Re: filmscanners: Vignetting?

[Arthur Entlich <artistic@ampsc.com>]

I could make some hubba-hubba male chauvinist comment, but I won't.

Yes, your image is probably showing vignetting, and it is probably
because you are using a wide angle lens and a Polarizer with too thick a
filter ring.  Also, I suspect the lens is not absolutely centered at the 
filter surface, causing uneven vignetting, (the lens barrel could have
some play in it) or you cropped off the right side a bit.

However, one possible explanation might be a wide sky area which is 
showing different degrees of polarization due to sun angle differences.


Suspecting vignetting, the reason the vignetting isn't more obvious is 
because you probably didn't have the depth of field that would made the 
filter ring in greater focus.  Ironically, stopping down might have 
either increased or decreased the effect, since the more you stop down, 
the less of the total filter surface is in play, so it depends a lot on 
how wide angle the shot is.

Now, having said all this, there are several other possible causes for
the problem you have on this image.  If this is a zoom lens, it is
possible that there is light falloff at certain positions in the zoom
range that are objectionable while there may be none or nearly none at
others.  The new compact lenses tend to have these problems more than
older ones.  This is due to the new computer designed lenses which often
use movable cams within the lens, which can cause internal light loss,
shadows, etc.  The variable f-stop lenses (ones which change f-stop
depending upon the zoom ratios) are more likely to show this fault.
This lens building method both allowed for much smaller and lighter
lenses, but also made them less expensive, due to less internal lenses
and smaller optics needed.  Further, some of these lens designs may not
have been tested for polarizers, and the polarized light may affect how
light bends and is reflected within the lens elements, again causing
more significant light falloff on the edges of the lens.  Some newer
lenses (mainly less expensive ones) use molded (rather than ground)
aspheric glass or even plastic lenses internally, which again allow for
amazingly wide zoom ratios in small lenses, but have other optical problems.

Lastly, in terms of possible correcting this problem on future images,
you might try using a circular polarizer rather than a linear one, as
sometimes this will help (if you are not already doing so).  Some
current metering systems require circular polarizers to read properly,
especially if they read through a semi-silvered mirror.

As to whether the "defect" in this image is objectionable, well, (smile)
I guess it depends what object you're concentrating on in the image.  I
think stock agencies have so many model shots to choose from that they
can be very picky.  If the model was very famous, or if the image told a
really good story, such defects would become secondary.  But let's face
it, it is just a pretty model in a bathing suit, and professional stock
photographers can set up such a shoot anytime and have no vignetting or
light falloff at all.

If you feel the image is special enough to have monetary value, fix it.
   That's the beauty of digital manipulation.  It isn't very difficult 
to even out a sky, but don't expect the stock company to want to bother
when they have these types of images coming out of their ears.

Finally, be happy the stock agent was honest with you and took the time
to explain why they rejected the image, many will just say "thanks, but
no thanks".  The information provided should help you to improve upon
your images for the future.  Believe me, a good stock agent knows what
clients want and need, and you're better off to follow their advice.

One final comment about stock photo agents... some agencies are very
specific in their catalogues.  If you send cheesecake to a lifestyles
stock agency, or an agent that only deals with scenics, don't be
surprised if you get a harsh letter back.  Research before submitting (I 
am not suggesting you didn't do this, BTW, I am just giving some basic
advice).

Art

Rob Geraghty wrote:

 > Apologies to those who are using the digest, because the attached picture
 > will appear as encoded ascii.  A while back I was in touch with a guy 
from a
 > stock photo company and I sent a low res jpeg of a photo of mine, 
which he
 > claimed showed vignetting.  Now to me, vignetting in the camera is 
caused by
 > a wide-angle lens "seeing" the edges of a filter.  Years ago I did 
make the
 > mistake of putting a polariser on the end of a lens which already had 
a UV
 > filter on it, and this certainly caused vignetting.  But the effect I
 > believe he was attributing to vignetting is caused by a polariser - 
the sky
 > tends to be darker at the edge of the photo, sometimes on one side,
 > sometimes both depending on the angle to the sun.
 >
 > Would anyone on the list call the variation in the sky in the 
attached jpeg
 > vignetting?  I don't find the effect objectionable, but are publishers
 > really likely to?
 >
 > Obscanning: images which have this kind of effect may actually enhance it
 > depending on the scanner settings used.
 >
 > Rob
 >