horizontal keystone, what is it?

i notice in many product descriptions projectors having horizontal keystone and people wanting it when buying a projector, what is horizontal keystone and should i be looking for it when purchasing a projector?

[geewhizz]

Keystone Correction

'Keystoning' is the name given to the effect on the projected image when the projector sits below or above the centre of the screen. Keystone correction counteracts this effect by digitally compressing the image at the bottom or top, resulting in a squarer, more professional image.

See Here
for more info

[cinesom]

Hi its always nice to have horizontal and vertical keystone - but for emergency setput ups only. Why, because otherwise you loose resolution! If you can, when you are seting up the projector, set all keystones to 0, correctly position the projector to the screen. A quick test to show you what I mean by lost resolutuon:
Put on test disc (Joe kane or whatever) with needle pulse display, keystones zero, now step the keystone up or down - see what i mean?!

[ELV]

It's the way the pj compensates if it's not on the same horizontal plane as the screen, it would be bigger on one side than the other,



so to get round this the pj lets you squre it up making the image square again, vertical keystone is more common but with many pj's mow having lens shift the need for horizontal keystone has lessened, it's better if you can get away with not using it at all, ie having the pj dead square with the screen as there is slight image loss when it's used to extremes, if you are gont to have bother sighting your pj with the screen then it would be handy,if not then it's not realy needed, but i would get one with lens shift if possible.