Focal length of the lens remains same. Intensity of image formed by lens is proportional to area exposed to incident light from object. i.e., Intensity ∝ area or
I2
I1
=
A2
A1
Initial area, A1=π(
d
2
)2=
πd2
4
After blocking, exposed area, A2=
πd2
4
−
π(
d
2
)2
4
=
πd2
4
−
πd2
16
=
3πd2
16
∴
I2
I1
=
A2
A1
=
3πd2
16
πd2
4
=
3
4
or I2=
3
4
I1=
3
4
I(∵I1=I) Hence, focal length of a lens =f, intensity of the image =