If you’re a RAW shooter you should already know about ETTR, Expose to the Right. The theory behind ETTR is that the best image capture for the most possible information is when the histogram is pushed to the right, to the “bright” side. Just keep adding exposure until that histogram is over to the right side of the graph.
OK, but how far to the right? You definitely don’t want to clip the highlights. DSLR cameras have a clipping warning display, the “blinking highlights,” or “blinkies” as they are commonly called, which shows up on the camera’s LCD. Many cameras will even display the blinkies for each individual color channel, besides the composite luminosity. But remember that the image displayed on the LCD is not the actual RAW file; it’s a jpeg thumbnail created on the fly by the camera. Camera manufacturers have coded in some headroom with the blinkies, as they don’t want customers to be angered at blown out highlights.
Fine. But I would suggest running a test to determine exactly what the correlation is between when the blinkies start, and the actual clipped highlights in the RAW file. You can easily run a test to determine this. Set your camera to Aperture Priority, lock it firmly on a tripod, and aim at any scene. Increase exposure until the first blinkies appear. Note this frame (probably the easiest solution would be to delete any previous frame you shot to get to this point). Now shoot several more frames, using Auto Compensation to add 1/3 stop to each successive frame. Open this series in your RAW file software, such as Lightroom or Adobe Camera Raw, turn on the clipping warning in the software, and check each frame. I’ll bet that the starting frame, the one the camera blinkies said was clipped, is actually not clipped at all. In fact, you might be surprised at just how much headroom you have. With my particular Nikon D800E and a medium toned test subject, I have to add 1.3 stops beyond the blinkies before the RAW file has clipped highlights.
So what’s the point of doing this? Why worry? Well, with digital capture, noise lives in the dark exposures. If you want the best possible data, start with the best possible exposure. With my particular camera I’ll add some extra exposure whenever the subject is such that I can, especially when working at higher ISO values where noice is always a problem. Recently I shot a landscape at ISO 1600. I shot at both the metered ETTR exposure, and at my “extra 1.3 stop” ETTR settings. The difference was remarkable. The first image needed noise reduction. When I looked at the second shot, the one when I had added 1.3 stops, the image on the LCD appeared almost washed out. But when I reduced the exposure in Lightroom (my standard RAW file software), all the noise was gone. In fact, my tests suggest there is even a slight difference when the camera is set at base ISO 100, where I use the camera the most.
Does this really matter? The answer depends on how compulsive you are about quality, the realities faced in the field, and on how the photograph is to be used. Just remember, ETTR is for RAW capture only. And you don’t want to lose an image by blowing out the highlights. There are indeed some scenes with the highlights already maxed out.
Once you know how the blinkies in your camera correlate to the actual RAW histogram, a simple and safe solution — particularly for static subjects such as landscapes — is to set your camera to bracket another frame that is 2/3 stop more exposure than the ETTR histogram on the LCD. When I shoot with my D800E, I know that I’ll be adding that extra bit of light whenever I can.