When I began studying gravitational lensing, I was told that a gravitational lens preserves a quantity called “surface brightness”. This is defined as the flux per unit area. In other words, if you look at the sun from out at Pluto, it’s very dim. But if you receive as much light at Pluto, it’s because the size of the sun in the sky is what it is here, not because it’s gotten intrinsically dimmer. The stars are just as bright here as they are right up close. They look really small, though (so small even a big space telescope like Hubble can’t see any details on a star (except the Sun, of course)).
Even though gravitational lensing can bend light and thereby make a star seem bigger in the sky, it cannot make it seem intrinsically brighter. The same is true for normal optics. A magnifying glass can make the words on a page larger, but if the lights in the room are dim the magnifying glass cannot make anything brighter.
Question: why not?
Hint: you don’t need to know anything about optics or gravity to answer this question, except that lenses and gravitational potential are completely passive. That is, they only bend light, not create it or change it.