Expose to the Right

Expose to the RightGreg McMillan

To embrace the term “expose to the right” you have to use what I would call the most important tool on your digital camera. That tool is the histogram. All digital SLRs will have it and it’s becoming a standard feature on most point and shoot digitals. This feature may not be for everyone, but if you’re ready to take the next step in your digital photography, using your histogram to improve your photos might be the thing to try.

The histogram is a graph that represents the brightness distribution in the image. The histogram on the camera and in photo editing programs like Photoshop represents the brightness distribution of an 8 bit JPEG image which has 256 levels of light, ranging from 0 (black) to 255 (white).

Exposing to the Right of the Histogram

Figure 1.

(1) You want to expose to the right to get the full range of the exposure. (2) This graph shows six stops across a 12 bit camera image. Most cameras these days are 12 bit cameras (their analog-to-digital converters have 12 bit resolution) unless you get into the real high priced models, then you get into 16 bit processors.

Linear Distribution

Figure 2.

So this first graph shows what the raw data looks like coming out of the chip. The first stop has lots of room. There are 2048 levels, which mean more accurate reproduction and smoother tonality. Now remember our histogram and how it shows its representation of the different levels of brightness in an image, it goes from black on the left to white on the right. This graph does the same thing. The next stop has 1024 levels, then 512, 256, 128 and down to 64 levels where you won’t get much tonality out of it at all.

The camera’s chip reacts to light in a linear way that the eye doesn’t, so we have to map the data, from this linear distribution of the chip, to a way that suits the eye. This is done by (3) remapping it into a Gamma corrected distribution by way of the image processor that’s in Adobe Camera RAW (for those who shoot in the RAW format) or by your camera’s processor if you shoot JPEG.

Gamm Corrected Distribution

Figure 3.

As you can see, when you get down to your last stop, you only have 64 choices, or places to put all that data, and when it gets stretched out, you begin to add noise, because something has to occupy that space. It’s like taking a piece of pizza dough and stretching it out; eventually you’re gong to get some holes in it and in digital imagery, these holes are noise.

Now I’ll show you two images. (4) One of them is stretched and the other is exposed to the right. You can’t really tell just yet which is which, right?

Figure 4.

Let’s take a closer look (5) into the shadow areas. The one that’s exposed to the right is much cleaner.

Figure 5.

Okay, for those of you who are shooting RAW, or would like to shoot RAW, one important thing to remember is that the only controls that affect the RAW data going to your image processing software, like the Adobe Camera RAW in Photoshop, are aperture, shutter speed, and ISO. Everything else doesn’t matter. The image you see on your camera’s preview screen actually gets processed much like a JPEG. With that being said, if you want to see a more accurate exposure reading using your histogram, go into your camera’s settings and turn your contrast slider all the way down. The histogram is built from processed data, not RAW data. But don’t do that when shooting JPEG. That would be disastrous.

There are a few things you can do to expose to the right. Okay, you’ve taken a picture, and now you look at your histogram. The photo looks to be pretty good. That’s what I thought when I took this image. (6)

Figure 6.

It looks okay, right. Maybe a little on the dark side, but in general, it looks okay. But look at the histogram. (7) Most the exposure information is on the left side, or in those smaller portions on the tonal ranges.

Figure 7.

Now for my second attempt, this is after I examined the histogram on the back of my camera. (8)

Figure 8.

Thinking back to Figure 2, remember where the greatest tonal range was? It was on the whole right side of the histogram. One way to achieve this is to adjust your exposure compensation. My first image was set at -1 and the good one was at +1 on the dial (9), even though it changed the way the exposure was made in terms of shutter speed and aperture.

Figure 9.

A better test for this would be to go to Manual and work from there because you can try using a little longer shutter speed or a wider aperture setting to get the same result, depending on the depth of field you want.

Now let’s compare the two histograms. (10) What kills the integrity of my first image is the fact that a great deal of the exposure information is on the left where the tonal ranges are quite small.

     

Figure 10.

One last thing; the original shots look different but yet my first two comparisons were actually quite similar, right? That’s because even though I exposed to the right, I still wanted to adjust the image in Photoshop to optimize it. By exposing to the right, I know I’m not going to end up with a bad file with some degree of noise in it like my first photo had. And my first photo was adjusted for optimization as well and we all can see what happened to it. Here are the two shots and their histograms after being adjusted to look the same. (11)

Exposed to the Left then optimised.      Exposed to the Right then optimised.

Histogram for Exposed to the Left then corrected.      Histogram for Exposed to the Right then corrected.

Figure 11.

Tip: Use a tripod. It forces you to take more time when thinking about your exposure, and composition, for that matter.

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  1. Pingback: Back to Basics (Episode 5): RAW vs. JPEG, Histograms and “Expose to the Right” | Learning DSLR

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