Bill, Full helicoid extension on the 100/4 macro gives 1:2 magnification and requires plus one stop exposure correction. Adding the 50mm tube permits a magnification range from 1:2 when the focus ring is on infinity (the same as the fully extended lens with no tube) to 1:1 when the helicoid is fully extended. At 1:1 the exposure correction is plus 2 stops. This assumes that you're not using TTL metering which would be more or less self correcting. However, beware of the effect of mirror cutoff at long extensions if your cameras meter is above the screen (less light will get to the finder than to the film, so the meter will cause overexposure). Below the mirror or OTF meter cells won't be affected by mirror cutoff.
The matched extension tubes for the old macro lenses would be nice to have for my two M series macro lenses. Alas I have to make do with the set of three manual tubes with the odd lengths that Pentax in their wisdom chose, perhaps they were getting into the "Limited" frame of mind as long ago as that. Regards, Anthony Farr ----- Original Message ----- From: "Bill Owens" <[EMAIL PROTECTED]> > Which brings another question to my feeble mind. Would adding an additional > 50mm of extension to a lens that is already extended increase exposure by > one stop, or is this too simple to be feasible? :-) > > Bill - 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 .
