You shouldn't ever (at least, as far as I know) be using ISO 800 if you're taking a picture with a tripod unless something in the shot is moving. Always use your lowest ISO, and adjust the shutter speed and aperture accordingly. I would use an aperture of around f/8 - f/13 unless you're trying for a more shallow depth of field, as that range is generally the "sweet spot" of the lens. That will probably require you to use a longer shutter speed (8-15 seconds, maybe?). You are reading the meter when determing your settings, right? Make sure the the tripod is absolutely secure, so that it's not moving at all. You should be fine, I wouldn't worry too much.