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We present a critical assessment of commonly used pre‐main‐sequence isochrones by comparing their predictions to a set of well‐calibrated colour–magnitude diagrams of the Pleiades in the wavelength range 0.4–   2.5    μ   m. Our analysis shows that for temperatures less than 4000 K, the models systematically overestimate the flux by a factor of 2 at    0.5    μ   m, though this decreases with wavelength, becoming negligible at    2.2    μ   m. In optical colours this will result in the ages for stars younger than 10 Myr being underestimated by factors of between 2 and 3.  We show that using observations of standard stars to transform the data into a standard system can introduce significant errors in the positioning of pre‐main sequences in colour–magnitude diagrams. Therefore, we have compared the models to the data in the natural photometric system in which the observations were taken. Thus we have constructed and tested a model of the system responses for the Wide‐Field Camera on the Isaac Newton Telescope.  As a benchmark test for the development of pre‐main‐sequence models, we provide both our system responses and the Pleiades sequence.

Pre‐main‐sequence isochrones – I. The Pleiades benchmark