09-07-2015 04:47 PM - edited 09-07-2015 04:49 PM
One possibility comes to mind...if your 50mm lens has a different maximum wide-open f-stop (i.e. 1.7 vs 3.5) than your 18-55 then the flash isn't firing because the lens is capable of opening up more and camera doesn't sense you even need the flash to fire.
09-07-2015 07:41 PM
I just read some articles about the XSi and there is an on going problem with the AF mode (Auto focus). Perhaps it is this since you said that now it doesn't work with either lense. Googled "canon xsi af problem".
Can you put this camera in Manual Focus mode and see if it works that way?
09-27-2015 12:02 AM
My Rebel XS also will shoot in MF mode but not in AF. I rotated/pulled out the front "auto focus" section of the lens, and then the camera would auto focus for 1 shot. I have to pull/rotate the front portion of the lens all the way back out again for the camera to take a shot. It will pull the lens back in to properly autofocus, but it doesn't seem to like to push it out. Is this just lens failure with no way around it but a new lens, or is there a way to clean it out inside or something? I did wipe that front AF portion of the lens very clean when it was fully extended, but to no avail.
09-28-2015 01:55 AM
09-29-2015 05:22 AM
09-30-2015 04:39 AM
In Digital Photo Professional or at http://regex.info/exif.cgi you can check your older pictures to check what firmware you have in the camera. If it is 1.0.5 or older, then maybe Canon service centre fixes it for free.