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The Myth of Correct Exposure

There's no easier way to ruin an image than to blow the exposure. But just what is "correct exposure" anyway?

Expose an image too little and it will turn out completely black or close to it. Not enough light will reach the sensor to record a viable image. Expose too much and you'll burn out enough pixels to create a useless, washed out image that approaches pure white. On the one side there's "too little" and on the other "too much." Somewhere in between these two extremes lies the correct exposure – that perhaps elusive combination of aperture and shutter speed that is "just right." Somewhat reminds me of Goldilocks and the Three Bears for some reason.

Of course you can always set your camera on auto-exposure mode and let it pick the settings. This way, the concept of "correct exposure" means whatever your camera says it means. That was easy. But you should be aware of how your camera decides what exposure it thinks is correct. Modern digital cameras are wonders of computer technology, but that still doesn't make them very smart. In the mind of your camera, "correct" exposure means "average" exposure. All camera meters since the early days have been calibrated to 18 percent medium gray. They will do their level best to give you an exposure that makes whatever you point them at render as medium. This simple rule works remarkably well, if you average out the results of enough image exposures, but quite a few specific examples prove exceptions to the rule. Glaringly white snow and dark volcanic rock are extreme examples, but I'm sure you can come up with quite a few others without much effort.

So even if your camera doesn't know what snow or dark rock is supposed to look like, you do. You know that the correct exposure for the former has to be brighter than medium and that that latter will look more realistic when rendered darker than medium. Exactly how much brighter or darker though can be a matter of opinion and intent. Do you want that dark rock to look as it does under midday sun or do you want to look more like late evening? Regardless of the time of day an image is actually shot, you can control what an image of it looks like by judicious use of over or under exposure with regard to what the camera meter tells you. In quite a few cases, the mood of an image can change significantly based on the exposure. It's up to you.

But even when you do have a vision of what you want an image to look like, it may not be possible to achieve that exposure. In photography, everything's a tradeoff. To get the depth of field you want, it's necessary to stop down the aperture. And when you do that, you have to compensate for the loss of light by leaving the shutter open longer in order to achieve the same exposure. But when you do, even a slight breeze may complicate your plans if your subject moves during the exposure. Can a blurred image ever have a correct exposure? Perhaps technically so, but it would be a hollow victory.

One could argue perhaps that the answer is to come back another day, one when the wind isn't blowing. But tomorrow it could be raining. Or your may need to be back in the office on Monday and can't come back then to try again. Wait too long and those beautiful wildflowers will no longer be blooming. Or in a different circumstance, fall eventually turns into winter and the mountain pass will close. Even discounting the factor of time and seasons, some situations are effectively once in a lifetime opportunities. That gorgeous deer fawn in the background likely will never be in that exact spot again, wind or no wind.
Often in outdoor photography the brightness range within a scene extends so much that it's simply not possible to render everything as you want. If you want to hold detail in the sky you have to accept forcing more of the foreground into shadow that you'd prefer. What "correct exposure" is for one part of a scene may not be correct (or usable) for other parts. There are tools such as graduated neutral density filters that can help you deal with this in the field, or you can combine multiple exposures later digitally, but either option means accepting that no one exposure is indeed "correct."

Everything's a tradeoff. Do you want the exposure, or the depth of field? You might just miss the shot entirely if you insist on both. This is the reality of things whether you wait and try again tomorrow or whether you go for it right now and let the wind and the wildlife determine your fate.

The only real answer to this whole issue then is "it depends." It depends on what you want the image to look like. Indeed, if you really want an image that looks like a solid black rectangle then by all means underexpose as much as possible. If this is what you want, then the "correct" exposure will be very different from what it would take to render the objects in front of your camera as recognizable subjects. Heck, just leave the lens cap on and you'll get what you want regardless of the aperture and shutter speed your camera is set to. This same line of reasoning extends to someone who wants a pure white image. This kind of shot is easy. But in most cases, the shot you'll likely be after is something more nuanced, and it will require a more carefully selected combination of aperture and shutter speed. And selecting these values often means factoring in far more than just the exposure. As I say, everything is a tradeoff.

So this whole idea of a "correct" exposure is far more than what it may seem upon casual glance. The idea that there's truly one exposure that is correct for an image is a myth. It depends on what you want, and what you can get away with.

Mt. Rainier from Reflection LakesMt. Rainier from Reflection LakesMt. Rainier from Reflection Lakes
So which of these is the "Correct" exposure?
Which one looks the way the scene looked to my eye as the sun started to rise that day? Only I know for sure.


Date posted: June 14, 2015

 

Copyright © 2015 Bob Johnson, Earthbound Light - all rights reserved.
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Related articles:
Exposure for Sunsets
Getting De-Programmed: Learning About Manual Exposure
The Exposure "Stops" Here
Program versus Aperture Priority versus Shutter Priority versus Manual Exposure
Some Thoughts on Exposure in the Era of Digital Photography
Is Exposure Bracketing Obsolete?
 

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