What SPF measures
SPF (Sun Protection Factor) measures protection against UVB, the rays mainly behind sunburn, under lab conditions. It isn’t a timer, and it isn’t linear: going from SPF 30 to SPF 50 takes you from roughly 97% to 98% of UVB filtered. Useful, but not double.
PA++++ is the other half
UVA goes deeper and drives tanning and photo-ageing. The PA rating (PA+ to PA++++) describes UVA protection; more plusses, more protection. In India’s sun, look for SPF 30+ and PA+++ or higher, labelled “broad spectrum”.
The mistake almost everyone makes
Labs test SPF at 2 mg of product per square centimetre of skin. Most people apply a fraction of that, which means real-world protection is lower than the label. For the face and neck, the common rule of thumb is two finger-lengths.
Reapply, really
Sweat, water, towels and touching your face all remove sunscreen. If you’re outdoors, reapply about every two hours, and after swimming or heavy sweating.