The Sunny 16 Rule is an easy way to get a good exposure without a light meter. When it's bright and sunny, use an aperture of ƒ/16 and match the ISO and the shutter speed. If you use ISO 100, use a shutter speed of 1/100 seconds. For ISO 200, it's a shutter speed of 1/200 seconds. Exposure on a sunny day = ƒ/16 1/ISO