The reason exposure compensation exists.

That little +/- function on your digital camera, do you ever use it? Usually available as a button, sometimes through a navigation control or menu, it allows you to alter the exposure the camera chooses. It would be wonderful if your camera's ability to capture an image was without flaw or imperfection, but the truth is that you often have to adjust some parameters. Chief among those is the exposure compensation button.

The exposure is a combination of sensor sensitivity (ISO), the amount of time that an exposure takes (shutter speed), and the amount of light that the lens allows into the camera (aperture). Each of these can change up and down by values we call a stop; a stop is just a doubling or halving of light. Change ISO from 100 to 200 means the sensor is twice as sensitive to light (needs one-half the light). Decreasing shutter speed from 1/250th of a second to 1/125th of a second means twice as much light hits the sensor. Altering aperture from f/8 to f/5.6 doubles the amount of light coming in from the lens. Put these all together at the same time and it means you are letting 3 extra stops of light into the camera, that's 8x as much before the changes.

Exposure compensation will affect one of these three parameters depending on what mode your camera is set to. Changing from 0 to +1 lets in one stop more light. Changes from 0 to -1 lets in one stop less light. The entire purpose of this is to alter the outcome of the image, to make a better picture. Examine the above photo composition and notice how the changes in exposure have altered the rendering of the statue's face. Too much light (a) causes light areas to be washed out. We say that the highlights have been blown out. Too little light (f and g) causes dark areas (shadows) to be flat with no or little detail.

Have a look at the areas surrounding d and e. The statue should be white, or certainly something close to it. Both those sections are too dark for what they are supposed to represent. We see this typically in scenes with snow and sand; particularly on sunny days. More light is required to make these brighter rather than darker. The +1 exposure setting brings out the highlights without causing them to be blown out as we see on the +2 photo. The best choice would be to adjust the exposure compensation feature to +1, maybe even +1⅓ (plus one and ond one-third stops). Verification could be done by referring to the photograph's histogram, but that is another lesson.

Such corrections are less important when shooting in raw exposure mode, although it is my firm belief to always bring the best image you can into any post-processing app such as Photoshop. Raw can correct for a myriad of flaws - keeping in mind that not all raw files are the same. Raw images from compacts that are capable of creating them tend not to be as flexible as those made from DSLRs and large-sensor mirrorless cameras. Although I shoot only in raw mode I still adjust my exposure to end up with the best image I can get. It never hurts to do your best.

Thanks for reading.     Ericspix     Eric Svendsen

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