Compared to traditional 2D image techniques, stereoscopic techniques provide additional information - the binocular depth, which strongly enhances the immersion. However, it may also cause visual comfort problems because it is still not a perfect representation of natural human vision but to some extent an illusion. How to fairly evaluate and understand the QoE (Quality of Experience) of stereoscopic images is still an open question. In this paper, we aim to explore the influence of binocular depth on the QoE of stereoscopic images by subjective quality assessment using methods that take into account the 3D concept. Firstly, quality indicators based on 3D concept including 2D image quality, depth rendering, depth quantity, visual comfort, naturalness and visual experience are defined. Synthetic scenes and natural scenes are designed and the camera parameters are calculated in order to create variation of perceived binocular depth range in terms of DoF (Depth of Focus). Subjective quality assessment based on the SAMVIQ method is used to evaluate the viewer's subjective opinions. The experiment results reveal how the variation of perceived binocular depth affects different quality aspects of 3D QoE. It leads to a proposal of modeling higher level concepts (depth rendering, naturalness and visual experience) of 3D QoE as a weighted sum of 2D image quality, depth rendering and visual experience in case of binocular depth variation.