Smartphone-based fundus photography (SBFP) allows for a cheap and mobile fundus examination with the potential to revolutionize eye care especially in low income settings. The purpose of this study was to assess the impact of pupil dilation on image quality in optic nerve head (ONH) imaging and vertical cup-to-disc ratio (vCDR) evaluation with SBFP. Eyes with glaucoma or suspected to have glaucoma were imaged with conventional digital fundus photography (CFP) and SBFP undilated and following dilation, all monoscopically. SBFP was possible in 74% of eyes without dilation and in 98% following dilation. Better image quality on SBFP was achieved with dilation and complete visualization of the optic disc rim was possible in 46% of images without dilation and on 94% of images with dilation. VCDR measurements on images obtained following dilation highly correlated with measurements on CFP (coefficient of correlation r = 0.91, p < 0.001), whereas vCDR on images obtained without dilation correlated less well with CFP (r = 0.70, p < 0.001). SBFP for ONH evaluation is promising, however dilation appears mandatory to achieve results comparable to optic disc evaluation on CFP. ONH imaging with smartphones without dilation might bear the risk of underestimating the CDR and hence overlooking patients at risk for glaucoma.