On Fri, 8 Nov 2002 15:14:36 +0000, David Littlewood
>In article
>
>>Mycroft wrote:
>>> The second way is if your lens is of high enough quality, you are
>>> focusing a $1000 lens thru a $29 filter. Where do you think lens
>>> aberations will occur?
>>Certainly not at the $29 filter.
>>A simple planparallel piece of glass will have no detectable effect at all.
>>(Unless you tint the glass, and then (what marvel!) we have a usefull gadget
>>called "filter"! ;-))
>No; an absolutely flat, optically perfect piece of glass, without
>coating, will add flare to the image of perhaps 3-6% of the image
>forming light. That will apply to reduce contrast over the whole image,
>not just to show artefacts around bright spots such as the sun.
>
>For the record, most filters, even cheap ones, have at least a single
>coating and the flare factor will be maybe around 1-2%. Multi-coated
>filters will reduce it to a small fraction of 1% (but still not zero)
>but you don't get too many of those for $29.
Hmmmm....
Depending on the size of the filter, many good-quality
multicoated filters are available for $29 or less - but
many popular filters are actually uncoated (look at
Tiffen's output...), and the ill effects are minimal
(a 6% overall flare rate will add virtually nothing
to the image densities that will be seen in practice).