----- 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
