Scott, Colour me dense, but I can't decide whether you're referring to the filter on the lens, or the CCD's filter, or both. Perhaps you should repost your description with the "filter's" position named.
"Diffuse" reflections can't be reformed into focusable images under any circumstance (except holography but that's interference patterns at work), only "specular" reflections have that ability (the "specular" reflections we associate with pinpoints of light are simply partial images from smooth surfaces with compound curves, rather than a more complete image being visible when mirrored off a flat surface). A cheap filter is the prime suspect when ghosting occurs, but it can also happen within a lens. I had a look at the FA50/1.4 diagram <http://kmp.bdimitrov.de/lenses/primes/_optics/50f1.4-iii.jpg> on Boz's site <http://kmp.bdimitrov.de/> and notice that the 3rd and 4th element from the front were strongly concave menisci (meniscuses?). Internal reflection of the incoming rays from the 4th element back to the 3rd element and then on to the focal plane would very likely show as an inverted image. Something to remember is that ghost images are extremely common, but are mostly extraneous images of the sun or bright lights when they're captured in the frame. Importantly, these images are inverted whenever they appear in the opposite quarter of the frame, but when it's the sun or a lamp being imaged nobody ever notices that simple fact! Digital photography ~did not~ introduce ghost images, and while it's theoretically possible for ghosts to reflect from the sensor, we should apply Occam's Razor to our theorising, in which case the digital solution is one level of complexity higher than the already proven theory of internal lens reflection, and so is one level ~less~ likely. regards, Anthony Farr ----- Original Message ----- From: "Scott Nelson" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Sunday, December 28, 2003 6:34 AM Subject: Re: another istD issue > On Fri, 2003-12-26 at 04:01, Jostein wrote: > > Quoting Anthony Farr <[EMAIL PROTECTED]>: > > > > > That's a "ghost image", a reflection from the front surface of one of your > > > lens elements onto the rear surface of the filter, and then back through the > > > lens in parallel with the desired image. It is most pronounced when a lens > > > has any uncoated elements, or the filter is uncoated. > > > > I thought ghosting between the front element and a filter would produce a > > reflection with the same orientation as the reflected object? > > No this is definately a ghost image from the filter. The CCD (or film) > doesn't act as a mirror, but diffusely reflects some light back towards > the front of the lens. This light travels back through the lens system > along the same path that it originally did, bounces off the filter > (which is acting as a mirror and creates the inversion of the images) > and is reformed on the CCD after another trip through the lens. In this > case, things are made worse because the lens is wide open, allowing most > of the rays relfected from the filter to be recaptured. > > -Scott > >
