> 24 Jan 02, Frantisek Vlcek <[EMAIL PROTECTED]> wrote: > > but with auto-aperture and semi-auto-aperture the speed with > > which the aperture closes and opens needs much less blades. > > That's why the 9 blades are maximum, and 5-6 blades are > > usual. There's less stress on the mechanism. With 18 > > blades, it wouldn't close as quickly as required for 5 fps > > motordrive or 1/2000 time.
24 Jan 02, Steve Larson <[EMAIL PROTECTED]> replied: > Never thought about that, and thanks for pointing it out. I think > a lens with say even 12 aperture blades would have a hard > time opening and closing at 1/2000 second shutter speed. > The knowledge on this list never ceases to amaze me. > Steve Larson I've been trying to rationalize that train of thought, and I keep disconnecting. I'm not a camera-tech or an authority on SLR design by any means, so feel free to point out the error of my ways. But here's what I'm thinking... With regard to limiting fast shutter speeds: A heavy aperture diaphragm (lots of blades) should have no impact on fast shutter speeds. The sequence of events in an SLR is: 1) Mirror opens / Aperture stops down. 2) Shutter fires. 3) Mirror closes / Aperture opens. Each step executes to completion on it's own, and upon completion triggers the next step. Step 2 alone controls the exposure time. Steps 1 & 3 can be timed with a calendar, and the shutter can still produce as fast an exposure as it's technology will allow. The only impact if steps 1 & 3 are slow is the amount of dark time your eye sees in the viewfinder. The film will see the exposure time allowed by the shutter in step 2. BTW, "Shutter Release" is a misnomer... it's really a mirror release. Then "mirror-up" triggers the shutter and "shutter-closed" triggers the mirror return. The aperture goes along on the mirror's ride. With regard to limiting ultimate motordrive frame rate: The aperture diaphragm moves in the same time frame as the mirror... usually driven by the same mechanism. Typically, a heavy diaphragm is still significantly lighter than the mirror and it's reflex mechanism. Getting the mirror up and down quickly is more of a limiter to motordrive frame rate than opening and closing a diaphragm with relatively more blades. A state of the art aperture, even with 12+ blades, should be capable of cycling within the time frame supported by the mirror mechanism (no, I can't support that with data). True, total mass is critical in any start-stop mechanism... no argument. Lighter is better and saving a milligram wherever you can is a noble effort. In that regard I agree minimizing the number of blades supports faster frame rates. I'm probably being too literal here, but I think saying more blades limits fast motordrive frame rates assigns the cause to the wrong link in the chain. But, if you're having trouble moving a box containing a horse and a fly, it's kind of hard to blame your difficulties on the fly. Enlighten me... Regards, Tim - 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 .
