Re: macro lens hotspot - help!!



David Littlewood wrote:

In article <442276E8.7A497B96@xxxxxxxxxxxxxxxxxx>, Colin D
<ColinD@xxxxxxxxxxxxxxxxxx> writes


steve keen wrote:

I'm getting a strong hot spot (overexposure) exactly in the center of my
macro images when the lens is stopped down to small apertures and the
exposures are long. The hot spot is a small fuzzy circle and are maybe 1/2 -
1 stop overexposed. I get the same thing on a D70 and a D200, and when
using either of 2 different 90mm Tamron macro lenses. Very little info on
this on the www. - can someone please give me an idea on this? I'd love to
keep these lenses as they are otherwise wonderfully sharp. thanks! steve

You may be experiencing a problem with lenses for film being used on a
digital camera. The Anti-aliasing filter in front of the sensor - part
of the sensor assembly - reflects more light back toward the lens than
does film, and where film has a matt surface, the filter is glass. This
causes reflections within or from the rear element of the lens, which
can show up as flare, or a hot-spot in the image.

Lenses designed for digital camera use have a different coating on the
rear element, and probably some internal elements, to suppress these
reflections.

I can't say whether this problem afflicts your Tamrons, but if they were
ok on film, then it's likely to be the cause. No cure except new lenses
{:-(

Colin D.

Interesting. You would think it should be possible to put a good
multi-layer AR coating on the AA filter - if Hoya and the likes can do
it for a pound/dollar or two on a filter, shouldn't be that much of a
problem.

David
--
David Littlewood

Yes, but maybe the action of the AA filter may preclude some coatings
being used. Don't really know, the above is what I have read but I have
no sources.

Colin D.
.

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