That's right. Apologies to Rob, whose knowledge I highly respect. I just got confused and read "relatively small area of the image forming rays" as meaning that they cause a relatively small shadow on the sensor itself (which is not true, the shadow is bigger but blurrier, that's why it's not visible)
Thanks all who posted. j On Wed, 9 Mar 2005 15:37:01 -0800, Godfrey DiGiorgi <[EMAIL PROTECTED]> wrote: > Rob said the same thing I did (almost) albeit in a particularly > tortured form of English. ;-) > > Again ... Shadows from a broad light source vs a point light source. > That's the answer. > > Godfrey > > > On Mar 9, 2005, at 3:23 PM, Juan Buhler wrote: > > > Actually, I'm buying Godfrey's explanation more than this one. I think > > the difference between the angles of incidence should be negligible... > > > > j > > > > > > On Thu, 10 Mar 2005 09:42:18 +1000, Rob Studdert > > <[EMAIL PROTECTED]> wrote: > >> On 9 Mar 2005 at 14:13, Juan Buhler wrote: > >> > >>> On Wed, 9 Mar 2005 15:14:32 EST, [EMAIL PROTECTED] > >>> <[EMAIL PROTECTED]> wrote: > >>> > >>>> Obviously a small (higher) f stop shows up fine detail that might > >>>> get blurred > >>>> with a shallower depth of field. > >>> > >>> > >>> This is precisely what is not obvious to me. If the dust is on the > >>> front element of the lens yes, it will be more visible at smaller > >>> apertures. But we are talking about sensor dust, which is right on > >>> the > >>> sensor, without a lens to "focus" it. > >> > >> Think of it this way and remember that the surface with the dust is > >> suspended > >> over the active area of the sensor: > >> > >> At wide apertures the image forming rays pass through the lens over > >> the entire > >> aperture opening and all focus at the film/sensor plane so the image > >> forming > >> rays are coming from a wide range of angles so any object suspended > >> above > >> obscures only a relatively small area of the image forming rays.. > >> > >> At narrow apertures the image forming rays pass though a narrow > >> opening hence > >> that are conformed to pass though the dusty filter above the sensor > >> surface at > >> limited angles hence anything in their path will be rendered as a > >> more obvious > >> shadow on the image forming plane. > >> > >> Cheers, > >> > >> > >> Rob Studdert > >> HURSTVILLE AUSTRALIA > >> Tel +61-2-9554-4110 > >> UTC(GMT) +10 Hours > >> [EMAIL PROTECTED] > >> http://members.ozemail.com.au/~distudio/publications/ > >> Pentax user since 1986, PDMLer since 1998 > >> > >> > > > > > > -- > > Juan Buhler > > http://www.jbuhler.com > > blog at http://www.jbuhler.com/blog > > > > -- Juan Buhler http://www.jbuhler.com blog at http://www.jbuhler.com/blog
