  We will mainly discuss the measurable angle (local angle) of the light ray
$\psi_P$ at the position of the observer $P$ instead of the total deflection
angle (global angle) $\alpha$ in Kerr spacetime. We will investigate not only
the effect of the gravito-magnetic field or frame dragging but also the
contribution of the motion of the observer with a coordinate radial velocity
$v^r$ and a coordinate transverse velocity $bv^{\phi}$ ($b$ is the impact
parameter and $v^{\phi}$ is a coordinate angular velocity) which are converted
from the components of the 4-velocity of the observer $u^r$ and $u^{\phi}$,
respectively. Because the motion of observer causes an aberration, we will
employ the general relativistic aberration equation to obtain the measurable
angle $\psi$. The measurable angle $\psi$ given in this paper can be applied
not only to the case of the observer located in an asymptotically flat region
but also to the case of the observer placed within the curved and
finite-distance region. Moreover, when the observer is in radial motion, the
total deflection angle $\alpha_{\rm radial}$ can be expressed by $\alpha_{\rm
radial} = (1 + v^r)\alpha_{\rm static}$ which is consistent with the overall
scaling factor $1 - v$ with respect to the total deflection angle $\alpha{\rm
static}$ in the static case. instead of $1 - 2v$ where $v$ is the velocity of
the lens object. On the other hand, when the observer is in transverse motion,
the total deflection angle is given by the form $\alpha_{\rm transverse} = (1 +
bv^{\phi}/2)\alpha_{\rm static}$ if we define the transverse velocity as having
the form $bv^{\phi}$.
