(faster shutter speed / less light). Aperture (or f-stop) refers to the size of the opening in the lens of the camera when a picture is taken. As you are focusing the camera, the lens is wide open with the largest aperture supported by your lens.
The harvest season typically falls between August & October in the Northern Hemisphere and February & April in the Southern Hemisphere. With various climate conditions, grape varieties, and wine styles the harvesting of grapes could happen in every month of the calendar year somewhere in the world. In the New World it is often referred to as the crush.
Yes, you can pick one grape, put it in your refractometer and take a sugar sample, but it won't be very representative of all your grapes. Instead, pick individual grapes from many clusters. Sample from both sides of the vine, high and low, in sunny areas and shaded areas, and pick from different parts of each cluster. An ideal sample might be 50 grapes. But if you only have a few vines, it'll be okay to take a smaller sample.