Why does the sun have to be nearly fully covered to notice any darkening in an eclipse? I was looking at eclipse footage and I noticed that it doesn't get any noticeably darker until the very end when it suddenly all the light is gone. As the moon blocks out the Sun, I would expect that the brightness would gradually decrease as less of the Sun became visible (e.g. 50% as bright when the Moon covers half of it) however judging from all the videos out there this is not true! I took a look at the Wikipedia article, and it says:
"Partial eclipses are virtually unnoticeable, as it takes well over 90% coverage to notice any darkening at all."
"Even at 99% it would be no darker than civil twilight."
Why would this be the case?
I also found this diagram that may help illustrate my question: 

I would expect the graph to be more of a linear shape rather than being so exponential!
 A: Human perception is generally logarithmic. For example, the perceived loudness of a sound is measured using decibels, where an decrease of $10 \text{ dB}$ divides the sound intensity by $10$. So if the eclipse were heard instead of seen, "90% coverage" might mean reducing the intensity from $120 \text{ dB}$ to $110 \text{ dB}$, a small change.
Perceived brightness is the same way. There's a huge range of light intensities that we see every day: direct sunlight is ~100 times brighter than indoor lighting, though both look fairly bright to us. So a 90% reduction wouldn't make the sky look dark at all.
The shape of the graph 'looks like an exponential' because the $y$-axis is the log of the intensity. This is done so the graph somewhat represents "perceived brightness" vs. time.
A: The graph looks exponential because the vertical axis is logarithmic! If you were to re-plot it as linear lumens per square meter, it would be much more v-like, or even u-like.
It so happens that a logarithmic plot matches our subjective perception of light intensity better than a linear one would. That's a result of our eyes having evolved to work well in an extremely wide range of different amounts of light.
A: Based on my own anecdotal evidence, it doesn't. Several years ago there was a partial solar eclipse in my area. I don't remember precisely how much of the sun's disk was covered - it wasn't much, surely nowhere near 90% - but I do remember getting out of the house in the morning, thinking "hmm, it's quite dark today", then having the eerie realization that the sky was perfectly clear, with none of the haze or clouds I was expecting. So yes, the darkening is noticeable.
A: Any shadow, either that of a small ball or a celestial body(especially spherical) consists of two parts umbra and penumbra. They are separated by a distinct border. Think of the moon(object) casting its shadow on the earth(screen) due to the sun(light source). Due to relative distances the moon's umbra is small, so only a small portion is in extreme darkness. On the other hand a relatively large penumbra of even darkness is formed.This darkness is the same in all places of sufficient proximity from the epicentre of the eclipse regardless of how much of the sun is blocked out by the moon when viewed from the location in question.
So, in response to your question when the sun appears nearly fully covered from the location in question it means that location comes under the umbra region of the shadow. That's why it suddenly gets dark.
