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 p.2 #12 · highlight tone priority and RAW test | |
When shooting a high contrast scene, ETTR is not generally an employable strategy, unless you choose to clip your highlights to a greater extent than what the camera's meter indicates.
For simplicity's sake, let's consider a monochromatic scene. The histogram represents the distribution of tones in the scene--a bin count of areas on the image of dark shades to light shades. Suppose the scene has a very wide distribution of such tones--i.e., there are parts that are very nearly completely dark (reflect almost no light), and there are parts that reflect a great deal of light.
Now, the sensor, being an electronic instrument, is only capable of measuring a fixed, continuous subinterval of the histogram. Anything to the left of the chosen interval is clipped to pure black (reflectance is so low the sensor registers no light); and anything to the right of the interval is clipped to pure white (reflectance is so high the sensor cannot distinguish whether it is just "bright" or "super nuclear blast bright").
You generally pick the interval to record by adjusting shutter speed and aperture. You can also adjust the ISO, but this falls under a different category which we'll talk about later.
The question of which shutter speed and aperture to choose for a given scene is answered differently by three methods: ETTR, HTP, and the camera's meter, which we'll abbreviate by CM. The answers are different because they have different intents:
ETTR's intent is to maximize signal-to-noise ratio. This is done by picking the right-hand endpoint (the white point) of the histogram so that you clip only "non-essential" highlights. In other words, pick the exposure for which everything that exposes as pure white is something you don't care about seeing any highlight detail.
CM's intent is to match average luminance to middle gray. That is, it picks the interval so that the midpoint is middle gray. Of course, evaluative or spot algorithms are not quite so simple but in all cases, the basis of comparison is the middle gray tone, not the highlight or shadow. So if your scene contrast is very high, the exposure selected by the meter will still cause clipping. This is why Canon offers HTP.
HTP's intent is to overcome the limitation of using a digital sensor to capture a high-contrast scene by trying to preserve as much highlight detail as possible. The idea is to use the CM to obtain the exposure, but then deliberately set the sensor's sensitivity one stop slower. The result is underexposed, but then the shadow and midtone data is pushed one stop back. (The highlights aren't pushed because then they'd clip to white.) Back in the days of using negative film, it was difficult if not impossible to overexpose to the point of completely losing all highlight detail, because the film just keeps getting more and more density the more light hits it. Underexposure was the thing to avoid, because an underexposed negative has no shadow information--it is simply not there.
HTP is simply an extra safeguard, one stop more highlight latitude, to be used in high-contrast situations when the camera's metering might select an exposure for which the highlights are clipped more than acceptable. But because the exposure is still chosen by the meter, it is really just a modified CM method, and ultimately relies on the assumption you want things to appear middle gray. The ETTR method doesn't care about middle gray--all you care about is pushing the histogram as far to the right as is reasonable. The result could be overexposed, or it could be underexposed. (If underexposed, you are stuck because you cannot clip more highlights by definition, yet your scene is too dark--this is where you use exposure bracketing and HDR.)
Changing ISO does not change the amount of light reaching the sensor. Changing shutter speed and aperture does affect the amount of light reaching the sensor. Thus, it is always preferable to use the latter controls to pick your exposure, but when you cannot, the ISO is the only parameter you can pick.
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