Tan and sun protection factor. How do you calculate SPF suits

Many people say they do not use sunscreen lotions that will not be so tan. Totally wrong!

Lotions protect the skin from UV rays without affecting however bronze.
What is SPF and how you calculate the right one for your skin?
Take the time that you could sit normally exposed to the sun without suffering burns (over 20 min. Is sufficient a paler skin to become red). This time period is multiplied by the factor SPF of the product that you want to use. For example, a cream with SPF 15, you can stay in the sun 15 * 20 min. = 300 min. = 5 h., Without getting burned. But because the skin of each of us is different, it is recommended to apply more often this period calculated lotion to make sure that we are protected.
A product with SPF 15 blocks 93.3% of UV rays, while SPF 30 blocks each with 3.4% more - ie 96.7%.Therefore, the difference is not significant given protection from sunlight, but the time you can spend in the sun without getting burned.
To find out what kind of sunscreen suits, you should know is your skin phototype:
- White skin (usually in people with red hair), always burns, never tans, many freckles - lasts five minutes without burning.- Light skin , always burns, sometimes catch a tan, quite a lot of freckles - lasts 10 minutes without burning- Leather open to closed , sometimes burns, always tans (medium tan), a few freckles - lasts 30 minutes without burning- Dark skin, never burns , always tans (heavy bronze) without freckles - lasts 45 minutes without burning- Leather intense closed , never burns, always tans (very dark tan), no freckles - lasts up to 60 minutes without burn- Black Leather , never burns, no freckles - maximum resistance to sunlight. 

Source

http://www.skincancer.org/prevention/sun-protection/sunscreen/choosing