Focus distance speed to brightness filter expression

Hi- I'd like to create an expression that links the speed at which a camera's focus distance changes to a brightness filter.

So, for example, if the focus distance changes abruptly and quickly, I want an image to suddenly get brighter proportionate to how fast the focus distance changed. Conversely, if the speed at which the focus distance changes is slow, then there should be very little change in the image's brightness.

Even better, if you could tell me how to make the brightness changes lag by about a half a second after the focus distance changes have occured, that would be great!

Thanks so much!