Length frequencies are essential data for fish stock assessments, particularly for longer-lived species. They are usually provided by commercial vessels, or by port sampling, observers, or sample-vessel programmes, but each of these has limitations. Collection by sample vessels might be the most balanced way if the data quality is verifiable. We introduce a photograph-based length-measurement approach for sample vessels to photograph fish images with a calibration board, using a regular digital camera to obtain length estimates that can be verified after the images are transformed to reduce errors of perspective distortions. We analyse this approach under ideal conditions, develop a set of objective criteria for choosing acceptable photographs from observers, and compare estimated lengths for bigeye tuna (Thunnus obesus) based on this approach with lengths measured by observers. The criteria can serve as guidelines for photographing: if images are captured following these guidelines, the approach shows the potential for obtaining cheaply a large quantity of length estimates that deviate around 3% (up and down) on average from the actual measurements taken by observers.