> I'll say this and no more: when you ~manually~ set the aperture, > the "variable aperture" becomes a "preset" (by you) aperture. > Nothing you do while zooming will (can) change the aperture > until (you) change it to a different setting. [snip] And it does > not matter whether you set it wide open or close it down all the > way. The aperture CANNOT (does not) change until you change > it-period.
No. In a constant-aperture zoom, the diaphragm opening is either effectively opened wider or closed down narrower as one zooms to longer and shorter focal lengths - this extra engineering is necessary to try to maintain a constant aperture. Since the aperture of a lens, in f-stops, is equal to focal length divided by iris opening, longer focal lengths require wider openings and shorter lengths require smaller openings to maintain the same ratio. However, in a variable-aperture zoom, the designer simply lets the iris opening stay fixed during focal length changes (or sometimes only partially compensates for focal length changes). If the diaphragm opening is fixed at a certain size, as an example, then zooming to a longer focal length will result in a larger f-stop ratio (i.e., a smaller aperture), while zooming to a shorter focal length will result in a smaller f-stop ratio (i.e., a larger aperture), even if the aperture ring is untouched by the photographer. Therefore, despite the statement that "the aperture CANNOT (does not) change until you change it", the aperture (as an f-stop ratio) DOES change as you zoom in a variable-aperture zoom (although it does NOT in a fixed-aperture zoom, but only because the designer compensated for zooming by employing a correspondingly "opposite" diaphragm change). You said "I'll say this and no more" - Well, I will say no more on this myself, but I'll bet that you can't - <big grin>. Fred - 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 .
