For those who might be interested, I did some tests yesterday with my AF 180/28 and my TC 16a - I had seen falloff in some shots taken last year, but couldn't be sure what f/ stop or shutter speed (or combination or othe two) had created the problem. I used a tripod mounted FM2T and Fuji Sensia 100 to run the test. Shots at f/2.8 had severe falloff at both 1/1000 and 1/500, and in spite of the very different exposures of the slides the falloff appears the same in both shots; f/4, at both 1/500 and 1/250 produced objectinable falloff, again with similar effect at both exposures. There was, however, no visible falloff at f/5.6, at 1/250 or 1/125, and a shot at f/8, 1/60 was also evenly exposed. (All aperture values are those indicated by the lens, not the effective value of the lens/TC combiination, and in each case of paired shots the lower shutter speed was indicated by the camera's meter and proved to be the correct exposure, while the higher shutter speed produced an underexposed image.) Since the problem went away at smaller f/ stop and was visible at f/4 at 1/250 but not at f/5.6 at 1/250, I conclude that it is related to aperture and has nothing to do with shutter movement. This result seems logical, and as someone else has pointed out, a problem related to shutter speed, would more likely result in uneven exposure across the image from one side to the other, caused by the shutter curtains beginning to open before the aperture blades have finished stopping down - and when you think about it it appears that this would probably be seen only at small apertures, so it would still be an aperture-related problem. Would smaller apertures have produced this type of problem? I didn't test for that because I would rarely, if ever, want to shoot this lens/TC combination at apertures smaller than (indicated) f/8. These results indicate that, unfortunately, this lens/TC combination is really usable only at indicated f/5.6 (effective aperture of about f/9.5) or smaller, a very small aperture for a tele. On the other hand, under a 5x loupe the slides appeared very sharp at all apertures, so if you are using a tripod or fast film the combination might still be useful. I hope this info helps someone - Bill Boyle
