Some Thoughts on Localized Adjustments During Raw Conversion
There once was a time when all adjustments made during raw image conversion were global, affecting the entire image equally. Nikon's Capture NX was one of the first raw converters to introduce localized adjustments back in 2006. Since then, Adobe Lightroom and Camera Raw (ACR), DxO Optic Pro and others have followed suit in varying degrees. But if Photoshop can do all this too, why should you care?
Before the explosion of digital photography, Adobe Photoshop was the standard software for image editing. For the most part it still is. But camera raw images presented Adobe with a dilemma since the inner workings of a raw file is vastly different from a normal multi-channel color image in either RGB or CMYK. A raw file is an odd beast indeed. Comprised of an ingenious arrangement of alternating pixels each representing just a single red, green or blue value rather than all three, it takes special software to intelligently interpolate the remaining values to form a usable image. Beyond that, the scale of values used for each pixel doesn't conform to the way our eyes register increasing brightness. Known as "linear gamma," this system is more amenable to exposure adjustment than color spaces such as sRGB or Adobe RGB.
Because of these differences, raw file edits are inherently non-destructive. Unlike traditional image editing, it's not really practical to directly change raw pixels since what you see on screen is the combined result of interpolating a number of neighboring pixels of different colors. All changes made in a raw converter exist only as a list of instructions to be applied to the image on the fly as it is rendered on screen, and baked into the final file created when the raw file is finally processed.
The closer you can get an image to the way you want it while still working in a linear gamma space the better. While this holds true first and foremost for exposure adjustments it also pertains to other adjustments as well. Removing a color cast or correcting white balance may not change overall exposure, but it might shift at least one individual red, green or blue channel value by an equivalent degree.
For a long time the problem was that such changes were only possible for an image as a whole. As raw converters became more sophisticated, what you could put in the instruction list for conversion became more sophisticated as well. As far as I know, the first software to provide a way to target steps in this instruction list to only portions of an image was Nikon's Capture NX, the innovation coming in large part from Nik Software's U-Point technology. Apple's Aperture did come out with a method that looked similar but wasn't really non-destructive, requiring that edits be rendered internally as actual pixel data. Adobe's Lightroom program was built from the ground up as a non-destructive editing program. Although the basis of how raw files get converted in Lightroom is the same as in the ACR Camera Raw plug-in for Photoshop, somehow new features always seemed to show up in Lightroom first.
There was a time when it seemed like Nikon was positioning their new Capture NX software as direct competition for Photoshop. Bragging about its capabilities, they actually put out a piece stating that you really didn't need Photoshop anymore for optimizing photos. That was several years ago now and Nikon has since taken the page down from their site, but it caused quite a stir at the time among Nikon shooters. Whatever all that was about, these days Nikon and Adobe are back to playing nice with each other, but it is interesting to note that shortly after this was when the first official release of Adobe's own non-destructive raw file editor Lightroom made its debut.
Since all raw adjustments are non-destructive, you can think of the steps in the instruction list built up during raw editing as the equivalent of Photoshop adjustment layers even though they don't really look as such in the raw convert user interface. As with adjustment layers, localized raw edits are targeted by means of layer masks, but just like the adjustment layers themselves, the masks are generally not explicitly shown to the user. Whether it be with control points in Capture NX, pins in Camera Raw and Lightroom, or the Multi-point Color Balance Tool in Dxo Optics, the ability to perform localized edits seems to be implemented along the same lines in most raw converters that are capable at all of targeted adjustments. The user picks points both to be modified and optionally to be locked from possible modification. Based on these points, the program internally builds a layer mask by looking for surrounding pixels with similar color and brightness. While these programs generally does a good job of building this mask, you probably won't get full control it &mdash a blessing for some users but a curse for those more skilled in Photoshop masking.
So which edits should you make in your raw converter and which in Photoshop? As with many things in the digital darkroom, the answer depends on your needs. Some users will only need the edits possible in a capable raw converter. Beyond some point though, you'll need the richer set of tools found in Photoshop. While the changes you make in a raw converter remain fully editable in that raw converter, once you press the button and convert the raw data to a normal RGB image those edits become locked in. As such, if your needs do include Photoshop, I'd recommend focusing your raw edits to those that both require the greatest degree of change and also those you are unlikely to need to undo or modify later. Global edits for white balance, exposure and the like almost always fit these requirements, but deciding which localized adjustments to do when may require more thought. Many can be done safely, but you will improve your chances of not regretting a change if you keep a light hand on raw adjustments. Strive to get changes in the ballpark without overdoing things. You are better off needing to add slightly to your raw edits later in Photoshop than to risk needing to undo something that is now baked in.
Over time, I would suspect that Photoshop, Adobe Camera Raw and Lightroom might just merge, as the originally stand-alone ImageReady became absorbed as part of Photoshop as the need for web graphics reached a critical mass. For now though, while these programs have a degree of overlap, they also have quite a few differences. Putting some thought into what edits to make in each will let you get the most out of all of them.