filters - Does longer exposure increase saturation?

I have read that longer exposure times lead to increase/improved saturation and that landscape photogs use ND filters to allow for longer exposures to get better saturation.

Is this true and does this technique apply to both film and digital. If true for digital, why? Surely, the sensels receive the same number of photons for a particular EV regardless of the time.

Also, is the quality of increased saturation much different than what you would get from just pumping up saturation in PS?

Answer

What you may have heard is that you should slightly over-expose your digital shots with modern DSLRs when wanting better saturation. Similar to film, where you'd under expose so that you don't "lose data", if you over expose your digital shot, without clipping the highlights, then you'll have a better saturation without noise when you adjust it in post-processing.

The reasoning behind this thought is that it reduces the noise is shadows since digital sensors are better at picking up information in the highlights than they are in the shadows. Because of the heat and electrical interference within the sensor the lowlights and shadows will have random data picked up due to the sensors own errors. This increases with the ISO setting.

If you under-expose a digital shot by 1/3 or 2/3 stop, like you would in film, then you increase your chance of having to deal with noise in the lowlights and shadows during post-processing. This is a result of having to boost values, and therefore the errors already present from the noise, in the dark areas of the composition.

For this reason, many digital professionals will tell you to over-expose by 1/3 or 2/3 stops so that you can then lower the stops in post production, since the sensor picks up more information in the highlights than it can in the lowlights and shadows. This only really works with the newer cameras with 14-bit RAWs, otherwise you ran the risk of clipping the highlights and losing just as much information as you would if you under-exposed the shot. This works because the highlights and actual like are strong enough, and the values are high enough, that it overrides and electrical and heat interference on the sensor. But again, the results are going to vary based on the ISO and exposure length used.