Re: Internal ND filters on the XH-A1 affecting focus


"Martin Heffels" <goofie@xxxxxxxxxxx> wrote in message
news:71o394t6n78g0tk557ltpsatqbg3qe127b@xxxxxxxxxx
On 30 Jul 2008 23:26:58 GMT, "Jacques E. Bouchard"
<invalid@xxxxxxxxxxxxxxx> wrote:

Is this a phenomenon common to ND filters, or just the internal ones on
the
XH-A1?

Internal filters actually do indeed affect the back-focus of the lens,
because they increase the length of the optical path. This will usually
only be visible on wide angles, and when trying to focus on infinity,
turns
out your image gets blurry. So either zoom in a bit, or use external
ND-filters or a higher shutter-speed.

Cheers

-martin-

?????????????
Unless the lens optics are moved to accommodate the filter thickness
(highly unlikely), there should be no effect on the optical performance
of the lens regardless of where it is placed (assuming high quality
filters). You may see a *slight* shift in focus with the addition of a
rear filter on a lens, though. Putting in place a ND filter with a fixed
shutter speed would force the aperture wider for the same exposure
under the same conditions (as would using a higher shutter speed). A
lens witth inherent problems would then be more likely to show what
JB has observed. Zooming in toward tele would also likely worsen
the problem seen here, since to me, it looks like it is caused by a
defective lens. It is hard to tell, but it looks like in the image without
the filters that the sharpness in the left 1/3rd or so isn't great, but the
center and right are OK. With the ND filters in place, the image is
kinda OK in the bottom-right (with only the chimney on the right OK),
but quite soft elsewhere. I would expect better performance (my
HV20 lens is quite sharp to the corners wide open in the first half
of the VF-marked zoom range, but it gets rather poorer toward the
corners as the long end is approached (but the decline is even around
the frame edges and corners). I would do further checking on the
lens, without the filters in place...
--David Ruether
www.donferrario.com/ruether
d_ruether@xxxxxxxxxxx


.

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