Steve, At Steve Larson's request, I compared my Vivitar Series 1 135/2.3 and 200/3.0 screwmount telephotos against my Pentax K-mount f/2.5s at the same ISO and f/stop. It appears that the Vivitars are "brighter"--that is, they delivered a faster shutter speed at the same f/stop. For the 200s, I used ISO 1000 and f/4. I aimed at a variety of targets in my basement. About half the time, the Vivitar beat the Pentax by 1 stop in my Super Program readout. The other half the time, the readouts were the same. That suggests the Vivitar is a half-stop brighter, f/stop for f/stop, so that its f/3 is as bright as a Pentax 2.5, since f/2.5 is 0.33 stop faster than f/2.8. For the record, the Vivitar uses a 72mm filter; the Pentax, a 77. For the 135s, I probably didn't use as reliable a method. ISO remained at 1000. But I set each lens to its maximum f/stop. Result? About 80 percent of the time, the Vivitar delivered a shutter speed that was 1 stop faster. It would be difficult to account for this by an f/2.3 vs. f/2.5 alone. The Vivitar uses a 72mm filter; the Pentax, a 58mm. I repeated the tests by switching cameras. Results were the same. I suspect that the Vivitars' brightness owes in part to the fact that they are only single-coated, and in part to a conservative f/stop claim. Paul Franklin Stregevsky - This message is from the Pentax-Discuss Mail List. To unsubscribe, go to http://www.pdml.net and follow the directions. Don't forget to visit the Pentax Users' Gallery at http://pug.komkon.org .
