Question

what is the reason you think for different
values focal length​

Answer 1

So the difference is: 1, boiling can occurs everywhere, depending on the heat source. Whereas evaporation only takes place at surface of the liquid. 2, boiling requires certain temperature at certain pressure, and continuous heating source, while evaporation works at any temperature and pressure.

Answer 2

There are following reasons for different  values focal length​:

1) The power of the different lenses is different and it is very clear that the power of any lens is inversely proportional to the focal length of the lens.

2) Different power of the lens means that it has a different focal length.