Video: What do you know about SPF?

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Summer means sun, fun, and of course, sunscreen. But with all the formulations out there, it can get confusing, even if you think you have a PhD in SPF.

Here's a question: Do you know the difference between a 15 and 30 SPF? The answer may not be as simple as you think.

"An SPF of 30 is twice as strong as an SPF 15," guessed one person.

Wrong - an SPF of 15 screens 93 percent of UVB rays only, while a 30 screens 97, just four percent more.

"UVB protection does not increase proportionally with the SPF number," said Dr. Elizabeth Hale, Skin Cancer Foundation.

There is a large jump between an SPF of 2, which only protects against 50% of UVB rays, and 15. After that, the increments are small. So 15 is a good place to start.

"This would be for casual wear, you're going to and from your car, you're putting out the garbage, you're picking up the kids at the bus stop," said Dr. Zoe Draelos, American Academy of Dermatology.

Doctors warn, though, the numbers mean nothing if you don't use the product correctly.

"Studies have definitely shown that people are not using enough sunscreen. In fact, the effective SPF might be far less than what is actually seen on the bottle because people are not using enough," said Hale.

How much is enough?

"At least the size of a quarter, something like that," guessed another person.

Wrong again - you'll need to use a shot glass of sunscreen to get the full benefit.

"If you know you're going to be spending prolonged periods outside at the beach or during the summer time, I say SPF 30 or higher," said Hale.

And don't forget UVA protection. Look for the words "broad spectrum" on the label. And yes, last year's leftovers are still good, but that's a clear sign you're not using enough.

So who needs an SPF 50 or higher? Dermatologists say those who are sensitive to the sun, are taking medications that make them more prone to sunburn, or have a history of skin cancer. Whatever your sunscreen, you should reapply it every two hours.