I’ve mentioned this technique to a number of people I know and referred them a couple of websites for an explanation. Most of the time I need to then discuss what those explanations meant. This is my attempt to create a simplified explanation of the technique known was “exposing (to the) right”. I thought I’d share it here as well – and create a discussion of its pros and cons.
So you’ve become a pretty competent photographer with your digital SLR camera. You know how to balance shutter speed, aperture and ISO to capture the kind of scene you want in your photos. Now it’s time to start looking for efficiencies. How can you capture more detailed, less noisy images with the equipment you already have?
One technique that’s made inroads as referred to as “exposing (to the) right”. The term is named thus because use of the technique leads to the histogram representation of the image being shifted to the right on the X axis (which is mapped to the brightness level of the photo). But to understand why we would want to expose to the right, first we need to learn a little more about how your digital SLR camera captures that image.
Why We Do It
Optical technology today still being far inferior to the human eye, your digital SLR has a much more limited range in which it is sensitive to light. In practical terms, your camera has 5-7 F-stops to work with. Anything lower or higher than this range is simply rendered as black or white (respectively).
Let’s refer again to the histogram representation of an image. A histogram is merely a graph displaying the distribution of brightness in your image, with the darkest part of the image represented on the left, to the brightest on the right. Logically, from what we already know about a digital SLR camera having 5-7 F-stops of usable range to work with, in order to capture the most detail in our image, we’d want to place the majority of the image squarely in the middle of the histogram with plenty of breathing room on each side. No chance we’ll bleed off detail that way.
But that’s not how your camera actually works. I’ll quote from the source material in which I’m adapting/dumbing down this entire FAQ to explain:
A 12 bit image is capable of recording 4,096 (2^12) discrete tonal values. One would think that therefore each F/Stop of the 5 stop range would be able to record some 850 (4096 / 5) of these steps. But, alas, this is not the case. The way that it really works is that the first (brightest) stop’s worth of data contains 2048 of these steps â€” fully half of those available. Why? Because CCD and CMOS chips are linear devices. And, of course, each F/Stop records half of the light of the previous one, and therefore half the remaining data space available.
This realization carries with it a number of important lessons, the most important of them being that if you do not use the right-hand fifth of the histogram for recording some of your image you are in fact wasting fully half of the available encoding levels of your camera.
How To Expose To The Right
So let’s attempt to make the most efficient use of our cameras. This means making use of the most step-heavy part of your camera’s sensor – the first (brightest) stop.
How do we do this? Well, it’s actually pretty simple to do. Refer to your camera’s manual to see how you can view the histogram of an image after it’s shot, and turn that option on. Line up the scene you want to capture as per normal, choosing the shutter speed, aperture and ISO settings you typically would. Since enormous memory cards are cheap these days, take the shot.
Now, go into review mode on your camera and take a look at the image’s histogram. Assuming you’ve taken a picture of a scene where the light levels are all relatively within the same range, you might end up with a histogram looking like this:
If you’re in a computer-controlled mode such as Tv (shutter priority) or Av (aperture priority), you’ll need to look up the “Exposure Correction” (EC) feature of your camera. On the Canon EOS 350D (Digital Rebel XT), the EC button is on the back of your camera next to a little square with a + and – sign. (The same button has a dual use in setting Av in Manual mode.) Hold down the button and use the jog dial on the top of the camera to move the exposure bar indicator in the positive direction (to the right). Moving it to the right about one full F-stop should be sufficient.
If in doubt, simply snap the photo and examine the histogram to see how far to the right you’ve shifted the exposure.
Very important: You absolutely do not want to go overboard and end up losing detail by shifting too far to the right (a phenomena known as “blown highlights”), so account for some breathing room on the right. Your camera will likely also warn you if you’ve blown highlights by displaying flashing black patches on the image itself.
Example of a histogram properly shifted to the right:
We’ve covered why we want to expose to the right – to capture more detail / make better use of the dynamic range of your camera. That’s a nice hypothetical, but how about a look at the real-world results?
Well, take a look below. The member PacAce on the Canon Digital Photography Forums took three exposures: A) was taken as the correct exposure according to his camera. B) was shot 1 F-stop underexposed. And C) was “exposed to the right” – the bottom left image being the original overexposed image, and the bottom right image with its brightness/contrast readjusted to look normal:
Now let’s take a look at a 100% crap of A), B) and C) (the brightness/contrast corrected version):
High ISO noise is virtually eliminated from the image he “exposed to the right”.
Make note: This technique is a wonderful way to make the best of low-light situations.
When Not To Expose To The Right
You don’t want to “expose to the right” if you prefer not to do much post-processing work. Each image taken using this technique will require brightness/contrast corrections to look normal again.
Following from the point made above, you don’t want to “expose to the right” if you don’t like to use RAW for your image file format. While the post-processing of the image will still be fairly effective on a JPEG, the nature of the format requires that the camera throw out the data it’s not making immediate use of. Exposure correction gets dicey.
You don’t especially need to “expose to the right” if the scene you are going to capture does not have a wide dynamic range. Save yourself the post-processing work and get the correct exposure the first time.