Measurements of Secchi disk depth are correlated with beam transmittance (B) in turbid coastal water. Both k (the irradiance attenuation coefficient) and α (the beam attenuance coefficient) can be estimated from the Secchi depth either on an empirical basis or by using the Duntley‐Preisendorfer equation of contrast reduction. Such estimates possess relatively large standard errors of estimate.The effects of the size of Secchi disk and viewing techniques are discussed. An estimate of contrast threshold of the human eye under field conditions has been obtained. A modification of the factor in the Poole and Atkins equation for estimating k from Secchi depth is suggested.