You could try stacking two polarizers.  With the two aligned, you'd get
only a few stops darkening, but rotating one 90 degrees would make it a
lot darker.  Align them while composing, then rotate one to take your
shot.

Here's a discussion you might find helpful:
<http://www.photosig.com/go/forums/read?id=207169>

Yes, you can get a Cokin filter holder and then get filters from any of
several manufacturers to work with it.  Check at B&H in the Filters
section.

Is colour fidelity an issue? If it isn't, or you don't mind mono
together with false colour rendition, you could always try having some
fun at the same time with an infrared filter.

Make your own from some gel filter material?

In article <1141253759.260604@ftpsrv1>, slowphoto <john@doe.com> writes

I have used the crossed polariser method mentioned by others (to
increase exposure time in photomicrography to take exposure time out of
the danger zone) and yes, even using top quality Zeiss polarisers, I got
a strong (blue) colour cast. I suspect, but cannot state categorically,
that this is a result of a frequency-dependent change in polarisation
efficiency.

A completely off the wall idea, which you can try for nothing: use a
slit in front of the lens. You would not want to stop down the lens, as
that would affect depth of field (and in extreme cases diffraction would
reduce resolution quite a lot). However, if you use the same approach as
that in cat's eyes you should maintain the optical properties of the
lens* but be able to attenuate the light by any amount - depends only on
the precision with which you make the slit.

*Except possibly for some astigmatism.

Try using a folding hinged pair of matt black** flaps mounted - an a
Cokin or Lee holder if you have one, or lash up some other way. You can
compose the picture with the "doors" open, then close them to take the
picture. Some trial and error will be involved to get the exposure
factor, but once you have it the factor should stay constant unless you
change the slit width.

**If the barn door arrangement is well sealed and no light can get
behind it, any opaque material will probably do.

Got to be worth a try, in view of the zero cost!

David

IMHO, there's no need to make this more complicated than needed to get the
results you want ..... just get the Cokin "P" holder and ND gel(s)....they
come in different "strengths (stops)" and you can stack 3 or 4 in the holder
if needed.  This will allow you to compose your image and then just slide
the filters into place.

An important consideration is that the Cokin holder is wider than the end of
your lens thus avoiding vignetting which undoubtedly would occur if you
stacked multiple glass filters on the end of your lens.  -- by the time you
compose, determine the settings, and screw in multiple glass filters, it
will probably be dark enough you wouldn't need any NDs anyway.  :)

Hope this helps.

Mike

To simplify the process of attaching the ND, get a gel-filter holder (Kenko
makes/made a nice one with detachable lens shade) and buy some NG gels to
place into the holder. Keep the holder on the lens, and swap out NDs as
needed. ND gels are fragile, and not *cheap*, but you can quickly stack 'em,
and get all sorts of degrees of darkening.

Hunt