Originally posted by timw4mail That also explains why film takes over-exposure better than digital.

Actually, the reason film handles over exposure is because of the statistical nature of the binary grains of silver halide. That is, each grain either develops to black if by statistical chance it was hit with photons or develops clear if it was not.

Imagine a piece of film with 100 grains of silver halide in it.

Now expose the film with just the faintest amount of light needed to get one grain in 100 to develop to black. Now there's 1 black grain , 99 clear grains.

Next, double the exposure so that 1 of the remaining 99 unexposed grains develops to black. Now there are 2 black grains , 98 clear grains.

Next, double the exposure a second time so that another 2 of the remaining 98 unexposed grains develops to black. Now there are 4 black grains, 96 clear grains.

Next, double the exposure a third time so that another 4 of the remaining 96 unexposed grains develops to black. Now there are 8 black grains, 92 clear grains.

Imagine increasing the exposure until about 50 of the 100 grains are developing to black and 50 remaining clear grains.

Next, what happens if you double the exposure again? What are the chances that the 50 remaining unexposed grains become black? The added light only ensures that each unexposed grain has another 50% chance of turning black so the result is that doubling the exposure only brings the black level from 50% black to 75% black. There are still 25 unexposed grains.

What happens if you double the exposure again to 4X the light needed to create a 50% black negative? Each of the remaining 25 unexposed grains has a 75% chance of turning black. Thus another doubling of the exposure only brings the black level from 75% black to about 94% black.

The point is that it takes exponentially more and more light to ensure that the few remaining unexposed grains become black. That implies that even the brightest highlights have some gradation in tonality.

In contrast, a silicon sensor is almost perfectly linear. Doubling the exposure when the sensor is already at 50% exposure level results in a 100% exposure level and saturation.

Note: I've greatly simplified the math, physics, and the chemistry for this. But the point remains that film has a nonlinear response to light which gives it extra latitude for over-exposure. Silicon sensors are linear but saturate which makes them unforgiving to over-exposure.