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The 18mm - 35mm (MSAKCF1) provides a wider range zoom option for more flexibility. This flexible zoom adjustable lens endoscopes adapter is also waterproof.
The 25mm (MSAKC25) coupler is the most popular coupler for endoscopy use. Compared to an eyepiece, the 25mm coupler gives very similar view.
If you desire a larger field of view the 14mm (MSAKC14) gives approximately a 30% larger FOV compared to the 25mm.