From: donl@glass.esd.sgi.com (donl mathis)
Newsgroups: rec.photo
Subject: Re: meter testing and lens flare (Pop Photo)
Date: 31 Aug 92 19:43:45 GMT
Reply-To: donl@glass.esd.sgi.com (donl mathis)
Distribution: usa
Organization: Silicon Graphics, Inc., Mountain View, CA, USA


In article <1992Aug29.050036.1337@cello.hpl.hp.com>,
jacobson@cello.hpl.hp.com (David Jacobson) writes:

|> First, on page 46 there is a piece on checking your camera's meter.
|> The procedure described is to shoot a series of transparencies of an
|> 18% grey card while bracketing the exposure, nominally by overriding
|> the ISO setting.  "When you've gotten the transparencies back, compare
|> the images to your gray card subject.  Determine which frame most
|> closely matches the gray card.  This is the ISO at which you should be
|> shooting that film and, probably, all films of that speed."
|> 
|> Is it true that ISO is defined such that a subject of uniform
|> brightness will come out 18% transmission?

No. ISO film speeds are based on finding the exposure that gives a
minimum workable density.  I don't know the details for transparency
films, but I'm reasonably sure that 18% is not part of the equation.

In any case, comparing densities in a transparency to densities in the
subject seems like apples and oranges, because in Real Life,
transparencies are never lit with the same light as the subject,
reflected 100% off a white card.  With an idealized illuminator for the
slides, you could make any of them match the card by varying the
intensity.

|> Second, on page 51 there is a discussion of lens testing.  It says "To
|> check flare, shoot a scene having a small, brightly lit, white or
|> reflective subject.  A white index card or sunlit reflections in an
|> automobile hood or fender will do fine.  Shoot at all apertures to
|> determine at what lens opening the bright card or reflection seems to
|> blur; the blur should go away at smaller apertures.  This will
|> indicate how far you should stoop you lens down to minimize flare."

I agree with you -- this seems a bit off-the-wall.  I can see how
extreme overexposure could affect sharpness, because at higher
exposures, internal reflections within the film will exceed the
exposure threshold, and begin to record as part of the image.  These
reflections are always larger than the bright point in the subject, so
bright areas will begin to get fuzzy as these large spots overlap.
This has nothing to do with lens flare, however.  I'm trying to decide
whether it could have anything to do with aperture or not, but I can't
figure it out.  You didn't say whether the shutter speed was changed
appropriately as the aperture was changed, and I haven't seen the article,
so I don't know if the overall exposure changed from frame to frame or
not.  If so, I vote for halation in the film.

|> [...]  I thought flare was manifest by two things.
|> First ghosts of a bright object or images of the diaphragm.  And
|> secondly, a general reduction in shadow detail caused by light
|> scattering into it.  [...]

I think you're right, though in the second case, I'd phrase it a little
more generally, to cover light scattering everywhere, and not
necessarily just in the shadows.  Perhaps a reasonable flare test, just
for fun, would be more along the lines of photographing a broad, very
white subject, with a small, very bright, very strongly colored element
in the frame somewhere.  The color should wash over the white parts of
the frame; that's flare.  In fact, half white and half some other color
should show a color cast in the white part.  This will require
transparencies, of course, since color in prints is at the mercy of the
printer.

A more accurate test for flare might involve gray scale exposures.  For
example, a gray scale made of patches placed in a black surround, one
at a time, and another made with them placed in a brighter white
surround.  I don't believe this test, or any other, would provide any
numbers that would be useful in real life.

In real life, I think the best you could hope for is a general sense of
comparing one lens to another.  The absolute values involved for any
given subject will be based on the subject, and will cover a range as
wide as your range of subjects.  I think of a few basic things when I
think of flare: make sure you've got good, coated optics; expect more
flare as the number of elements in the lens goes up (all else being
equal, which it isn't); keep the lenses clean; and most important, keep
the sun off the front of the lens -- make a shadow on the glass.
Aperture doesn't seem to be a consideration.

- donl mathis at Silicon Graphics Computer Systems, Mountain View, CA
donl@sgi.com