[rollei_list] Re: Veiling Flare

Dr. Owl, as my many British friends would say:
"Brilliant!".

John

--- John Owlett <owl@xxxxxxxxxxxxxxxx> wrote:

> "What are all those pieces of paper?" asked Pooh.
> 
> The animals were gathered in a clearing in the
> Forest by the tree in which Owl's house was.  Owl
> had laid out the pieces on the grass outside his
> house and was fluttering between them, studying each
> piece carefully.  Tigger, who thought this was a
> grand game, was bouncing from piece to piece,
> gradually reducing them to papier maché.
> 
> "They're replies from the RUG to his question about
> the flare on his Whiteface pictures," explained
> Rabbit.
> 
> "Why has Owl been talking to a rug?" asked Piglet.
> 
> "Oh it's not a real rug: it's a cyber-rug."
> 
> Pooh and Piglet thought about this.  Neither of them
> thought that Rabbit's explanation helped at all. 
> Actually, very few of his explanations helped at
> all.  This was probably because he had Brain.
> 
> Owl interrupted the thoughtful silence.  "I had a
> Whiteface camera for a fortnight and saw what looked
> like veiling flare on the test photographs," he
> explained.  "So I asked whether I might do better
> with a multicoated lens.  But the short version of
> what they recommend is that the veiling could well
> be caused by haze on the lens elements, and that I'd
> be better off learning how to use an electric torch
> to check for haze rather than worrying about lens
> coatings."
> 
> "If there's a short version," put in Eeyore, "then,
> knowing Owl, there will be a long version ... and an
> interminable version as like as not.  But never
> mind.  Mustn't grumble.  We'll probably have heavy
> rain to put a stop to it.  Even Owl has to stop
> talking if the Forest is flooded."  That thought
> seemed to cheer him somewhat.
> 
> "Well, there are some more things that could be
> said," went on Owl, saying them.  "I've not had to
> worry about lens-element haze in the past: I've
> bought most of my secondhand Nikkor lenses from Gray
> Squirrel of Westminster, and his lenses don't have
> haze.
> 
> "The replies I've had seem to be very even-handed
> about the lenses available for the Rolleiflex.  Some
> of the writers have their personal favourites, but
> they seem to take the view that 'reasonable men may
> differ'."
> 
> "Oh, come on Owl," splurted out Piglet, surprising
> himself with his boldness, "'men' is hardly
> inclusive language.  You should say 'animals'.  Or
> perhaps, since that word has acquired negative
> connotations, 'beings' would be better."
> 
> "Mum," asked Roo quietly, "what on Earth is Piglet
> on about?"
> 
> "He disapproves of speciesism," replied Kanga, "he's
> very PC."
> 
> "What's PC?"
> 
> "Pooh Correct."
> 
> "Reasonable beings may differ, then," carried on
> Owl, unstoppably.  "In a comprehensive reply,
> Richard distinguishes between veiling and ghost
> images.  That's a distinction I've met before.  The
> uncoated 69-year-old Carl Zeiss Jena Tessar on my
> other Rolleiflex has quite a lot of veiling flare
> but, unlike the Nikkors, I've never known it produce
> ghost images.  Richard says that flare increases
> nearly geometrically with the number of surfaces,
> but I prefer to count second-order veiling factors."
> 
> "You what?" asked Pooh.
> 
> "Don't!" cried Eeyore.  "If you ask questions,
> you'll only encourage him!"  But it was too late.
> 
> "An uncoated glass-air surface reflects about 4% of
> the light that hits it.  So, with a one-element
> lens, about 4% of 4% of the light bounces off the
> second surface AND then the first surface to be
> scattered randomly over the film.  That's only about
> one-sixth of a percent of the total light and
> doesn't matter too much.
> 
> "But if you have two groups in the lens, there are
> six times as many zigzags.  That's because there's
> light bounced back off the third surface and
> forwards off either the first or the second surface;
> plus light bounced back off the fourth surface and
> forwards off the first, second, or third surface.
> 
> "In the same way, with three groups there are
> fifteen times as many zigzags; with four groups
> there are twenty-eight times as many zigzags -- the
> triangular numbers.  With four groups nearly 5% of
> the total light is distributed as flare, so contrast
> in inevitably low."
> 
> Pooh was feeling queasy by now, so he lay back in
> the grass and closed his eyes.  Some of these
> numbers were too big for him to visualize, but he
> thought he understood the number 6 and he couldn't
> see how it was triangular in shape.
> 
> "Owl," said Kanga accusingly, "you telling us that
> flare in three-group lens is about double that in a
> two-group lens, and that flare in a four-group lens
> is about double that in a three-group lens?"
> 
> "Yes."
> 
> "Wouldn't it be simpler just to say that Richard was
> right?"
> 
> "Well, I suppose so, but Les did seem to say that
> prolixity was important."
> 
> "Hang on a moment," said Rabbit who had been
> studying one of the pieces of paper.  "You haven't
> mentioned that Patric suspects you of being a
> collector in wanting to have a Whiteface."
> 
> "Ah, yes," stuttered Owl, "I'm not a collector,
> really I'm not.  I use all my cameras, not just have
> them on a shelf.  It's just that part of my
> enjoyment in photography is using a classic piece of
> engineering from the past.  And surely a camera with
> a white face is most suitable for an owl."
> 
> "Fifty percent collector, I'd say," said Rabbit. 
> "Wouldn't you agree Kanga?"
> 
> "At least," she replied.
> 
> 
> 
>
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