More on the Subject of Blinkies
I've received a lot of feedback and questions since my recent article on the blinking highlight display most cameras are capable of. As with many good things, blinkies aren't quite perfect. And you have to know your camera to understand what it is trying to tell you.
One frequent topic of questions relates to the source of the blinking highlight display. You might think that this would depend on the image file mode you are shooting in, but this isn't so. Whether you are shooting in Raw, JPEG only, or Raw + JPEG, the camera always builds the entire camera back preview display for the LCD screen out of a JPEG rendition. No doubt in part this allows the camera manufacturers to keep things simple, but it's more than just that. Raw files are downright complicated beasts and don't lend themselves to easy analysis. If you think about it, since each photosite (pixel) records a single channel of red, green or blue light, the camera might be able to evaluate each one in the context of its filtered color and display it as blinking or not based on how close to maximizing the available headroom it may be. But that would only take care of one third of the total image since the remaining two colors for each recorded pixel has to be interpolated in order to create the final image. That is, for a given red pixel, the camera could theoretically show you whether the red channel were close to being burned out, but since no blue or green light even got recorded at that point, it couldn't easily do likewise for the other two colors. The same is true for every pixel in the image. Even as the recorded color may vary, there will always be two colors that weren't recorded. That's just the way that raw image capture works with a Bayer mosaic sensor.
In order to show you a plausible image at all, the camera must interpolate values for those two missing channels at each point by looking at the values for those channels that were recorded by adjacent pixels, and factor in whatever trends and patterns the camera manufacturer was smart enough to write into the software that drives the camera. In other words, the camera must build some kind of de-mosaicked composite image in order to calculate highlight clipping at any given point. While it could not about one channel, the other two take some work. And since it is already interpolating the raw data into an image to show you what you just shot on the camera back LCD, why not simply use that one for the blinkies as well? And that's exactly what it does. There's really no reason to even contemplate building a different interpolated image for blinkies than the one it already generates for playback.
Now, it may a natural assumption that this is also the same image recorded to your memory card when you shoot JPEG or Raw + JPEG, and clearly all these JPEG renditions must be related, but keep in mind that you can select the size for JPEG capture to be anything up to the full resolution of the camera. You don't have to save full size JPEG images, neither are you constrained to only shoot JPEG's the size of your LCD camera back display. Yet regardless of what size JPEG you choose, the camera (at least Nikon) only shows blinking highlights when not zoomed in. If you want to zoom in to check exactly where and what is blinking, you can't. So the blinking highlights are at best built from a down-sampled preview JPEG, not necessarily the final one saved to your memory card. That means there can be quite a few tiny, specular highlights that are burned out that you could never see via the blinking highlights on your camera. You have to assume that the ones your camera can't show you at that zoom size are either small enough not to be relevant in the final image, or that other areas that are big enough to show will tell the tale of overexposure if you have a problem.
But of course, do you have a problem at all? JPEG images on your camera back have to be rendered as sRGB images since the LCD simply isn't capable of doing justice to a wider gamut color space. Only very expensive desktop monitors can approach the full gamut of Adobe RGB, and none that I'm aware of (or at least that I could afford) can faithfully show you the full ProPhoto RGB color space. So just because a particular part of an image would be burned out when constrained to the sRGB color space gamut, it may still fit comfortably inside ProPhoto RGB. Adobe Lightroom always uses ProPhoto RGB internally, and Adobe Camera Raw gives you a choice of the target color space to convert raw images into. And you are using ProPhoto aren't you?
So areas your camera thinks will become burned out highlights may in fact be safe after all if you work on your images in a wider gamut color space. I don't believe this means you can cavalierly ignore the blinking highlights though since there could always be smaller areas that are in danger of maxing out and losing highlight detail. But it does mean the blinking highlights should be taken as a warning, not as an absolute certainty in terms of exposure problems.
Some readers have argued that highlight recovery can save the day to retrieve lost highlight detail in otherwise burned out areas. That is true up to a point, but it's important to keep in mind that this recovered detail is only a best guess as to the detail that was lost. Once any color channel hits its limit, everything above that limit will be lost permanently. The detail "recovered" isn't somehow pulled back from some secret extra part of the raw file. More correctly, this detail is interpolated along with the color channels not recorded at each point. So if a red pixel is burned out (a typical scenario when photographing strongly colored red subjects) but the blue and green channels contain usable data, the lost red detail may be able to be guessed at with at least some degree of confidence. The highlight recovery feature available during raw conversion is really just more elaborate interpolation added to the complex guesswork already done by the software. If you rely on this to save your blinkies, you are depriving yourself of the full image capture resolution your camera is capable of.
So what should you do? My advice is to get to know your camera. Learn what it is trying to tell you via the blinking highlight display. Learn it either the hard way through trial and error, success and failure, or take the time to learn it methodically before you need to actually put it to the test in the field. If you really want to push "Expose to the Right" as close to the edge as possible, shoot some controlled tests and find out what your camera is capable of. Find a good contrasty scene and set your camera exposure so that blinking highlights just start to show up. Take a shot. Then bracket additional shots above and even slightly below that point. Go as far as a couple of stops above the blinkie danger signal to make sure you have enough images to work with, then open the lot of them on your computer and find out what you actually have to work with. You will likely find that at least in a wide gamut color space such as ProPhoto RGB the first in-camera blinkie shot is in fact completely free of burned out highlights. Examine the remaining shots similarly to discover your true point of no return. It's best to run this series of tests with multiple subjects and under different lighting conditions too so as not to base everything on one particular situation. But with modern cameras having such excellent exposure latitude you should find that, in a pinch, you can go at least somewhat beyond the blinkie threshold without undue fear of actually losing detail.
Should you give up worrying about blinkies? I wouldn't. Not unless you like skating on thin ice to temp your fate. But knowing as much as you can about what those blinkies are telling you can allow you to make the most of the image capture capabilities built into your camera.