Install this Shorter M42 mount adapter can reduce ATR3CMOS26000KPA/KMA camera's backfocus distance 5mm. so it's backfocus distance can be 12.5mm
The buyer can change the M42 mount adapter by yourself.
The left one is default standard camera
the Right one is ATR3CMOS26000KPA/KMA installed Shorter M42 mount adapter
This adapter
External thread size: M42x0.75
Internal thread size: M38x0.75