The problem is ... there shouldn't be "more" or "less" usuable shots due to f/2.8 vs f/4. Unless your are not focusing on the subject you are taking an image of, or you require a greater DOF to get more subjects in the image. So questions if no image with EXIF - Which AutoFocus mode are you using. Page 121. - in your custom menu, what is A1 and A2 set to - Release/Focus ? - What AF Area mode are you using. Page 123 - 126 (good diagram on 126) . This can have a dramatic effect on what's "in focus" and what's "out of focus" at f/2.8 - Which Metering Mode are you using. Check your Metering. Pg 139 in the manual - can you confirm in image preview that your focus point is on the subject that is out of focus ?