Because we’ve moved the film plane by adding the back, you need to reset where infinity focus is.
Luckily the 110A, and a few other models, allow for adjustment of infinity with relative ease.
When you pull the lens forward it locks onto this metal stop. Moving this stop back a bit, to match the new film plane, will set infinity.
There are 2 rivets holding the stop, and 2 screws to loosen but not remove.
Get yourself some ground glass, or if you can’t get any use some tracing paper, and put it into an empty film pack.
This is how we will focus infinity so make sure its tight and flat.
With a sturdy tripod set up the camera to view something at least 75ft away which can be easily focused on.
To view the image on the ground glass the back needs to be in a darker place than the rest of the camera.
Your best bet is to shoot out a window in a darkened room. For extra darkness I used a black t-shirt as a hood over the back of the camera.
Make sure the focus knob is set to infinity and that the aperture is set to wide open, f4.7 on this shutter.
Once you have your subject on the ground glass, move the lensboard locked to the stop back and forth until you have sharp focus.
Tighten the two screws down. You have now reset infinity.
To really test your results, focus on something close with the finder and check it on the ground glass for accuracy.
If you find that your split image is off from the actual focus of the film plane, you will need to tweak the screws on the back of the focus mirror.
Lower right moves the split image vertically, the upper left moves horizontally.