"Bruce Murphy" <pack-news@rattus.net> wrote in message news:m2llut8fxj.fsf@fruitbat.rattus.net...

> "David Ruether" <rpn1@no-junk.cornell.edu> writes:

> > "Bruce Murphy" <pack-news@rattus.net> wrote in message news:m2ispzm6oj.fsf@fruitbat.rattus.net...

> > > "David Ruether" <rpn1@no-junk.cornell.edu> writes:

> > > > I will often use step-up rings and large, wide, deep shades made for longer

> > > > lenses on my short lenses... (I make sure they do not vignette by checking

> > > > the corners of the near 100% coverage F3 VF with the lens stopped down

> > > > all the way using the DOF preview).

> > > Interesting. You don't feel a lens is more likely to vignette when it's

> > > wide open?

> > (I knew that would come up again...;-)

> > No. As the lens is stopped down (focused to infinity with most, but

> > to closest-focus with some zooms), DOF brings external obstructions

> > more into apparent focus, showing a fairly "hard" corner-cutting at

> > the smallest stop unless the lens FL is long. With wide stops, the

> > argument can be made that the too-small shade will affect wide stops

> > more, but in practice lens internal obstructions predominate, leaving

> > the shade obstruction in the "shadow" of the lens' own internal

> > obstructions.

> And the reasons that internal obstructions are not also brought into a

> harder vignetting focus as the lens is stopped down is precisely the

> reason why as you stop a lens down, external obstructions will be more

> significant, but have to be placed in a narrower cone.

Hmmmmm.....;-)

> > This can be checked by aiming the lens wide-open at

> > a uniform light area and placing a finger at the edge of the shade

> > where it would affect the corner if moved inward. If the shade does

> > not cut into the image at the smallest stop (viewed with a 100%

> > coverage VF), there will likely be no effect from moving the finger

> > tip very slightly inside the shade edge at the frame corners...

> > 'Course, there may be exceptions...;-)

> I think this is much more likely to be the case on wideangle lenses

> where the physical magnitude of the aperture doesn't vary all that

> much. Longer lenses in nparticular where the collection aperture can

> be several centimetres wide will behave somewhat differently.

Yes, but it is also easy with long lenses (well, not that hard with all

lenses...) to look in the front of the lens with the shade on and see if the

shade will obstruct the aperture when open or at minimum stop when

viewing the VF screen corners of an SLR...

> So in short, the aperture at which it is best to check to vignetting

> will be 'f/it depends' :)

>

> B>

Yes, but most of the time, minimum is fine (I have found no exceptions,

in all the combinations I've used...;-). BTW, you can also compare

wide-open exposures with/without the shade and with a constant, large,

and even light source to also check for shade effects with teles and

fast medium FLs...

--

David Ruether

d_ruether@hotmail.com

http://www.David-Ruether-Photography.com

Hey, take a gander at www.visitithaca.com, too...!