I'm doing a friends family portraits today, and was wondering if f/2.8 would be to large? I want to acheive the blurred background, but definately do not want to blur any family members....Suggestions? Oh and I'm shooting with a 35mm camera and 50mm lens.
I am wondering why you have seemingly arbitrarily chosen the aperture before doing any testing?
Because many new to photography don't understand how depth-of-field (DoF) actually works, and wrongly assume lens aperture is the only factor that affects DoF.
The DoF an image has is also dependent on the lens focal length, and the distance to the point of focus. If the background is to be blurred the distance from the point of focus to the background also has to be considered.
35 mm film is the same as a full frame image sensor, so using a 50 mm prime lens @ f/2.8 the total depth of field at various focus point distances will depend on the size/arrangement of the group. As the point of focus distance increases, so does the DoF:
Point of focus @10 feet - total DoF =
2.06 ft (0.92 ft in front of, and 1.13 ft behind the point of focus.)
Point of focus @15 feet - total DoF =
4.72 ft (2 ft in front of, and 2.72 ft behind the point of focus.)
Point of focus @20 feet - total DoF =
8.57 ft (3.41 ft in front of, and 5.16 ft behind the point of focus.)
Point of focus @25 feet - total DoF =
13.8 ft (5.1 ft in front of, and 8.6 ft behind the point of focus.)