They make the Ultra Sheer in SPF 55 too, I get so blotchy from the sun I figured I should get the 70. (they were the same price)
From skincancer.org
Q. Many people mistakenly believe that an SPF 30 rating gives twice as much sun protection as an SPF 15 and an SPF 50 more than three times that much. What is really the difference?
A. In vitro tests have shown that SPF 15 sunscreens filter out 93% of UVB rays, while SPF 30 protects against 97% and SPF 50 98%.
Q. How does photosensitivity add to the risk?
A. Phototoxicity can result when a person uses one or another of a wide range of photosensitizing products and then is exposed to UVR. Typically, hours after the exposure the initial reaction, resembling a severe sunburn, appears. Many of the sensitizing agents are drugs, such as the antibiotics tetracycline, doxycycline and ciprofloxacin; the nonsteroidal anti-inflammatory ibuprofen; and the diuretic furosemide, as well as retinoids. Photoallergy, like other allergies, occurs in previously sensitized individuals. Repeat exposure to the same allergen plus UVR exposure prompts a typical pruritic and eczematous reaction. In some individuals, even certain sunscreen ingredients cause photoallergy; these people should switch sunscreens. [See Table 1.]
Anyone with a known history of photosensitivity should stay out of the sun as much as possible and rigorously use sunscreen and other sun-protective measures.
Photoprotection is also advised for patients with photo-aggravated diseases such as lupus erythematosus or dermatomyositis.