The relationship between time perception and brightness perception remains poorly understood. We developed a computational model linking the two domains, grounded in established principles of neural information processing in visual cortex. A nonlinear transducer maps luminance to population spike rate, and correlated gain fluctuations impose an upper bound on the achievable signal-to-noise ratio. Perceptual magnitudes in both domains are decoded from the same spike-count statistics, yielding a reciprocal trade-off in perceptual resolution: brighter stimuli improve temporal precision but impair brightness sensitivity, whereas longer stimuli enhance brightness sensitivity but degrade temporal resolution. We tested these predictions in two psychophysical experiments manipulating stimulus duration and luminance. Observed data closely matched model predictions, revealing a fundamental coding limit in vision—the time-intensity uncertainty principle. This limit provides a unified explanation of the mechanisms behind Weber’s law, Bloch’s law, and luminance-dependent shifts in duration thresholds.