A godsend to APS-C camera owners, speedboosters allow photographers using adapted full frame lenses to regain some of the image lost in the crop factor as well as increase the light gathered by the sensor. Every lens is different and not every speedbooster will work great with every lens. However, there is a way you may be able to adjust this to achieve better performance with your speedbooster.
This could be a helpful trick if you get your adapter in and you have issues achieving infinity focus with the lens you intend to use it on. Thankfully too, the fix is rather simple, though trial and error will definitely be needed in order to really dial it in correctly.
As you can see in the video below, Aleksandar Roman demonstrates the process, which basically just involved unscrewing one screw on your adapter and then rotating the rear element of the adapter to get it to move either closer or further away from the camera sensor.
There is no way to know while you are adjusting if it will work better with your adjustment, so you will need to adjust, then test, and then adjust and then test. The process could be long, or short, depending on how much adjustment your adapter needs. But we would probably recommend making small adjustments as a little movement can go a long way. Also, don’t forget to mark the original position of the element so that you can return the adapter to its stock configuration if necessary.
It should also be noted that most speedboosters shouldn’t need this adjustment out of the box, but if you have bought one specifically for your lens and you are not quite able to achieve infinity focus then it could be worth at least giving this method a try before returning the adapter.
Shout out to Aleksander Roman for the video, make sure to check out his channel on Youtube.