visual observation
(1)目察法，直观研究法
(2)目视，观测

• front focus
• frontiers
• front illumination
• front index plate
• front lens
• front lesn mount
• front light
• front operating aperture
• front operationaperture
• front panel
• front-reflection
• front sight
• front-silvered mirror
• front surface
• front surface mirror
• front vertex focal distance 1
• front vertex power
• front view
• frosted bulb
• frosted glass
• frosted screen
• Frounhofer grating
• frozedn stress
• frozen fringe
• frustrated total reflection