Recently my canon 700d has been acting strangely. When using preset modes such as landscape, the photos come out fine when taken using the flip screen, but when taken through the viewfinder the photos come out extremely overexposed. The iso seems to be constantly high at 6400 and shutter speed low. This has never happened before and its meaning I cant take any quick snaps using auto modes. Can anyone help?
When you use Live View the camera meters differently than when shooting via the viewfinder.
The exposure meter used when shooting through the viewfinder is a dedicated unit located up inside the prism housing. When you're using Live View, the dedicated light meter is not receiving any light because the mirror is flipped up. In that case the camera measures the amount of light detected directly on the main imaging sensor.
It sounds like your light meter is malfunctioning or has lost connection to the camera's logic circuits that calculate exposure.
Most likely explanation: That camera uses different metering devices depending on whether you have it in live view with the mirror up (can't use a metering cell glued to the pentaprism that way) or OVF mode (can't use the sensor as a meter) - and the metering cells on the pentaprism, or related electronics, have recently given up the ghost.
It might be that you have by mistake changed a Exposure Compensation for that mode that overexposes everything. Check this maybe: