Ongoing Saga - a brief interlude
2002-04-11 by royvharrington
Austin et al,
I don't know if you are listening to me any more but I'll try.
Here's a light hearted attempt. Maybe it'll be amusing enough
to bother reading and ease the tension.
Briefly, I'd like to get really simple -- let's get away from
DyR and talk about elementary school rulers. Dumb question:
How many inches in a 12 inch ruler? 12. How do you know?
(12 inches/1 inch) = 12 steps. How many quarter inches in the
same ruler? 48 = (12 / (1/4)). How many inches in a 36 inch
yard stick? 36. blah blah blah. This so obvious that its
painful. --- please no more examples!
This is approximately what you've been feeling in some of the
posts. Close maybe?
Austin: this is so obvious why are we talking about it.
Roy: I know, I know, but... I have a different ruler ...
Austin: no, no, who cares if its metric or time or anything
just take the last number (12) by the first number (1)
and you'll know how many units. it always works.
Roy: Well it works a lot of the time, but what about ...
Austin: come on, really, this is idiotic.
Roy: Well I've a couple of rulers that're different. Can
I tell you about them?
Austin: (you'll have to supply this one!)
Roy: This ruler I have goes from 1 to 16.
Austin: Great (16/1) = 16. It must have 16 steps.
Roy: Well I only see 8 even marks on it, but maybe they just
missed every other one.
Austin: Yep.
Roy: Hey, I have another that also goes from 1 to 16,
but it only has 4 steps.
Austin: Jees, Why don't you just go buy some decent rulers.
Or at least add the marks they left out and just
relabel them with all the numbers. it'll be good as new.
its so simple: 1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16
Roy: Well, I sure like these rulers, I've used them for
years and they've always worked perfectly. Maybe
I'll start with cleaning them up and polishing.
Austin: Great. Go for it.
Roy: Hey, guess what that first ruler actually goes all
the way to 32 now. Cool, clear from 1 to 32.
Austin: Well that sounds a lot longer. How the heck did
you miss the whole other half of the ruler?
Roy: I didn't miss half! It's just a little longer than
I thought. In fact there's only 2 more marks on it.
So its 10 marks long not the 8 marks I thought it was.
Austin: This is getting nuttier and nuttier. How are you
ever going to get all 32 marks you need on that
dumb ruler. Buy a new one.
Roy: Well I'm going to clean some more and see if there's
any more stuff written under all this gunk.
Austin: You're wasting your time.
Roy: Well its really starting to look good now. In fact
it turns out that there really are numbers on all
the marks. But, they sure don't look like we
expected. Here's what's written on the marks:
1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32.
Boy, it sure is good I didn't put those other
numbers on it. It would have confused things royally.
My other ruler goes: 1, 2, 4, 8, 16
Its a lot shorter, they didn't bother with the
intermediate numbers.
----------------------------
Well obviously the rulers are on my lens, The first one
is the aperture ring and the second one is the shutter
speed. I wanted to illustate how different an exponential
series is from a linear series. Linear is so natural to
our thinking many of its properties just seem universal.
Any kid can answer the questions in my first paragraph
without any deep thought. Exponential series on the
other hand are very non-intuitive.
Quick Question: How many stops are between f5.6 and f64?
Ask experienced photographers who've known the series for years.
How many know the answer instantly?
How many can calculate stops = 2*log2(f1/f2) in their head?
How many are reduced to kindergarten counting on fingers?
See my point. It is not intuitive.
Going back to my two "rulers", we know that the aperture
scale is a series of equivalent intervals. They don't
really "look" equivalent but they sure work that way.
Similarly, the intervals of the shutter speed series are
all equivalent. In going from 1 to 16 there are 8 intervals
on the aperture scale but only 4 intervals on the shutter
scale. Are the intervals bigger on the shutters? No,
as photographers we all know that intervals on both scales are
also equivalent to each other. I.e. every interval = one stop.
Notice that the end points of 1 and 16 tell us nothing
about how many steps there are. For the same overall
ratio of (16/1) we come up with two different numbers of
steps even when we've used the same "photographic" sense
of one step = one stop. Actually could also have tick
marks on the lens for fractional stops, giving more
possible "number of steps" values.
Mathematically, within an exponential series a ratio
of any two elements only gives you a relative measure
of "how far apart they are" not any sense of "how much
stuff (how many elements) are between them".
The identical analogy in a linear series would be subtracting
two elements. Using the ruler, I can say 5 - 3 = 2 inch
interval tells me nothing about how many tick marks there
are in the 2 inches.
------------------------------------------
The DynRange formula is always based on a ratio of two
measured values. The formula doesn't "care" which kind
of underlying scale is used for the two values.
But the meaning of the underlying scale effects the
meaning of the ratio which finally effects the meaning
of DynRange.
You've stated quite a few times that the "meaning" of
DynRange is number of steps. I entirely agree if the
underlying scale means something as a linear scale.
But I think the meaning of DynRange loses that connotation
when applied to a scale whose meaning is only as an
exponential scale.
Kind of dense wordage so here's an example.
I consider the fstop scale exponential; but is that really
builtin? After all its based on a measuring the aperture
size in mm and the focal length in mm. What if it wasn't?
How about a lens that had: f1,f2,f3,f4,f5,f6,f7...f16.
It wouldn't be very useful because photographically
the intervals would all be different sizes. So if
you were to apply DynRange the "meaning" of the
results would have no notion of "number of steps".
--------------------------------------
So going way back. I think the DynRange formula for
sound is fairly well documented and agreed on. Its based
on the ratio of power levels delivered to sounds waves and
to our ears. The power scale must be exponential to
give an evenly spaced perception of sound. Therefore
the DynRange of that sound has no notion of number of
sounds levels. In a much earlier post I showed my analogies
from sound to light and all the same scaling existed
in light as well as sound. So I keep coming to the
conclusion that number of gray tones in a print can't
be a property measured by DynRange.
Roy