your focal point will change depending on the depth of field, aperture, and lens of choice. selecting the sharpest aperture will also depend on your lens of choice and your environment, i.e. indoors or out. i'd suggest, as a starting point, using your 50mm (manual focus shouldn't be a problem if you're doing portraits), f8 or f11 if outside, preferably not direct sunlight. at that aperture your depth of field is large enough to overcome placing your subjects within a few feet of each other, but you can always check a depth of field calculator for exact numbers. as a general rule of thumb i try to focus at a point a third into my frame and adjust accordingly.