Im having problems when im using the auto iso mode in my canon rebel t3i whith the optical viewfinder but not when i use the lcd monitor. whe i use the optical viewfinder the exposition and iso change and the picture take so long to take. but not when i change to the lcd screen to take the picture.
The question would be why is it taking a long time when using the optical viewfinder. Different metering is used for optical (using the metering sensor) since the mirror is down vs direct measurement using the CMOS sensor when the mirror is up (LiveView). Something may be wrong with your metering sensor. First I'd check to make sure your metering settings are on the defaults, if they are and it is still causing problems, I'd try another lens if you have one, if the problem still persists, it might be worth having a camera shop or a Canon service center take a look at it.