To more fully answer your question first lets determine the aperture at 80mm/f11, that is 7.27mm. Now at 320mm, that is 29 mm. If you enlarge both to give you identical 8x10's (the 80mm one severly cropped) you have the same overall magnification (the subject is the same size on both prints), yes? Now the one taken at 80mm used an aperture of slightly less than 8mm and the one taken at 320mm used an aperture of almost 30mm. You answer, what is the difference, if any, in the DOF?
Now go out and take two photos, make two prints, then come back and tell everyone what the results are. Does one have approximately 4x the DOF of the other? A further experiment, once again take two photo one at 80mm/f11, and one at 320mm/fll, this time with the subject the same size in the viewfinder (the focusing distances will be very different but the magnification will be the same). Now make identical sized full frame prints of both. What is the result? About now you will be getting a feel for this DOF thing, and you are not taking anybodies word for it, but checking it out yourself. Ciao, Graywolf http://pages.prodigy.net/graywolfphoto ---------------------------------------------------------------- ----- Original Message ----- From: Chris Brogden <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Tuesday, April 09, 2002 3:02 PM Subject: Re: Using external lightmeter with a Zoom lens... > On Tue, 9 Apr 2002, T Rittenhouse wrote: > > > Actually, a constant f-stop zoom is a variable aperture zoom. The > > aperture has to vary over the zoom range to keep the f-stop constant. > > Variable f-stop lenses may have a fixed aperture (that is the aperture > > stays the same across the entire zoom range), or a stepped aperture > > (one that stays the same over part of the zoom range) which allows a > > smaller f-stop variation across the entire zoom range but is still > > cheaper than a fixed f-stop lens. Basically a fixed f-stop zoom lens > > is bigger, heavier, and more expensive than a variable f-stop lens. > > Once again as I have pointed out before, aperture and f-stop are not > > really interchangeable terms. > > So if aperture and f-stop are two different terms, which controls DOF? > That is, pretend I'm using my variable-aperture 80-320/4.5-5.6, and that I > have the lens set to f8 but zoomed in all the way, causing the viewfinder > to display f11. Now, do I get the DOF of f8, or f11? In other words, if > I take two shots at 80mm (one at f8 and the other at f11), then crop and > enlarge the shots so they approximate a 320mm view, which one of the > shots--f8 or f11--will be closer to what I'd get by shooting at 320mm to > begin with? > > chris > - > 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 . - 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 .
