As a photographer it is important to know that there is a physics principle that we most likely learnt in school many years ago (or more recently depending on age) that governs the effective light from a flash or other source. That principle is the amount of light fall off as the distance from the source increases. Now we know that it is an inverse square rule which means if we double the distance we get one quarter the amount of light.
Controlling the amount of light is where it gets interesting. We can add more light, increase the ISO, open the lens i.e. a smaller f stop number, or decrease the speed. Now sometimes using one or all of those techniques is ok. Another way is to provide a focussed light on the subject.
How I hear you ask, well you can do what I did this morning and build yourself a fresnel magnifier for your flash for about $5 AU. How, two bits of cardboard and a fresnel lens. A fresnel lens what is that, well in short it is a flat magnifying glass and one can often find them in bookshops or $2 shops readily.
Attach the lens to the flash at about 25cm’s using two bits of cardboard and the have fun playing around. There will be a little bit of juggling to get the flash to focus in the right spot but once there one can get this effect. Normally to get this I would have to play around with Lightroom and vignetting etc to get the highlighted look. A unfocussed flash would have too much spread.
Now it is a useful tool just remember if you make one to use it when required and not over use it.