Filters and ghosting

I'm popping in here due to an ongoing discussion elsewhere about filters causing ghosting when they're used on lenses which have a well recessed front element....ie, the further apart the filter and the front element, the higher the likelyhood of ghosting.
Would anybody here have the science to support the theory/observation?

Cheers
Bob

Bob Latham said:

I'm popping in here due to an ongoing discussion elsewhere about filters causing ghosting when they're used on lenses which have a well recessed front element....ie, the further apart the filter and the front element, the higher the likelyhood of ghosting.
Would anybody here have the science to support the theory/observation?

Cheers
Bob

Click to expand...

Bob,

Whenever an edge gets in the way of a beam, there's a special shadow called a penumbra. If that reaches the film or silica chip, then an extra edge is seen. There's a brief reference to these effects, here also look at this discussion on "funkiness" which to my surprise gives the same link to "penumbra" as I selected for you!

Asher

Thanks for replying Asher although I don't really follow the logic of "an edge" and the resultant penumbra

On a run of the mill lens, the filter thread is usually just a little way forwards of the front element (enough that the convex element doesn't contact an installed round filter (B&W, Hoya Pro1D etc). Some lens constructions have the from element well back, several centimetres, from the filter thread. Installing a filter on these lenses would seem to more likely induce ghosting than lenses where the filtter is almost in contact with the front element.

Perhaps you understood the question and I don't understand the relevence of the answer or maybe I didn't/haven't explained the setup too well.

Bob

Bob Latham said:

I'm popping in here due to an ongoing discussion elsewhere about filters causing ghosting when they're used on lenses which have a well recessed front element....ie, the further apart the filter and the front element, the higher the likelyhood of ghosting.
Would anybody here have the science to support the theory/observation?

Click to expand...

Hi Bob,

Without knowing the exact situation, I can imagine that a well recessed front element potentially has a front element with a rather flat front surface. That would make it a good candidate to pick up filter reflections. The relatively large distance between the filter and the first element also gets that potential reflection closer to being in focus.

I rarely use filters myself, because they are usually more trouble than help, a polarizer being an obvious exception. ND filters are also different in that they absorb a lot of light, including internal reflections between the filter surfaces.

Cheers,
Bart

Thanks Bart.
The discussion was based around the Zeiss 50/2 Makro Planar ZE which has a front element a full 30mm behind the filter thread.
Like you, I only use filters when I need the effect from them (CPL, ND, Grad and a few specials) but others like to have "protection" and fit UV or Skylights as a matter of course.

Cheers
Bob

I occasionally see ghosting in off-axis sun images where the filter is thick. Many of my filters are cut from 3mm coloured perspex obtained at the local plastics shop and a double sun image is inevitable. When I use a very thin gel the same distance in front of the lens there is no double image. I think the lens to filter distance plays little or no part in ghosting.

But there is another problem that has bitten me in the past. A filter a long way in front of the lens may come into partial focus when the lens iris is stopped way down. Any dust or dew on that filter then images as vague spots or blobs. The frustrating part is that it is virtually impossible to closely inspect a very dark stopped down image for those artifacts. Instead I clean the filter, shade it if possible and shoot; fingers crossed.