SPF is a measure of how much UVB protection a sunscreen gives when you apply it properly. The number is not a strength dial and it is not a duration. It is the ratio of how long it takes protected skin to redden versus unprotected skin, measured at a thick, even, laboratory-standard layer.
What the number measures
At the standard application amount, SPF 30 filters about 97% of UVB. SPF 50 filters about 98%. SPF 70 filters a little over that again. The jumps look large on the label and small in reality: the difference between 30 and 50 is roughly one percentage point of UVB reaching your skin.
SPF 30 vs 50 vs 70, the real difference
Because the curve flattens, no sunscreen reaches 100%, and the gains above 50 are marginal. A higher number is not wrong, but it buys less than the packaging implies. What changes the outcome far more is whether the layer is thick enough and reapplied. Most people apply a quarter to a half of the tested amount, which drops a labelled SPF 50 to an effective SPF in the teens or twenties.
The best SPF is the adequate number you will actually apply generously and reapply, not the highest number you apply thinly.
Why application beats the number
For the face and neck, the rule of thumb is about two finger lengths of product, or a third of a teaspoon. Reapply every two hours of meaningful sun exposure, and after swimming or sweating. A well-applied SPF 30 outperforms a thinly applied SPF 70 every time.
What to choose
For daily life, broad-spectrum SPF 30 or higher is the sensible floor; SPF 50 is a reasonable default and gives a small buffer for the under-application most of us are guilty of. Above 50, choose on texture and broad-spectrum quality rather than the number, because the number itself is doing very little extra work. Broad-spectrum, which covers UVA as well as UVB, matters more than chasing a higher SPF figure.