Canon xti and canon 420ex flash giving

What are the issues your having? Without an idea of what the photos look like and what settings you are using there's not much help we can offer.

yes it is compatible

You can get the user manual for this device from this link

http://www.scribd.com/Speedlite-420EX-Manual/d/11516507

You could try increasing the exposure level (press AV+/- while rotating the selector wheel to the right) but this will increase the time the shutter is open and increase the possibility of picture blur. Only other thing to do when shooting indoors is to change to a high ISO - 1600 for instance. This reduces the time the shutter is open so working together with the first suggestion you may not notice any difference!

If still too dark, you need a faster lens - you can pickup a 50mm f/1.8 lens quite cheap - this is an excellent indoor lens as the low F number means it lets in lots of light. The fixed focal length means you will have to move around to frame the shot, but they are indispensible for indoor work and a lot cheaper than a f2.8 zoom.

I also own a Vivitar 285 that i used with my Canon F1 manual 35mm camera. Most older cameras before the advent of digital, had a hot shoe on top of the camera, or like in my case, the hot shoe went over the rewind knob. I also had to plug in a power cord from the 285 flash foot and the cameras PC connection in order for the flash to work on some other cameras. The Canon Rebel XTi has a TTL (through the lens) hot shoe usable with Canon's own EX flash units. You can buy a "hot shoe to PC connector" if the TTL camera hot shoe doesn't work, but be extremely careful, the only problem that exists with the older manual flash units was the trigger voltage is as high as 200+volts which would fry digital camera hot shoes. I tried my unit with a Wein flash trigger device on the PC cord of the flash and my Canon D60 with it's own flash. All shots were overexposed and washed out highlights.  Here's a copy of the hot-shoe review:h The EOS 400D's hot-shoe can be used with Canon and third party flashes (although sync only on most third party units). The hot-shoe is E-TTL II compatible. Compatible flashes include Speedlite 220EX, 380EX, 420EX, 430EX, 550EX, 580EX, Macro-Ring Lite, MR-14EX, Macro Twin Lite MT-24EX and Speedlite Transmitter ST-E2.  The Canon Rebel XTi doesn't have a PC socket to plug in a manual flash trigger cord which common sense tells me there is no provision to step up the voltage to trigger manual flashes - so my answer is this combination is not a good idea on a Canon Rebel XTi. Hope this helps.

Could be the metering mode you are in. Could be the subject is too far away. Could be ISO is set wrong. Could be the lens aperture can't open up enough. Could be the exposure compensation isn't set correctly. Could be FE Lock is still on and the composition has changed. Does the <flash> icon continue to blink? Then you are too far away.

all canons tend to underexpose to avoid blowing the highlights. I still think this is a poor design / calibration of the light meter. A search of the net will reveal some folks who sent their xti's in to canon to be fixed and got them back better. Canon doesn't recognize this as a "problem" and states they did this on purpose but this is the 2nd canon I've had that did this and my next camera will be a Nikon!!

I found the answer and fixed my flash. Look on http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00ODWf

This will describe how, if you have been using a speedlite alot, than the little tab under the shutter release side of the hot shoe can get bent down thus constantly pressing down (toward the bottom of the camera) the small button which deactivates the pop-up flash even when the speedlite is not attached. Just bend the tab up a little with a jewler's screwdriver, turn the camera off and on and voila!! Fixed!!! Dan describes pushing the tab down, but on the xti, the tab needs to be bent up to release the flash deactivation switch and fix the problem. Thanks to Dan D on photonet for saving me a service call.

If you are not using canon lenses then the low light focusing aid will not work correctly. Go into the menu and turn of the flash infra red light

ALL Canon digital EOS cameras will work with E-TTL only. Solution: you need EX flash for full auto.