Exposure is not how dark or how light an image looks!

Agreed. How would you define over and underexposure? Or should the terms best be avoided ;-).

From a beginner's perspective, "Over-Exposure" is an exposure higher than what you wanted, and "Under-Exposure" is an exposure lower than what you wanted.

The exposure that you want, will vary with your goals for that shot.

For instance, your goal might be to find the maximum exposure that yields sufficient depth of field without unwanted motion blur. If your aperture is too small, or your shutter speed too fast, you may have a lower exposure than what you wanted. This can reasonably be called "Under Exposure". Note that if your ISO was set too high, the resulting camera produced JPEG may look much too bright, even though your image was under-exposed.

The term “overexposure” really ought to only be applied to an excess of sensor exposure (Scene lighting + Aperture + Shutter Speed) resulting in unintended clipped highlight detail at base ISO. While undesirable highlight detail clipping can also be caused by an excess of pre-ADC ISO brightening being applied to a (non-clipped) base sensor exposure, this needs to be considered independently, IMO.

While “optimal” exposure could be considered as the brightest important highlight detail being recorded just below clipping at base ISO, with significant exposure below that being considered “underexposed”, if there isn’t enough available light to fully saturate the sensor, I would also consider setting the maximum amount of sensor exposure possible without compromising the necessary SS/Aperture settings required for avoiding DOF and/or motion blur issues to be “optimal” exposure in that situation. Using a lower than optimal exposure here with an unnecessarily high ISO setting should also be considered “underexposure” in my book. Whether the resulting mid tones initially appearing too bright or too dark have nothing to do with being optimally exposed (at least for the RAW shooter). The jpeg shooter should also try to maximize sensor exposure when possible, but sometimes compromises will have to be made if an optimal SOOC jpeg is of primary importance.

Density increases with exposure regardless of whether it is transparency or negative film. Negative film simply offers more latitude for error with acceptable results.

Digital image data is derived from recording and converting an analog sensor's signal intensity which increases proportionally to the exposure received.

All of the above have density or saturation limits which are directly related to increasing exposure and which may produce over exposed images which are too light relative to the desired final image. They all also have sensitivity limits which with decreasing exposure may produce under exposed images which are too dark relative to the desired final image.

I see no conflict at all between those two statements. Exposure is measured in lux-seconds. ISO setting has nothing to do with it. The best exposure for the situation will depend on the ISO chosen, although I think that doing it in that order puts the cart before the horse. Decide on the exposure, then pick the ISO.

The first sentence above is incorrect. At zero exposure, negative films have their lowest density, development held constant. At zero exposure, 'chromes, reversal films, transparency films (whatever you want to call them) have their highest density.

True. And furthermore:

From a expert's perspective, "Over-Exposure" is an exposure higher than what you wanted, and "Under-Exposure" is an exposure lower than what you wanted.

Over exposure produces images which are lighter than desired in the final image and is the result of capturing more light than required. Under exposure produces images which are darker than desired in the final image and is the result of not capturing enough light to produce the desired final image. Over and under exposure can occur regardless of the ISO / EI (exposure index) chosen to produce the desired final image.

Producing the desired final image requires accurate exposure for the lighting conditions combined with the required signal or image processing. Optimum exposure is one component of the choices and compromises made by the photographer to balance light, exposure, and processing for the best achievable result.

Normal latent image has no density to speak of ;)

I simply said that "Density increases with exposure" but didn't specify whether that meant greater or lesser exposure. The implication which was intended is that exposure density is directly related to the amount received (how much or how little) by any type of film.

If you're using negative film with standard negative processing, increasing exposure will increase density; and with film suitable for producing transapancies, decreasing exposure will increase density if using a reversal process to reverse the density from highlight areas to shadow areas. As the process link provided demonstrates, reversal processing can be done with a variety of films including some black & white negative films.

Yes, obviously too much or too little light reaching the sensor will make an image lighter or darker than desired.
But too much or too little light reaching the sensor is not the only reason images can be too light or dark.
The exposure* can be the best possible for the dof and blur requirements but the image can still be too light or dark if an inappropriate iso was set. In this case the exposure* did not cause the image to be too light or dark so it is not correct to say the image is over/under exposed.

When someone says an image is over or under exposed they are in effect just guessing as to whether the best exposure was used or not by the photographer unless they are certain of the photographer's artistic intentions.

What they really mean when they describe an image as over/under exposed is that they see the image as being too light or too dark.

So for the sake of clarity and consistency it is safer and clearer in meaning if the terms too light or too dark are used when describing image lightness that is not desired.

* exposure - amount of light striking the sensor per unit area while the shutter is open.

I don't know that we have any disagreement. It's a balancing act of choices made by the photographer to achieve a desired result. Any errors or poor choices will have a negative effect on the final image you've envisioned.

I was expanding on your post to describe situations where saying an image is over/under exposed is not appropriate or correct.

Well... I guess we'll have to wait and see what develops.

Thanks. There are definitely many ways to spoil an otherwise perfectly good image — and I suspect we've all taken our turn at doing so on occasion.

That's just plain weird. You could also say velocity increases with brake pedal application. Or sunburn risk decreases with exposure to sunlight.

While underexposure and overexposure will make an image lighter or darker, to ensure the best possible signal to noise ratio and maximum dynamic range, optimal exposure is about recording as much sensor exposure as possible without blowing any important highlights (or compromising DOF/motion blur etc.) and can easily result in an initially too light or too dark image.

Clearly, this RAW image initially appears too dark, but was it underexposed?

Not if you wanted the highlight detail intact it wasn't...

Sure, but image brightness and exposure should be considered and handled independently, they are not the same thing.

Please see my previous reply. It's the processing, standard or reversal, which determines where density accumulates in an image. It's not weird that we have the option of reversal processing of film which also reverses the effect of exposure increases or decreases.

Fortunately, there is no switch in your car that reverses the function of the accelerator and brake pedal.