AIM: To simulate the effect of changes of pupil size and optical media with diffusing filters on visual acuity, and to compare these effects on Humphrey field analyser(HFA) and frequency doubling technology (FDT) perimetry. METHODS: The left eyes of 15 normal subjects aged 23-39 years underwent: 1) Snellen visual acuity and Pelli-Robson(P-R) chart testing; 2) white-on-white automatic perimetry of HFA model 630 using the 30-2 programme and size III stimulus; 3) FDT perimetry using N-30 programme. For each procedure, no filter, 1, 2, 3, 4 and 5 diffusing filters were mounted in random order in front of the eye to be tested.RESULTS: For Snellen visual acuity, 1 filter had no effect on all but 2 subjects who lost 1 line; 2 filters had little effect on all subjects and had no effect on 2 subjects (9 subjects lost 1 line and 4 lost 2 lines); 3 filters had the same effect on all subjects who lost 3 lines; 4 filters caused 5 subjects to lose 4 lines, 5 subjects to lose 5 lines, 3 subjects to lose 6 lines and 2 subjects to lose 7 lines; 5 filters caused 9 lines loss in all the subjects. For P-R chart , 3 filters usage caused 13 subjects to see none of the groups of letters and while 2 subjects could see only 1 group of letters. Both mean deviation(MD) of HFA and FDT were reduced with increased numbers of filters. For HFA, 5 filters caused nearly absolute scotomas, while for FDT, 3 filters caused the same result. The pattern standard deviation(PSD) for both HFA and FDT was uniformly consistent; the short-term fluctuation(SF) and corrected pattern standard deviation(CPSD) were all uniformly consistent.CONCLUSION: These findings showed that luminance reduction and light scattering caused by diffusing filters have marked effect on white-on-white and frequency doubling perimetry; the visual field loss of HFA and FDT was relatively diffuse, which could be effectively reduced through the use of pattern deviation analysis in separating visual field loss caused by glaucoma from that caused by cataract; the effect of media opacities on FDT was greater than it on HFA.