Hi Elena, here's my thoughts, and thank you for reading my answer patiently :)
The Grandpa misunderstood the photographer.
To understand this joke, it's better that you understand the meaning of "光".
"光" has many meanings:
1. light (as in 侧光side light, 逆光backlight and 全光all light)
2. "光” means exposure, not wearing clothes, or being exposed to the public( for example, this celebrity's scandal was exposed，这个名人的丑闻曝光了)
Here in this joke, the photographer and the Grandpa have different understanding of "侧光，逆光和全光". The Grandpa misunderstood the photographer perhaps he rarely goes to photo studios and has never heard of those terms used in photography. He thought the photographer told him to either expose the side of his body, or the back of his body, or all of his body. That's why he told the photographer that he was okay with the exposure. But he wanted to protect his wife, asking to leave his wife a pair of shorts.
Hope it helps!