Archive for the ‘Digital Darkroom’ Category

Getting More Highlight Detail from RAW Files

May 3, 2007

by Steve J. Murray

RAW format, when available, is a great advantage, especially when trying to capture all the digital information from an image. I am going to refer here to working with Nikon RAW files (NEF), because that is what I use, but the same advantages apply no matter what brand of camera you use.

Raw format basically captures as much image data as you can get from the camera’s sensor. It is in 12 bit too, compared to JPGS, which are 8 bit. This gives much more gradation in tones and in color variations. It also means there is more information captured in the highlights and the shadows. As I will show later, you lose all this information if you shoot in JPG mode. In JPG mode, the camera applies all your pre-set adjustments like saturation, contrast, white balance, and so forth. Once the camera has made the adjustments, the extra image information is lost as the image is converted into a jpg. When you shoot in RAW, you get to keep all the image information and apply different interpretations to the image as you see fit. It’s like having a “negative” to work with that you can always come back to and try a different approach.

This article is mainly about highlight detail captured in RAW format. I have three images taken of my neighbor’s white house shot in direct sunlight giving an extreme contrast. The first one is taken at a setting that does not allow any “clipping” of the white, sunlit parts of the image, hence, no “blowing out” of the highlights. Notice that by doing this the shadows are quite dark, and lacking detail. You see a lot of images like this as people often underexpose in order not to blow out the highlights.

figure_1.jpg

Fig.1 RAW file shot at f11 and 1/250s

The second image is exposed at f8, one f-stop more. This image initially appears to have some clipping, and indeed, the camera’s “clipping” indicator shows this by flashing on the white parts of the house. There is better shadow detail, and you can see the edge of the nearby house on the right edge of the image. It looks pretty good here, even though the really white parts are “blown out” and you don’t see much texture in the small house in the background.

figure_2.jpg

Fig.2 RAW file shot at f8 and 1/250s

The third image is exposed yet another f-stop more (f5.6), really looking blown-out in the highlights, yet, the shadows are much more exposed and have lots of detail.

figure_3.jpg

Fig.3 RAW file shot at f5.6 and 1/250s

Now, if these were JPGS, the only images that would be workable would be the first two. The first image has good highlight detail, but the shadows are very dark. One could lighten the shadows, but you get increased digital noise by doing so.

The second image shot at one f-stop more (Fig.2) has more shadow detail, and if the highlights can be brought down so they are not clipped, it will be a decent image. I did just that in the Adobe PS Raw converter (ACR) by moving the “exposure” slider to the left to darken the entire image including the highlights. Then I moved the “brightness” slider towards the right to bring up the shadow and mid ranges. You now get texture in the small house and the detail on the steps of the front house looks normal. This is probably the best shot because you have shadow detail and the highlights are controlled enough to get detail as well (Fig.2a).

figure_2a.jpg

Fig.2a RAW file exposed at f8 & 1/250s after corrections in ACR

You can work on that image more. I used ACR (Adobe Converter RAW) with the RAW image (Fig. 2) to make two different versions: a dark one and a light one. Then, in PS using layers and a mask I combined them to get the maximum shadow and highlight detail. I put the lighter version on top of the darker one as a layer with a layer mask, and simply “erased” the mask just over the tree, to expose the lighter layer. Thus, using one RAW file I was able to utilize all the highlight and shadow detail in the file to give a more accurate range of details from shadows to highlights. If you are not familiar with the use of layers and masks, this can be another article in the future.

figure_2b.jpg

Fig 2b RAW file exposed at f8 & 1/250s after corrections in ACR and PS

In this next example I tried to fix the JPG of the shot that was two stops over exposed (Fig.3) without benefit of having a RAW file to work with. I had to use some extreme curves to darken the highlights. Texture is still lost: notice the lack of detail in the steps and the small house. The colors are getting weird too.

figure_4.jpg

Fig.4 JPEG file exposed at f5.6 & 1/250s after corrections in PS

In this example I used the RAW image that was two stops overexposed (Fig.3.) I adjusted the “exposure” and “brightness” sliders in ACR to the max. You can see the improvement, even though its not perfect, it was an extremely overexposed image to start with, and it is certainly better than the JPG version above I tried to fix in PS.

figure_5.jpg

Fig.5 RAW file exposed at f5.6 & 1/250s after corrections in ACR

I used Adobe ACR in these examples to convert from raw to jpg and to adjust the exposure and brightness before converting. ACR is a free plug-in for both Adobe Elements and Photoshop, but you have to have a current enough version. I’m not sure about which Elements you need, but PS/CS is the oldest Photoshop version you can use. Even then, to convert RAW (NEF) files from more recent cameras, like the D200, D80 and D40 you need ACR 3.5 I believe, which only works with CS2 or CS3. It does come bundled with the free “digital negative” converter which you can use to convert the NEF files to DNG format without losing any image information. Once converted to DNG, PS/CS can open it with the older ACR (2.4). That’s all a little confusing, but once you get it all figured out, the process is smooth and works very well.

A little less intuitive, IMO, is Nikon’s raw converter: Nikon Capture NX, which is about $100. Nikon Capture has a large amount of control, like ACR, but the interface is different. I am completely unfamiliar with Canon’s RAW conversion software, sorry. There are some free raw conversion programs out there, but most of them do some sharpening whether you like it or not, and the pixels are compressed or altered so that more enlarging or sharpening doesn’t look as good. These free programs often don’t have the level of image control for exposure and so forth that the better programs have. The nice thing about ACR and Nikon Capture is that you get very smooth and detailed pixel rendition which allows you to get the most out of your enlargements when resizing and sharpening. DXO is supposed to be good as well, but I’m not familiar with it and it is not cheap either.

In summary, shooting in RAW format gives you more highlight and shadow information that you can “extract” using a good RAW conversion program, such as Adobe’s ACR or Nikon’s Capture NX. If you shoot in JPG format, the camera settings are applied instantly, and any image information that is not used is lost, and cannot be recovered. Now, if you are going to shoot 2000 images in one weekend, as some people do, you probably don’t want to shoot in RAW format; which would be way too much work to post process all those images, although ACR and Capture can do batch processing if set up correctly. Even still, some images need individual work to get the best out of them.

Links for more information:
http://www.sjmurrayphoto.com
Digital camera raw file support
Nikon Digital
Image Science by DXO

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