Understanding the development of biofouling on the surface of reverse osmosis and nanofiltration membranes is crucial for a safe and reliable operation of membrane systems. Current monitoring techniques however either cannot account for detailed temporal or spatial fouling trends (e.g., feed channel pressure drop, FCPD) or involve elevated cost and manual data post-processing (e.g., optical coherence tomography). In order to complement existing methods, we developed a low-cost and easy-to-use optical system for biofouling monitoring based on a consumer-grade DSLR camera equipped with a macro lens and freely available post-processing software. Applying our system in accelerated biofouling tests, we demonstrated that it allows for visual detection of incipient biofouling and qualitative real-time assessment of spatial biofilm growth patterns. We calculated the Pixel Intensity Difference ΔPI , a quantitative image-based proxy for biofouling, and demonstrated that it captures temporal biofouling dynamics, particularly the early biofilm growth that is hardly detected by FCPD. However, being limited to 2D top view planar images, ΔPI only partially discerns 3D biofilm growth in later biofouling stages. Additionally, our optical system enabled quantitative spatial fouling characterization, from in-detail assessment of minute pixel clusters to comparison of magnitude and temporal trends of biofouling in feed channel subsections. Biofilm growth was found to be substantially stronger on the feed spacer than on the free membrane surface. We share technical information on our system (e.g., drawings, code) so that further research can benefit from this affordable and easy-to-use method for image-based biofouling investigation in lab-scale and full-scale settings.