Single-Image Focal Length Estimators Have a Telephoto Blind Spot, and Their Benchmarks Share It

Single-image focal length estimators are evaluated almost entirely on wide-to-normal lenses. We audit the four benchmarks recent calibration methods report on: none contains a photograph beyond 243 mm, and on two a constant fitted to the test labels beats every published method. Beyond this range published methods fail. On our benchmark and on three third-party sets that share no images or labels with it, every method trained on rendered panorama crops places at most 2% of telephoto (≥135 mm) photographs within 25% of their true focal length. The failure follows the training range. Fine-tuning only AnyCalib's decoder on photographs spanning 8–1200 mm raises its telephoto accuracy from 0% to 40–48% on all four sets, whereas the same fine-tune on photographs below 110 mm collapses again beyond that limit, so rendering is not the cause. Covering the full range with one model costs accuracy at 20–70 mm. To make the range measurable we release FocalCommons-100K, 100,000 Wikimedia Commons photographs spanning 8–1200 mm, and FocalBench, a 3,177-image benchmark, and we measure how often EXIF focal tags disagree with structure from motion. The label-only statistics behind our audit apply to any regression benchmark and expose such coverage gaps before a model is run.