The images presented were shot at the same f stop, not at the same aperture diameter. The difference in focal length accounted for the perceived difference in sharpness of the 70mm lens. And that's the way it is. Paul On Apr 28, 2007, at 7:06 PM, John Francis wrote:
> > No he isn't. > > If you keep the same camera position, and the same aperture > (not the same f-stop; the same aperture diameter), and you > magnify the central portion of the image from the shorter > focal length lens to the same final size as the image from > the longer focal length lens the DOF will be the same. > The lens focal length doesn't enter into the equation. > > On Sat, Apr 28, 2007 at 06:44:41PM -0400, Paul Stenquist wrote: >> JCO is correct. >> Paul >> On Apr 28, 2007, at 11:02 AM, J. C. O'Connell wrote: >> >>> Secondly, whether hes making a scientific statement >>> or dogmatic whatever, or just generalizing, its >>> still WRONG, because the DOF isnt kept same OR similar >>> by changing focal lengths if you keep the same camara >>> postion, it **changes*** with focal length if you >>> do that. >>> >>> jco >>> >>> -----Original Message----- >>> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On >>> Behalf Of >>> Tom C >>> Sent: Saturday, April 28, 2007 10:43 AM >>> To: [email protected] >>> Subject: RE: M85mm f2.0 bokeh >>> >>> >>> Certainly what you state regarding DOF is true. I believe though >>> the >>> writer >>> was not making a dogmatic absolute statement of scientific fact. >>> He was >>> >>> generalizing. >>> >>>> As the camera: subject/background ratio wasn't altered, DOF >>>> should be >>>> SIMILAR for all four lenses. >>> >>> I'm pretty sure he knows that actual DOF is not changed by altering >>> the >>> subjects distance from the focal plane. That's only moving >>> subjects in >>> to, >>> out of, or within the range referred to as DOF. I think he means >>> that >>> the >>> *perceived* DOF will be *similar*, which is true for lenses close >>> to the >>> >>> same focal length used at close to the same aperture. Not the same, >>> but >>> similar. >>> >>> Tom C. >>> >>> >>>> From: >Reply-To: Pentax-Discuss Mail List <[email protected]> >>>> To: "'Pentax-Discuss Mail List'" <[email protected]> >>>> Subject: RE: M85mm f2.0 bokeh >>>> Date: Sat, 28 Apr 2007 10:26:25 -0400 >>>> >>>> No,this assumption is wrong, DOF is >>>> constant only for same fstop and >>>> MAGIFICATION (in camera ). If he >>>> used same camera position and fstop >>>> and only changed lenses, the shorter >>>> lenses will have same perspective >>>> in the shots but with MORE Depth >>>> of field than the longer lenses. >>>> DOF is a function of magnification, >>>> NOT the "subject/background" ratio. >>> >>> Mr. J. >>> >>>>> >>>> -----Original Message----- >>>> From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On >>>> Behalf Of >>> >>>> William Robb >>>> Sent: Saturday, April 28, 2007 9:10 AM >>>> To: Pentax-Discuss Mail List >>>> Subject: Re: M85mm f2.0 bokeh >>>> >>>> >>>> >>>> ----- Original Message ----- >>>> From: "Paul Stenquist" >>>> Subject: Re: M85mm f2.0 bokeh >>>> >>>> >>>>> Of course the 70 gives you more DOF. Thus, a bit crisper. >>>> >>>> I left the camera position static and cropped the 70mm and 77mm >>>> images >>>> to be similar to the 85mm images. >>>> As the camera: subject/background ratio wasn't altered, DOF >>>> should be >>>> similar for all four lenses. >>>> >>>> William Robb >>> >>> >>> >>> -- >>> PDML Pentax-Discuss Mail List >>> [email protected] >>> http://pdml.net/mailman/listinfo/pdml_pdml.net >>> >>> >>> -- >>> PDML Pentax-Discuss Mail List >>> [email protected] >>> http://pdml.net/mailman/listinfo/pdml_pdml.net >> >> >> -- >> PDML Pentax-Discuss Mail List >> [email protected] >> http://pdml.net/mailman/listinfo/pdml_pdml.net > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net
