This is difficult to diagnose without seeing any images. I don't know if you're shooting film or digital, either. I can think of a few possibilities. Please understand I have no idea of your level of hardware expertise, I'm only stating the only possibilities I can think of, without benefit of seeing the images.
1. Incorrect hood (only if it's a generic hood)
2. Correct hood, incorrectly installed (not likely, unless it's off-center?)
3. Correct hood, petals are bent toward center (least likely, but possible?)
4. Installed UV or UVIR filter is vignetting (very likely if you used a std. filter)
5. You're seeing "normal" light fall-off common in ultrawides (possible)
6. Something is very wrong with the lens (hopefully not)
To make sure it's not the lens, shoot without hood or filter. If you see corner darkening, and this is the first time you've shot anything wider than, say 24mm, you might just be seeing normal corner light fall-off. The darkening will be relatively even, without distinct lines or edges.
If you've shot ultrawides with film, but not digital, and this is a digital camera, the ultrawide light falloff is often more pronounced on a digital sensor.
Some folks shoot ultrawides with center filters, neutral density in the center only, to even out the corner fall-off. Corners can be darker by 1-2 stops, depending on lens design. I haven't played much with the Zeiss 18mm, don't know the usual fall-off for the this lens.
If you're very familiar with ultrawides, and, without filter or hood, you're seeing marked vignetting, more distinct and darker than normal ultrawide vignetting, something is likely wrong with the lens.
If the naked lens looks fine, and you used a filter, try with the filter and without the hood. If you see the darkening with the filter, it's the filter.
Ultrawide lenses often need thin-mount wideangle filters. Typically the front of the filter ring isn't threaded and the filter is flush with ring at the front. The mount itself is thinner/narrower than a standard filter.
If it's not the lens or lens + filter, it must be the hood.
IIRC, there are no other Zeiss hoods that fit this lens, so I don't think you could have an incorrect Zeiss hood. If you have a generic hood, not Zeiss, and the hood threads into the front of the lens, then it could very easily be the hood. If you're threading a generic hood into a filter that's mounted on the lens, it is extremely likely to vignette.
If it's a Zeiss hood, mounted with the long petals top/bottom and the short petals on the side, and if the hood mounts evenly/square, it's hard to imagine it could be the hood. If it's a Zeiss hood mounted any other way, it could likely vignette.
The bent hood scenario is very unlikely, but if the petal corners are bent inward, they'll vignette.
If anybody can think of any other possilbilities, feel free to chime in.
If loneranger can post some pix and provide more details, we might be able to make a better guess. What kind of camera? Aperture? Evenly dark in all 4 corners? Etc?
Good luck,
Paul
