Optical fields become comparable only after pixel mapping closes. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Magnification fixes the image field
A 3 m sample segment at magnification 4 gives image length 12 m.
Limage=3m⋅4=12m
Fields are addressable optical tiles
The field has 6 pixels, so one pixel spans 2 m. The checked tile count is 4 fields.