The spatial resolution that can be obtained with image intensifiers depends largely on the spatial resolution of the microchannel plate (MCP)-anode assembly. Existing models on MCPs never include the nonuniform anode field penetration into the channels. The objective of this article is to show that this field penetration leads to a lens effect which has a crucial influence on the electron trajectories and the spatial resolution that can be obtained. From a simple model the relevant parameters determining the spatial resolution are identified as the anode gap, the anode field, the end-spoiling penetration depth, and the channel diameter. These parameters are varied in a Monte-Carlo simulation in which 20 000 electrons are traced through a channel, the end-spoiling, and the anode gap. The spatial resolution as obtained from these simulations is shown to depend heavily on the nonuniform field at the end spoiling. Especially low-energy electrons generated at the end spoiling deteriorate the spatial resolution. The best resolution is obtained for a small anode gap (300 mum) and small anode voltage (3 kV). This indicates that, above a certain value of the electric field in the anode gap, the effect of the lens is dominant. Doubling the end-spoiling penetration depth from 10 to 20 mum increases the resolution especially at high spatial frequencies. Furthermore, the resolution is enhanced considerably by decreasing the channel diameter. These results can be fully understood from the simple model presented. In addition, results are presented from experiments in which the spatial resolution is measured for various anode gaps and anode voltages. The dependencies of the spatial resolution on these parameters are in good qualitative agreement with the simulation results.