However, I often end up with adapters that are a little loose. This is bad for a couple reasons. The first issue is the adapter if too loose may jiggle a little making focusing manually a bit more difficult, and inaccurate. The second issue is if the lens isn't held in place properly, it can end up at an angle to the sensor, preventing optimal sharpness in the plane you expect.
Lets take a look at a cheap adapter for Nikon G to Canon EF mount.
The adapter is from "Fotasy" which is a cheap chinese company that may simply resell adapters from another company. Here the adapter is attached to the lens first to "convert" the mount of the lens to EF. The fotasy lens adapter is pretty slack. I get better fittings from other brands "fotodiox" for example.
The inner lugs of the adapter that grip the lens has a slit. Take a good look at the space here. This is a closeup of the adapter's lug.
This is meant to act as a "spring" to apply pressure to the camera's mount. This "fotasy" adapter has extremely weak "springs".
My goal is to increase the force of these springs to have a tighter fitting adapter. I use a screw driver and firmly push it in the space. Do it slowly to gently make the gap wider and increase the effect of the spring.
Wiggle the screwdriver into the slit, expanding it slowly.
Don't make it too large. A little will help a lot.
On the Fotasy, this is a rather weak spring, and it helps a little. Stacking strips of tape or paper, inserting it and trimming it with an Xacto or razor works great to cushion the spring a bit. I used a post-it note to "fix", folding it a few times and squeezing it in, and cutting off the rest.
Here's what a pricier adapter looks like:
The spring of a pricier option - this is a Canon EF-MFT adapter, so in addition to costing more, there's also more room for better springs.
Once back on, the adapter holds firmer, having little or no play. Now go out and shoot!