How to turn off the LG Optimus L5 screen magnifier? That is the problem. I've already made a hard reset (press power button + vol. down key then pressing the home key) without luck. The screen is stuck magnified and I can't do anything.


Screenshot (click image for larger variant)

What can I do to get it back to normal?
Since you already did a factory reset, then I think that the problem lies in the /system/build.prop.

Are you rooted? If so, connect your phone to your PC, make sure you have ADB, and do this in a terminal:

adb pull /system/build.prop


In the build.prop file you just copied from the phone, look for ro.sf.lcd_density.
Change the number to something low, like 160 or 240. Save the file, and do this in a terminal to put it back onto the phone:

adb push build.prop /sdcard/build.prop
adb shell
su
cp /sdcard/build.prop /system/build.prop
chmod 0644 /system/build.prop
reboot


If you're not rooted, then you would probably have to do a firmware reflash; although I'm not familiar with LG's way of doing that.