What is the difference between an under exposed and an over exposed image?

Answer 1

The simple answer is that an underexposed image will appear excessively dark due to insufficient light, whereas an overexposed image may appear excessively bright or even washed out with light.

Answer 2

Very broadly, an underexposed image is too dark, while an overexposed image is too light. Underexposed images generally have "blocked" shadows (no visible detail), while overexposed images generally have blocked highlights.

That said, everything is relative. If you are making a low key portrait or other image, you may deliberately underexpose to block the shadows and have the highlights be dark for a moody look. High key is just the opposite, the object often being that there should be no blacks at all, only grays and whites.

Usually, though, in general Photography, the effect we're after is called "long scale," where there is some dark detail visible in all but the deepest shadow, and some light detail visible in all but the specular highlights, such as the reflection of sun off chrome. This is labeled "correct" exposure for the whole scene. Depending on whether you are working in film or digital, transparency or negative, black and white or color, "correct" exposure may be different for each despite the scene remaining the same!

As an old black-and-white man, I was trained to "expose for the shadow and develop for the highlight," which is a form of image manipulation not available if shooting in color. Conversely, when shooting transparency (slides) we usually expose for the highlights and let the shadows "fall." In my experience, this technique also works well with digital, which tends to "blow out" or "block" the highlights, while we can usually open up the shadows because …

… to further complicate things, today we have image manipulation programs such as Photoshop® which allow us to take considerable liberties with the original, whether it was under, over, or "correctly" exposed. (A tip: if your camera allows it, when in doubt try "bracketing," which is making at least three exposures; one at the exposure you think is right, then one over and one under, how far over/under depending on what your medium is, but usually not more than one full f/stop.)

Answer 3

Severe overexposure is when too much light has reached the film, causing the negative density to exceed the tolerances which allow it to still make a usable print. The denser negative will reduce light transmission onto the paper, causing it to be lighter. Compensating by reducing print exposure will not help much. Highlights will be "blocked up" (basically turn into white patches with no detail) and contrast will be low. Changing the paper grade may help somewhat. Slight overexposure will still render a usable print that may be enhanced by changing the paper grade but can often be detected by the trained eye.

With respect to transparencies (slides) too much exposure lightens the slide and there is nothing that can be done to it since it is the final product meant for viewing. Worth noting is that slide film has less tolerance to exposure error than negative film.

Answer 4

Shadows will be too dark and highlights too light with result: lack of details. Technically it is mapping midtones and 3/4 tones to or almost pure black/white colors without details.