Transmittance and Veiling Glare Index measurements in conjunction with the Optical Transfer Function can give a set of objective measurements for specifying and providing quantitative data on the performance of components and complete optical systems. The main application of the work described, is to directly viewed telescopic systems. Transmittance, the ratio of transmitted radiance to incident irradiance, of telescopic instruments is usually measured with a small diameter collimated beam, integrating sphere and photo multiplier detector. An alternative method in which the full aperture of the objective is filled by the incident beam will reveal any adverse effects of vignetting or variability of anti reflection coatings and is of importance in subjective/objective comparisons when the eye pupil is greater than the exit pupil of the telescopic system. The MVEE 'audit' and Standard telescopes which were developed for investigation of the application of OTF measurement to visual instruments were found to have markedly different Veiling Glare Index values when measured on a wide glare field apparatus. Subsequently a series of measurements of these telescopes, using several current techniques for the measurement of stray light, indicated the importance of unwanted radiation which came from outside the field of view and the inadequacy of routine VGI measurements with a glare field equal to the field of view of the specimen being tested. For a single measurement test it was concluded that a black spot on a wide field (>±45 degrees) was desirable; this is substantially test condition C2 of British Standard 4995. The Standard telescope, as tested above, had the lowest VGI and the Audit telescope the highest VGI of all systems tested, with a modern 10 x 60 sight having a VGI value close to the former. The final section of the paper discusses possible combined instruments for measuring all three parameters, to give the most effective utilization of apparatus.