The issue with some lenses and infrared is that they will not properly focus at some or all focal lengths in infrared, because the focuser just doesnt go far enough. This is because infrared light always focuses slightly behind visible light (ie if you focus for visible light, the IR light will be focused behind the camera sensor). Most lenses still have enough room in their focus range to allow proper focusing in IR, but others do not. I noticed this was an issue first with my Rokinon 8mm fisheye so I started looking for ways to address it.
I borrowed the idea from Pete Ganzel who makes a conversion kit to convert Rokkor mount 58/1.2 lenses to A-mount. In the conversion kit from Pete are a small assortment of spacers which you will need to test to find the proper spacing of the replacement a-mount ring so you can properly focus. Well I applied the same principle to an aftermarket a-mount to e-mount adapter (this probably would not be a good idea to try with the LA-EA1 because of the electrical contacts, and not wanting to destroy a much more expensive adapter, of course).
So here is what I did:
First I removed the A-mount ring from this adapter, and then added some spacers where the screws mount to the ring. These are just test spacers which will get replaced with complete rings once I have time to make them, it was just easier to cut small strips for testing. There is no set rule to how much space youll need to add, try adding one spacers, put the assembly back together and test the lens you are having trouble with. If you really want to get it perfect you can keep trying until the distance markings on the focus ring line up perfectly when focused in IR.
The other thing to keep in mind is that the manual aperture lever in the adapter (if it has one) still works, since adding spacers may prevent it from reaching the lenses aperture lever. Also, you will want to check that the lens locking pin will still lock, I added a small screw to act as a spacer behind the spring which provides pressure to the lock pin to hold it in place when the lens is mounted. You will have to do trial an error to get these things right, and not all third party adapters will lens themselves well to this adjustment process.
Here is what mine ended up looking like with the test spacers in place:
And here is what the adapter looks look on my IR converted NEX-5, with Minolta 28-135 mounted:
Lastly, the result? I can now properly focus the Minolta 28-135 and Rokinon 8mm in infrared, this shot below is with the Minolta at 28mm: