Mental effort is a construct that is widely used in our daily lives and is prevalent in attention, decision-making, and intelligence research. Therefore, operationalizing it has been of great interest in the cognitive sciences. Both subjective and objective measures have been used to index mental effort. Although several studies have shown discrepancies between these indices, some have found that subjective and objective measures are closely related. With respect to this, the present thesis aims to investigate how the subjective measure of mental effort, through ratings on the NASA task-load (NTLX) scale, relates to the objective measure of effort through the online measurement of pupil dilation during a multiple object tracking (MOT) task in which tracking load varied between 1 and 5 objects. Also, individual differences in fluid intelligence, MOT accuracy and personality and motivation traits were investigated as possible moderators of Load on Pupil dilation and NTLX ratings. Results showed that pupil dilation and NTLX scores only correlated at Load 1 (non-effortful condition). Significant interaction effects between Load and MOT accuracy were found for both Pupil dilation and NTLX. However, this moderation for NTLX scores showed showing High Accuracy participants starting off with less pupil dilation from load 1 and higher pupil dilation as load increased. Meanwhile, Low accuracy participants showed the inverse effect, with slightly higher pupil dilation at Load 1 and gradually, a lesser increase in pupil dilation as Load increased in Load 5. In addition, MOT accuracy and perceived NTLX performance were the only factors that moderated Pupil dilation. It was observed that personality traits and Load had no interaction effects on pupil dilation.