If the focal length of objective lens is increased then magnifying power of

$\text{ Magnifying power of a microscope, }m=\left(\frac{L}{f_o}\right)\left(\frac{D}{f_e}\right)$

where $f_0$ and $f_e$ are the focal lengths of the objective and eyepiece respectively and L is the distance between their focal points and D is the least distance of distinct vision. If $f_0$ increases, then m will decrease. 
$\text{ Magnifying power of a telescope, }m=\frac{f_0}{f_e}$

where $f_0$ and $f_e$ are the focal lengths of the objective and eyepiece respectively. If $f_0$ increases, then m will increase.