As Wayne noted, 3mm makes a big difference.
According to a FOV calculator a 21mm has an 81 degree view (measured on the 36mm
side) versus 93 degrees for a 17mm. So the strange effects you see around the
edges will be enhanced (for better or worse).
Some of my experiences with Super wide.
Grid screen helps to keep verticals and horizontals lined up, but I also use a
dual axis bubble level that mounts on the hot shoe.
I use a geared head (advice from this list got me using that)
Specific Tamron notes:
Although you mention OM / OM2 you may want to get an adaptall that is OM4 safe.
And hopefully you get the lens hood with the lens. Tracking those down
separately is painful. It is heavy, and the Fstop selection doesn't have that
feel of a Zuiko. I believe there are two models of the Tamron 17mm. I've got
the one that uses 82mm filters that are screwed into the hood. Which is why the
hood becomes important beyond the flare issue. It does have built in 81B, 80B
and Y2 filters.
And for a heretical statement, I like my Vivitar 19-35 for the flexibility of
the zoom. I can say the FOV difference between the 21mm and the 19mm of the
Vivitar is not nearly as dramatic as the difference between the 21mm and the
17mm Tamron. I found that 2mm is not a big thing, but 4mm is.
Jay
>I have a OMZ 21mm, which is fun to use, but needs reasonably careful
>composition to avoid strange effects around the edges. I've never used
>a 17 or 18mm superwide, so what does the extra 3-4mm of FL give you -
>both good and bad? I realize this type of lens is probably a specialty
>lens and its usefulness depends on individual tastes and shooting style.
>I'd be grateful for any comments from list members, based on their
>experiences, whether lenses of this FL offer that much more than a 21mm
>(given the price of an OMZ 18mm, I would likely be looking at a Tamron).
>
>
>Martin
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