I would probably not change anything and let it continue to focus normally. You could try 'macro' mode, or setting the focus to minimum distance, but I think it will not work as well. The barcode needs to stay at least far enough to be in focus. So you can change the UI to make sure the user does that -- make the scan area appear to be smaller, for example.
Zoom makes sense, but digital zoom can cause image problems. Your devices don't have optical zoom. Exposure should not be a factor I think, but sure you can play with it. The Android app tends to over-expose to scan in dark conditions, but, will under-expose when the front-light is on, to even out the light level.