I'm a little bit obsessed with preventing aging and I hope to be a dermatologist or plastic surgeon one day, so I thought I'd share what I've learned about sunscreen after many sleepless nights. I'm not sure what is general knowledge (little of it was to me) so I've made it detailed.
The sun produces UVA, UVB and UVC rays, although UVC is absorbed by the environment so not a concern. UVB burns the skin and causes skin cancer. UVA penetrates deeper breaking down collagen, elastin, damaging skin cells and also contributes to skin cancer. UVA passes through clouds, windows and clothing more readily than UVB so just because you may not burn doesnt mean your not damaging your skin. This includes darker skin tones.
SPF measures protection against UVB only in two ways. The main purpose of SPF is to tell you how much longer your protected as opposed to no sunscreen. A pale person can be out in the sun for about 10 minutes safely, so SPF 15 would protect you for 1.5 hours. However this is where it gets confusing - you shouldnt count on any sunscreen to protect you for over 2 hours because photodegradation breaks down the sunblocks/sunscreens. I dont really understand how that works with spf's that are supposed to last longer than 2 hrs but I prefer to apply too often than not enough. SPF also measures the amount of UVB protection - 15 absorbs/blocks about 93%, 30 adds 4% and 50 adds another 2%.
There are physical sunblocks that reflect light and chemical sunscreens that absorb it. Physical/mineral sunscreens tend to be less irritating but chemical sunscreens are usually required for higher spf and better UVA protection. There are health risks associated with different ingredients but the general consensus is that the benefits outweigh the side effects so I didnt look too much into it. I've read about sunscreens penetrating the skin and trapping uv rays under the skin and the damaging by-products given off as they photodegrade. Theres also a chemical I forgot the name of thats present in 97% of the population's blood according to the CDC and may cause birth defects, cell damage, and interfere with hormones. There doesnt seam to be a lot of research about any of this and there is such an overwhelming amount of harmful chemicals today that I just decided to not worry about it.
PFA (protection factor of UVA) was recently created to measure the amount of UVA protection, it's measured as PA which is starting to pop up on sun screen bottles. PA+ = PFA 2-4, PA++ = PFA 4-8, PA+++ = PFA 8+. UVA protection is nowhere near UVB but there are sunscreens that get up to PFA 25. Mexoryl is a chemical recently approved by the FDA that has additional protection against short UVA rays and it's one I'd look for in sunscreen, but it's patented by loreal so it's not in many.
Based on what I could find I think Anthelios sunscreens are the best, although every one I've tried except the spf 15 leave a thick white film. I'm trying to find the right balance between protection and texture, so far the xl 50 fluide is my favorite since its high spf/pfa, not as thick and has both the oil and water soluble forms of Mexoryl making it stay on better. I use Shideido spf 35/PA+++ powder through out the day to reapply sunscreen without having to redo my makeup. Antioxidants are also important for sun protection, I like revive serum protectif, dr sebagh high maintenance serum and kinerase c8 cream but there are cheaper options that are good too.
It's "specially formulate for face" and spf 70. Also, it's only around $10, and you can buy it from any drugstore, which makes it even better. 
