[rollei_list] Re: Veiling Flare

"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 machÃ.

"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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