RGB channel thumbnail to select the highlights of your lower, lighter layer. What you’ve got now is a so-called Luminosity Mask.

8 ) Go back to the Layer Palette, turn on your top darker layer (select it if it isn’t selected), and click the ‘Add layer mask’ icon.

9 ) You should now see a composite of the two layers that will have expanded dynamic range, but also will look a bit lifeless and with poor contrast.

10 ) To add local contrast, you need to select your Layer Mask, and add Filter>Blur>Gaussian Blur. How much depends on a number of factors, including file size and detail frequency in your image. Typical key figures for Amount are in the 90-127 range. However, for some images, such as the sunset example in the end of this article, you may need to go as low as 1! This is up to your best judgement, and needs some experimentation and careful examination to set right for every image. You’ll want to achieve natural contrast and at the same time avoid visible halos and artifacts. Note that what you are doing here is essentially how Unsharp Mask worked in the dark rooms of yore. Only this digital equivalent is much, MUCH easier.

11 ) Finally, you may need to adjust the tonalities affected. You can do this by adding a Curves adjustment (Image>Adjustments>Curves) to the Layer Mask. Typically, you want to darken the mask a bit, by creating a point in the middle of the curve and dragging down about 2/5th of the way. You may also use other tools and adjustments on the mask to localize the effect – all non-destructively.

12 ) For a final kicker, now press Ctrl-Alt-Shift-e to create a new composite layer on top of the others, then add Filter>Sharpen>Unsharp Mask, Amount: 6-12, Radius: 90-127, Threshold: 0, to add a bit of extra local contrast. Typically I overdo this a little, then immediately go Edit>Fade to 66%. Alternatively, you can set the layer opacity of this composited contrast layer as you like.

End result:

The final result - highlights have now been smoothly recovered.

Disclaimer: What you see above is no miracle cure for blown highlights. You will still need to expose your photo in a way so that all data can be recovered, and you might have to take care of increased shadow noise from pushing the shadow end.

Here’s another example comparing ACR’s Highlight Recovery to this method:

Dynamic Range expansion - top: image developed for the shadows, middle: Recovery at maximum in ACR*, bottom: my method

* Recovery at 100, with positive exposure compensation of +2. A closer approximation of ‘my method’ could be achieved by maxing out both Recovery and Fill Light in ACR, leaving Exposure slider at 0, but doing so introduces nasty edge artifacts and very low contrast.

Enjoy!

I’ve compiled this technique into a Photoshop action, which you may download and study here.

Copyright 2008 Mathias Vejerslev