The evaluation of image quality parameters directly from the image is often required when the imaging system point spread is not known. The derivation of the parameter values from microdensitometer scans across the images of inferred edges in the scene format is a popular technique. This paper presents the results of a study to determine the errors incurred and the simplification that results from making certain approximations in deriving the values of two particular parameters. The measures of image quality sought are "resolution", which is defined as the area under the transfer function, and "passband", which is the area under the square of the transfer function. An expression is derived for low contrast edges which provides a simple relation between the edge density trace and the line spread function which is independent of film gamma, and the error in resolution and passband resulting from its use as an approximation for edges of higher contrast is given. The second approximation considered is due to the practical requirement that the density trace of the edge is truncated. Closed-form edge-density functions derived from transfer functions consisting of weighted superpositions of exponential and gaussian terms were terminated by "visual" inspection and used to evaluate the errors introduced in the computations of the resolution and passband.