Yes it is. Look at it this way, if you set a 50mm at f/2.0 it has a 25mm opening, now if you set a 100mm at f/2.0 it has a 50mm opening, but at f/4.0 it has the same 25mm opening that the 50mm does at f/2.0. So, if the magnification of your viewing image (print) is the same (which Bill took care of by slightly enlarging his 70mm image) then the DOF will be the same. (Note that you can control the magnification by changing distance also, but that will change the perspective of the image.)
Sounds pretty complicated, but it is not actually. These are some of the controls you have over your images, at the camera. And in actual use it soon becomes pretty much unconscious. In the mean time you can cut and paste the following and print it on a 3x5 card to keep in your camera bag. 1) Magnification and aperture diameter control DOF 2) Focal length, subject distance, enlargement factor, etc. control magnification. Note, how many things affect magnification. 3) Distance controls perspective. 4) F-stop and shutter speed control exposure. 5) Shutter speed controls motion blur. 6) ISO adjusts for light level. NOTE: There are many interdependencies there, in other words, changing one thing also changes others. -graywolf [EMAIL PROTECTED] wrote: > In a message dated 4/29/2007 7:36:03 A.M. Pacific Daylight Time, > [EMAIL PROTECTED] writes: > ----- Original Message ----- > From: "graywolf" > Subject: Re: M85mm f2.0 bokeh > > >> Most folks do confuse f-stop with aperture, I sure used to. However, >> they are not the same thing. So you are mistaken here (I have not looked >> at the Wikipedia article to see if it is correct or not). >> >> I am not going to go through all this again, folks, look in the >> archives; or better yet, get the formulas and calculate DOF using f-stop >> and aperture diameter, go out and shot some photos and compare them with >> your calculations. Then, like me, you will never make that mistake again. > > ========= > Huh. Haven't been paying much attention to this thread. > > But I would presume while the diameter of an aperture opening could be set > the same from lens to lens if one fiddled around, a f stop is dependent on > the > focal length of the lens. So a f/8 on a 16mm and a f/8 on a 200mm say, would > actually have a different size of aperture opening if one measured the > diameter. > > Is that what you are talking about? > > Thanks. > > Marnie aka Doe :-) > > > > > ************************************** See what's free at http://www.aol.com. > -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
