ISO impact on SHARPNESS ? Why

[David]

I get it that when you slow down the SS and expose the image for longer the NOISE risk/factor increases because of the longer exposure time. I get it that a faster Apeture (2.8 cf 5.6 for example) reduces the risk of NOISE for the same reason: less exposure time consequential to a wider apeture and therefore more light getting into the shot.

But what I dont get is when people say higher ISO means less sharpness overall in said image- Why dont I get it ? Because an increase in ISO means you end up with a faster SS (less exposure time) and the ISO based increase of the amount of light hitting the sensor compensates for the lack of light coming from the Apeture (smaller hole = less light) - no ?

Questions Im asking myself include

A. If that is factual, why ?

B. Does it apply to all focal lengths from say 10mm-400mm

C. Does it apply to all lenses (including L series Canons and other higher quality lenses)

D. Does it apply to all light conditions and

E. Is it the NOISE factor alone that messes with sharpness with higher ISO settings

I dont shoot above 200 ISO even with my 50D which copes with ISO alot better than my 400D did but I leave the AUTOISO alone (sometimes why I dont use it) so ISO/NOISE issues wont be a problem for me unless/until I am doing something like star trails with long long exposure times which I can overcome to a degree with the in camera NR function etc etc.

Thoughts, comments and rueful shaking of the head at my probably misinformed ignorance most welcome