Focal length and filter distance from lens at

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Old May 17th, 2004, 01:46 AM   #1
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Focal length and filter distance from lens


I was looking at some sunshades/filterholders and matteboxes that could be used to mount a few contrast reducing filters in front of my PDX10. It has a focal length of 3.6mm at the widest angle of view. This 3.6mm wideangle (~50mm equiv.) - or should we call it "narrowangle" - is quite unusable for certain shots, so a wideangle adaptor must be applied. Now after that the focal length is 0.7x or 0.5x so it becomes:

f=2.52mm or f=1.80mm

With wideangle shots there are often contrast problems, at least in my shots, so I need some filters.

*** What happens if I put some filters in front of the lens and the distance between the lens and the filter(s) is greater than or equal to the focal length? ***

Problems? What kind of problems? Focusing on the filter? Picture quality degradation? Is there a way to circumvent those problems?

At least the Chrosziel filterholders are rated as follows:
f=7.8mm -> max 3 filters
f=5.2mm -> max 2 filters
f=4.5mm -> max 1 filters

so f=2.52 or f=1.80 looks really bad :(

I kind of guess why the Sony lens hoods only allow a single filter to be used...
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Old May 17th, 2004, 02:06 AM   #2
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Two issues can arise, although I cannot say how they relate to specific focal lengths. One, depending on the nature of the mounts, the corners of extremely wide shots might become clipped by a deep stack of filters.

Second, given the depth of wide-angle frames, focusing on a filter (and any dust of smudges on it) can become an issue.

I believe that these are probably the key matters at issue.
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