Canon 7d - view finder

sounds like that your mirror has a hair or mark on it. I had the same problem but I just used a blower brush to clean it. You can see it in your view finder because the mirror bounces the light into it, but when you take the photo the mirror swings up so the sensor absorbs the light. Be very careful with cleaning the mirror, if you damage the sensor that would be very bad. Take the lens off the camera and look to see if there are any marks on it.