It might do well to know the Petzval principle. S. ----- Original Message ----- From: "karl shah-jenner" <shahjen@xxxxxxxxxxxx> To: "List for Photo/Imaging Educators - Professionals - Students" <photoforum@xxxxxxxxxxxxxxxxxxxxxx> Sent: Sunday, August 22, 2004 2:57 AM Subject: Re: Digital lens question > > ----- Original Message ----- > From: "steves" <sgshiya@xxxxxxxxxxxx> > To: "List for Photo/Imaging Educators - Professionals - Students" > <photoforum@xxxxxxxxxxxxxxxxxxxxxx> > Sent: Sunday, August 22, 2004 12:52 PM > Subject: Re: Digital lens question > > > > Okay Karl, you can explain that to the Department of Defense that installs > > lenses on their satilites for digital imaging. I got my information from > > the lens designer while working in a photo gallery in Carmel. > > > > Go well. > > > > I'm quite sure Steve that the gentleman you refer to is well aware of the > diference between virtual and real images.. Lenses used for image formation > must converge the light to a focal point, lenses used for viewing through > (like a telescope) allow our eye to be the part of the optical path that > does the convergance. > > Either way for an image to form the light must converge. > > Auxilliary lenses however need not converge the light, neither need a *part* > of a lens system whether called a lens in it's own right or not - but the > optical system, all the lenses that make the whole must finally converge the > light to a point of focus. > > let me put it this way. The light reflecting from any given point will > radiate outwards, this light diverges and makes no image. Light travelling > from a bright source at a huge distance produces diverging rays that as far > as we are concerned approaches being parallel - it too makes no image. > > If it did we'd have images of stars all over our walls at night! as soon as > we use our eyes to look at these things we focus - a process that changes > the focal length of our lens and we converge the light to make an image on > our retina. Same thing with a camera, except we change the position of the > lens in relation to the film and thus bring the light to focus on the film. > > point a camera (digital or not) at the source of parallel rays (say a star) > with no lens mounted and snap a pic.. all you'd get would be fog. no image. > that parallel light is no good, the light needs focussing > > karl > > k > > >
