My internal light meter/indicator was working properly a few weeks ago. Now, when I half press the shutter, instead of the meter offering the correct exposure through the viewfinder, it always flashes at 1/4. I thought the shutter speed was locked at 1/500 of a second when the shutter was released because when I set the shutter to longer speeds (2-8 seconds) I didn't hear the shutter close after firing. However, when I set the mirror lockup, I can clearly hear/see the shutter open and close for the proper duration. I know that when the battery is low or inserted in reverse, it will only fire at 1/500. My battery is new and the battery checker indicates with a red light that it is sufficiently charged. So I think the shutter is firing as it should (at least with mirror lockup set). The camera is in like new condition. I got it from an acquaintance who almost never used it. I also have three lenses and changing them doesn't fix/change this issue. What might be wrong with my camera? Thanks!