>>3679905>Speedboosters didn't exist until 2006A speedbooster is just another convex lens, also known as a Shapley-lens.
These things have been used in telescopes before, same as Barlow-lenses aka teleconverters that are just concave lenses.
These Shapley lenses haven't been used in DSLRs because they reduce the flange distance.
So they don't realy work with DSLRs as you can't physicly move the lens back any further due to the mirror.
That is unless you have a long focus lens designed for a far longer flange distance than your camera body has, but that's kind of rare...
Due to the advent of crop-sensor mirrorless they got pretty usefull as many lenses offered larger image circles than the sensor and realy short flange distances.
And since 1,5x crop divided by a 1,4x would essentialy make a crop-sensor camera behave like a full frame sensor camera, they are pretty usefull.
Especialy with older lenses from DSLRs designed for longer flange distances of about 45 mm compared to the sub 20 mm of mirrorless.
Combined with a glassless adapter, you essentialy doubble your collection of primes as well.
So:
>28/2.8>50/2>135/2.8turns into
>20/2>36/1.4>97/2Giving you a nice set of primes that cover most situations.
Combine that with some kit lens or superzoom and you basicly have any lens you could ever need while carrying only 4 lenses and two adapters.
