Lightweight sunscreen that offers strong broad spectrum protection while seamlessly sinking into the skin and being totally invisible is a dream of any daily sunscreen wearer. More and more brands release "ultra light" formulas, and every skincare influencer seems to be on a quest to find the lightest one.
There might be a problem with this approach. To offer a proper protection, a sunscreen needs to form a uniform film on the surface of the skin. The reason it is so important is that our skin is not even, its surface is made up of hills and valleys. A thin layer applied over these hilly surface may result in uneven coverage where “valleys” are filled/covered, but “peaks” are not (think about painting a wall with uneven surface with only a little bit of paint on your brush). The lighter the texture, the easier (and more tempting) it is to apply a thin layer. Remember though that in the SPF testing, 2 mg of sunscreen is applied for every square centimeter of the skin. To get a feel for how much it is in relation to an average face, squeeze out an amount of product that is slightly larger than a US quarter coin. That's a lot of sunscreen to spread over a face. You might even need to layer it twice if the product consistency is truly thin. If you are applying less than that, you are certainly not getting the protection advertised on the product label. It is entirely possible that if a product advertised with SPF 50 would be applied in an amount less than 2 mg per square cm, it wouldn't be able to pass the minimum SPF rating of 15 because of the failure to cover the "skin hills".
If you don't feel any sunscreen residue on the skin after applying it, chances are you didn't apply enough and the film isn't formed. As the result, most of the skin surface is not protected. This can be especially dangerous because you think that since you've applied a sunscreen, it is safe to spend time in the sun.
Sunscreens that are thicker in consistency and/or leave you with a "heavier" feel are more likely to form a uniform film covering the full skin surface. While for many of us, thick formulas simply won't work for every day wear, it is a good idea to opt for a heavier consistency when spending significant amount of time in the sun (in other words, don't rely on your city everyday sunscreen for a beach time or sports outside).