De or het lensopening?

The correct Dutch article is:

de lensopening

Rule Words ending in -ing: almost always de. See the rule

How to use lensopening

the lensopeningde lensopening
this lensopeningdeze lensopening
that lensopeningdie lensopening
our lensopeningonze lensopening
your lensopeningjouw lensopening
every lensopeningelke lensopening
which lensopening?welke lensopening?
a big lensopening / the big lensopeningeen grote lensopening, de grote lensopening