A few remarks on 'correct exposure'

Started Jul 20, 2012 | Discussions thread
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Great Bustard
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A few remarks on 'correct exposure'
Jul 20, 2012

There seem to be a fair number of people who seem to be missing the mark on what exposure is, and this comes from the misuse of the term "underexposed".

Many (most?) seem to think that the exposure is how birght or dark the photo appears. Well, as well all know, we can display the photo as bright or dark as we want. The exposure is the density of the light falling on the sensor. More light, more exposure, less light, less exposure, regardless of how bright the photo is displayed.

In a very real sense, everytime you use anything other than base ISO, you are "underexposing" the photo, since less light is falling on the sensor at higher ISOs than would otherwise fall on the sensor at base ISO using a wider aperture or longer shutter speed.

Let's consider the simple case first -- when the DR of the scene fits within the DR of the sensor.

To get more light on the sensor, you have to use a longer exposure time, which increases the risk/amount of motion blur and/or camera shake, or you have to use a wider aperture, which necessarily results in a more shallow DOF and often a less sharp photo.

So, in this case, when the DR of the scene fits within the DR of the sensor, the "correct exposure" is the longest exposure that fits within the motion blur / DOF constraints imposed by the necessary shutter speed.

On the other hand, more light means less noise, so even if the DR of the scene fits within the DR of the sensor, if the darker portions of the scene matter more, then the photographer may intentionally blow the highlights to make the darker portions of the scene less noisy.

But let's say that there's enough light so that motion blur, camera shake, and DOF are not an issue -- there's still the matter of when the DR of the scene exceeds the DR of the sensor.

In this case, the competent photographer has to choose which portion of the DR to capture, unless they are taking and merging multiple exposures. So, the "correct exposure" is entirely subjective, as it depends on the portion of the DR that matters most to the photographer.

So, when someone presents a photo that has the shadows pushed way up, that doesn't mean the initial photo was "underexposed", since the reason for using a lower exposure and pushing the shadows was to preserve the highlights in the photo that would have been blown with a greater exposure.

One can argue, from a subjective point of view, that a "better" photo would have been made with a longer exposure and blown highlights, than a lower exposure and pushed shadows. One can also say that the photographer "should have" taken multiple exposures and merged (the motion in the scene and use of a tripod become critical factors in this scenario).

However, it is important to understand that "correct exposure" is largely subjective, and dependent not only on the scene, but the capabilities of the camera and the artistic intent of the photographer.

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