That question is hard to answer, can't you just Google it or read the 'prescription' haha?
Seems as if the Speedlite 420ex can't keep up with the speed of the 50d. Many missed flashes.
Yes, you can. It has a Hot Shoe Mount.
Any Speedlite!
It does not work with my xsi - the picture is over exposed
yes
Speedlite is the trade name for Canon's line of external flashes. The series of flashes include the Speedlite 90 EX, 220 EX, 270 EX and 270 EX II, 320 EX and the 380 EX.
Yes most defenitley.
No ofcourse I cannot. It's a pretty silly question. Do try to be more sensible.
Yep!
Yes, it is. :-)
Per the instruction manual, page 2, item #7, the Cannon Speedlite 540EZ can be set for seven manual output levels, from full 1/1 to the lowest setting of 1/128th power.
Yes, It will.