This Extra Lens Is Like a Megaphone For Your DSLR's Pop-up Flash
It might occasionally come in handy as a bit of fill light for a shot, but your DSRL's pop-up flash is a poor substitute for a dedicated flash perched atop your camera. It makes sense why it sucks; it's designed to be small and compact enough to fold away. But with Rogue's Safari Flash Booster added to the mix, all of a sudden your DSLR's pop-up flash isn't so crappy any more.
Using a cleverly designed fresnel lens, the Safari booster promises to give you up to eight times more light from your camera's built-in flash, without the need for extra batteries. It does mean you have to carry around an extra accessory that attaches to your camera's hot shoe, but if it delivers as promised, at $35 it's far cheaper than adding a full-sized flash to your kit.
It looks more like a gimmick to me.
Flash needs capacitors, more powerful bulbs, etc to produce more power.
Anything you put in front of a flash typically kills light rather than add.
Also, most photo-gear that has "real" value usually cost money. For $35, I wouldn't expect anything.
I would suggest saving up a bit and get a hot shoe flash (OEM or 3rd party).
I think it will save fuel too Kidding aside, everytime light passes through anything, it's power diminishes. Even when light passes through a convex lens (magnifying lens) , its power diminishes though the lens would bring the beams together. From the photo, it looks like a linear Fresnel lens similar to the type used to enlarge television screens back in the 80's.