On 4/22/2010 3:41 PM, usher99@xxxxxxx wrote:
> Eagle eyed Chuck did notice that the Canyon 100/2.8 macro is IF AND lost a
> third of a stop MORE light than the only standard helicoid lens on the list.
> Not sure how these fallacies get started
> and perpetuated
Some of it is undoubtedly sloppy work. Web forums, blogs, etc. certainly
make it easy for sloppy results to be presented to a wide audience of
the credulous and/or those who would like the truth to be as proposed on
the sites.
Another, much older factor is the natural human tendency to attribute
results to wrong causes, then generalize. Simple example; someone
measures characteristics of their first IF lens. It differs from what
they expected based on non-IF lenses, so they come to the apparently
logical conclusion that it is a result of IF, whereas it may be the
result of other design decisions.
Then either the originator or other(s) generalize it as a characteristic
of IF lenses in general - and we are off to the races. The nature of the
web makes this process happen faster and propagate widely.
> and even made it into print (pg 74 JS first macro book)
I'm not ready to dismiss Shaw's results without further consideration,as
they appear at first glance to be the result of empirical observation
and he seems to be fairly careful as experimenter and observer.
> , the equations with non symmetric lenses get quite hairy quite quickly
> though. Enjoyed your post summarizing JS's mix and match TC/extension tube
> use to either maximize mag or working distance. I am glad I have 3 of JS's
> books--still fun to read and overall superb despite one mistake it appears. I
> first heard about these books on this list.
>
Funny thing about that, as a counterpoint to what follows. It has been
my observation about life that some parts of it follow scientific and
logical principles, while others seem to operate on other principles.
While this thread was going on, I thought it would be nice to have a
copy of "Closeups in Nature". Last night, we went to a (wonderful) play
at Berkeley Rep. Often when the plays let out early enough, we wander
into the Half Price Books on the corner afterward.
Over many years I've only once seen, and bought, a John Shaw book there.
Last night, there was a very nice copy of "Closeups in Nature", so now I
have my own.
> So Moose with vorpal sword in hand and synaptic prowess has slayed two
> fallacies in so many weeks.
"One, two! One, two! and through and through
The vorpal blade went snicker-snack!
He left it dead, and with its head
He went galumphing back."
But did I leave it dead and with its head, come galumphing back?
Unlike with synchronicity or the generally deeply misunderstood "Law of
Attraction", certain parts of the universe are very reliably described
by the "Laws" of physics. Having once, long ago, been a physics major at
Cal, certain of the principals of classical physics still rattle around
in my head.
One of those is the first "law" of thermodynamics "Energy can neither be
created nor destroyed. It can only change forms." Even Einstein's famous
extension to the equivalence relationship between matter and energy,
E=MC^2 rests on this foundation.
So ... We have the case of a fixed opening through which the light
reflected from objects passes to be focused as a projected image. No
matter how accomplished, through principal lens focal length,
teleconverter(s) and/or lens extension, if subject and object sizes
remain are same, differences in brightness of the projected image can
only come from different amounts of loss in the system.
In properly designed optical systems, the only losses come from surface
reflection and actual absorbtion in air and glass/plastic, i.e.,
imperfect transparency, and from reflections. All the light energy that
goes in has to be accounted for, either as definable losses or in the
projected image.
So how do I reconcile this theory with Shaw's empirical experience?
There is always the possibility of some error in his results, as you
suggest. On the other hand, his results may well be accurate, but the
reasons not stated. I don't believe he even tries to say why, only what
happens.
I suspect the likeliest culprit is inadequate diameter openings/elements
somewhere in the chain of ad hoc optical connection. You know, of
course, that a diameter restriction in the right place, such as the
aperture diaphragm in the primary lens, does not vignette, as logic
might suggest, but lowers the overall brightness of the projected image.
Similarly, other restrictions in the light path as components are mixed
and matched may restrict the passage of some of the light.
Some practical measurements:
Extension tube clear diameters:
Oly:
7 mm tube = 40 mm
14 mm tube = 37 mm
25 mm tube = 37 mm
All minimum diameters are at baffles at the rear of the tube.
Vivitar:
The 20 & 36 mm tubes are just under 29 mm interior minimum diameter.
12 mm tube = 30.5 mm
The smallest diameter of 12 & 20 mm tubes is at the rear. The 36 mm tube
has restrictions both front and rear.
Oly 2X-A teleconverter entrance lens diameter = 18.8 mm
Kiron 7 element teleconverter entrance lens diameter = 19.8 mm
I'm not going to speculate much further or do any testing, but it is
possible from the above that using the Vivitar 36 mm tube together with
other tubes, you might measure less light with them in one order than
another.
It's also not unreasonable to assume that using the Viv 36mm tube and
Oly 2x extender could lose more light than using the Oly tubes and Kiron
extender with some primary lenses.
> The easier to hand hold shorter FL macro lens at same mag (my antenna always
> went up with that one) one (at least not due to any difference in FL but
> perhaps lens weight only) and now this.
Well again, the theory is sound, but other factors external to those
accounted for by theory may be significant, if One is able to hold a
smaller lens steadier.
> Bravo.
>
Snicker-snack? :-)
A. Vorpal Moose
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