long exposure error

[José Rebelo]

When taking long exposures shoots i´m getting a 1/3 of the frame darker than the rest.
I´m using a EOS 5D mark2 ; cokin holder P sistem with filters (round CP + ND8 + ND2); shoting in RAW; long exposure; I can´t understand if is an error in the camera (like curtains) or something in the filters.
If possible i would like opinions to solve the problem.

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[tirediron]

A little more information (exposure time, lens) and a sample image or two would help greatly.

[O|||||||O]

Is one of those ND filters graduated? Or, if not graduated - is it only covering part of the frame?

[José Rebelo]

[IMG][/IMG]

taken with 5d mark2, 70/200 2.8, about 160s, f:22, cokin holder with round cp + nd8

[KmH]

That looks like a shutter curtain, the second one.

The image on the sensor is inverted by the lens, and assuming you have the regular shutter release at the top of the camera in the vertical mode, is why I say it's the second curtain.

[Stosh]

Based on your exposure time I would say it wasn't the shutter.
My guess is some kind of light leak or internal reflections. With that high of a ND filter and that long of an exposure, any kind of minuscule stray light will ruin an exposure. I'm not familiar with the cokin holders, but my first guess is some kind of light is slipping by the filters.

Another possibility is light coming through the prism and getting by the mirror that's supposed to block that entrance. They provide a "cover" of sorts that you slip on over the viewfinder for such long exposures (after first removing the rubber eye cup). Did you use this cover?

[zamanakhan]

iam thinkin its ur filter since the cokin filters are made of resin plastic and are often prone to odd reflections..

try a long exposure in a dark room without any filters, if its still darker u will know if its the camera or filter

[José Rebelo]

After done more tests, i think the problem is a leak betwen the filters.
If not, i will post again.
Thank you all.