Whilst SPF 50 will help prevent tanning more than other SPF numbers, if you are out in the sun then it is inevitable that your skin is at risk of tanning. Ensure that you are reapplying your SPF50 as much as you can, and try and stay in the shade if you want to prevent tanning. SPF 50 is a great idea regardless, as although you may still darken slightly, SPF 50 stops you from burning as much and will help keep your skin protected and hydrated which could cause more serious problems if not done properly.