The first instinct when buying extension tubes is “the longer, the better” because a longer extensions allows a bigger magnification.
However, not all images require large magnification.
I shot these two photos with two Nikkors, the 20/3.5 UD AI and the 50/1.2 AI-S on a Sony NEX-5.
The theory says that if I extend a 20mm focal-length lens with a 20mm tube, I can obtain a reproduction ratio of 1:1. In order to achieve this, though, a 20mm designed for reflex cameras, that is with a retrofocus optical scheme, would need to have the subject too close: actually, inside the lens itself.
I used a Nikon PK-11A that provides an extension of just 8mm. This allowed a dramatic perspective when mounted on the 20mm, and a moderate close-up (my hand rather than my fingernail!) with the 50mm.
The images are on sale on iStock.