Expose to the Right, Don't Trust your Luminance Histogram

So, I just read this great post on Luminous Landscape about noise and posterization and why you should expose to the right. I decided to test this out with a sunset shot, which is where I usually tend to get posterization with even the lightest hand in post. I wasn't surprised to find that I could do much more with my colors, since I used to encounter problems when exposing my sunsets to the left to "saturate" the sky. I was surprised to find how little you can actually trust your luminance histogram.

Why expose to the right? You should really read the link the the Luminous Landscape post, because they explain it much better than I do, but it goes a little something like this.

Your histogram shows you how much light you have in your dark, mid and light tones. Anything on the left side of the histogram is black, anything on the right is white and anything in between is red, green, blue and a mix thereof. What I didn't know until I read that post is that an image with the histogram peak sitting to the left has less detail compared to one sitting to the right due to the way your camera's sensor records light. The shadows get roughly 1/16th of the available data space compared to the highlights. That, therefore, means if you make your image a little brighter instead of a little darker you'll have more headroom when you start pushing your pixels in post, and less noise to boot. You just need to be very careful not to go too far to the right, or you clip your highlights and then you just loose all detail to the point of no return.

This is where you need to be very weary of what your luminance histogram is telling you! Yes, expose to the right and keep an eye on that right hand slope, but if you don't know what it's really saying you may be clipping one of your RGB channels.

The image at the top of this post is a perfect example of when you need to keep an eye on your channels. Basically, in a sunset image you have a dark blue sky and some glowing orange highlights. This puts your blue channel somewhere near the bottom of the histogram, your green near the middle and the red all the way up at the top. If you just look at the luminance histogram, which is an average of your RGB histograms, you could be clipping your reds and still think you have some room to spare. I thought I had at least another 2/3 of a stop I could have gone to the right, but when I loaded this into Aperture I found my red channel highlights went right up against the wall. Luckily, shooting in RAW gave me the capability of reeling those highlights in. If I would have gone that extra 2/3 stop, though, I would have had some really obvious red clipping in form of a big orange spot at the hottest part of the picture.

Before I go, just let me say that exposing to the right works especially well with higher ISOs to avoid excessive noise and in images with color detail in the shadows which you might want to recover. It's not something you always need to do though. You camera's auto exposure probably does a good enough job in most cases where you can trust what your meter is telling you, but if you ever find you're getting really blotchy or noisy pictures, just give this a try.