On 30 Aug 1998 23:31:47 GMT, hrfoto@aol.com (HRfoto) wrote:

>> Re: Looking for the best filter F5

>> Good quality filters are optically flat, and have no effect upon image
>>quality if they're coated.

>Partially correct. Good filters indeed are optically flat. But they do effect
>the image. Light, when traveling though a filter, will be refracted. The
>effect is the same as holding a straight stick below the surface of water. We
>all know that the stick appears to be bent. This also happens when light
>travels from the less dense medium of air through the much denser medium of
>glass. The refraction effect is dependent on the angle of incidence. Light
>hitting the filter at a 90 degree angle is not efected at all. With the
>inclrease of the angle of incidence, the effects of refraction on the image
>increases. Thus filters will effect photographs taken with wide angle lenses
>more when compared to lenses with longer focal lengths. The only way to
>minimize these effects is to keep a filter as thin as possible. Another example
>in this regard are some of the older Canon lenses incorporating lens elements
>made of calcium fluoride. There were some which had the front lens element
>made of calcium fluoride. Since this is a very soft material, Canon
>specifically designed the lens to incorporate a flat glass element in the front
>for protection. The point is that the lens design incorporated the refractive
>properties of this flat element to avoid any negative effects on image quality.
>While filters often are a necessity, it is important to realize that they are
>not without negative effects to the performance of a lens.

For a front filter, the "refractive" effect appears to be non-existent -
there is no perceptible change in either focus or corner performance
when adding a filter to a wide-angle. A fast tele should show the ill
effects of a filter the most (think of the image quality effect of
shooting through window glass with a wide angle vs. a very long
tele - the first will give acceptable quality, the second won't...),
and my tests with various brands show that Hoya, Nikkor, and the
German brands are indistinguishable from each other and from no filter
at all under most conditions. I freely use filters for protection on
any of my lenses that will take filters, sharpness nut that I am.