Hi Moose, Ken, Mike, Chuck and all,
From: Moose <olymoose@xxxxxxxxx>
>Undoubtedly someone has gone to the trouble of designing an internal
>focusing prime with close to constant effective focal length
'Effective' is the key word here. If I understand it correctly, one thing is
the focal length as defined by the optical power of the elements
combination, and another is the _effective_ focal length, which adds the
offset for focusing anything closer than infinity. In fact, at infinity both
focal lengths are the same.
>In any case, most of them do change focal length as focused closer.
'Classic' lenses, where the whole optical block moves back and forth while
focusing, have constant 'optical' (?) focal length, but as we all know, do
increase the _effective_ focal length at close focus -- the well known
'breathing effect' on movie lenses. Maybe negligible with non-macro lenses
(reproduction ratios below 1:6 or so) but could be an issue for higher
magnifications.
>The Canon 100/2.8 macro is IF. So as
>one focuses closer, the working distance gets even shorter than would be
>expected with conventional design.
IF lenses do modify their optical layout while focusing, so they _could_
shorten their 'optical' focal length at closer range. The _effective_ FL
would be almost the same as at infinity; some optical trickery may avoid the
elongation of the lens' body... with the side effect you describe.
>With any lens at 1:1, the the focal plane to subject distance is 4x the
>focal length.
...assuming the delta-PP' parameter is _zero_. Most standard and even short
teles will be close to it, but retrofocus (positive) and long teles
(negative) definitely won't.
>The Canon 100 mm, instead of 400 mm, has a close focus of
>1:1 @ 310mm, for an effective FL of 77 mm.
Don't know the optical data for this particular lens, but it could be the
combination of the two above factors: the shortening of 'optical' FL due to
IF, and the (most likely negative) delta-PP'
From: Ken Norton <ken@xxxxxxxxxxx>
>If you
>placed a normal 2X teleconverter on the back of a 50mm lens and then
>extended the pair from the film plane, it acts like a 100mm lens in working
>distances, etc., but when you extend the 50mm lens first and then multiply,
>it acts pretty much like a 50mm lens.
For a given magnification, putting the extension between the converter and
the film would need _more_ extension, because as you say, it's behaving like
a 100mm lens. But working distance will be higher too, if I'm not mislead.
From: Mike (usher99@xxxxxxx)
>Well it is a good point that the no free lunch principle applies--more
>mag means more light loss and a smaller fraction of the total image
>circle is used.
That's for the 'classic' case of block-focusing lenses... but if some
optical trickery is used, the light loss could be minimised. See below ;-)
>The exposure correction factor should be able to be calculated from
>(M/P+1)^2 where M is the magnification and P is the lens pupillary
>magnification --I think about 1 for a standard lens).
I don't have at hand my books on Optical Instrumentation, but for a safely
assumed lens with pupillary magnification of 1, that formula becomes simply
(1+M)^2... which is the exposure increase as expected by the mere
_effective_ FL increase of a block-focusing lens -- it goes squared because,
the diameter of the iris being constant*, the f-number will increase
linearly with FL, of course.
*) There was an old 'compensating' Micro-Nikkor lens that did alter the
physical aperture while focusing, in order to "compensate" for the light
loss -- a clever way to greatly simplify exposure calculation, in the era
before TTL (flash) metering...
>For IF lenses things are complicated. . These lenses focus by moving
>the rear node away from the film/sensor this results in some shortening
>of the focal length and a change in pupillary magnification results.
Can't say for sure, but I think the final result is that the effective light
loss depends _only_ on the increase of _effective_ FL, more or less
compensated by a reduction in 'optical' FL.
If a lens could focus down to 1:1 by _halving_ 'optical' FL, the _effective_
FL would remain the same, and so would the _effective_ F-number (f/D), so no
light loss would happen...
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
>Well, the inverse square law isn't broken but in John Colishaw's
>"Closeups in Nature" he gives examples using a 50mm lens, a 2X
>teleconverter and various extension tubes applied alone or ahead of or
>behind the converter that all result in a 1:1 image but have differing
>working distances and differing degrees of light loss... ranging through
>2, 3, 3-1/2 and 4 stops. Sounds a bit strange at first until you
>realize that the effective focal length is also changing which is why
>the working distance is also changing.
I think this matches "my" theory...
>Google has some of Shaw's book on line which can be gotten to by this
>link. Click the highlighted text (which doesn't really take you to the
>right spot) and scroll down to and read pages 110-111 for details.
><
http://books.google.com/books?id=RDy0zlRFFDsC&pg=PA7&lpg=PA7&dq=%22teleconverters+for+closeups%22&source=bl&ots=50DhqwJgnt&sig=DRRBGWuOhcZxxjVsTwYNF9SKh0Y&hl=en&ei=Pe3OS834IYrs9QSi3-GgDw&sa=X&oi=book_result&ct=result&resnum=1&ved=0CAgQ6AEwAA#v=onepage&q=%22teleconverters%20for%20closeups%22&f=false
>
I get a 'no preview available' message... :-(
>The Vivitar MFTC varies its length by varying the extension between the
>lens and TC lens. When the MFTC is at 1:1 this is equivalent to Shaw's
>third method of reaching 1:1 by adding 25mm of extension between the
>50mm lens and the 2X converter. This method increases working distance
>to 6" but also costs 3 stops instead of 2. (1 to the extension, 2 to the
>converter). But it may be better than Shaw's second method which is to
>add 100mm of extension behind the lens mounted directly to the
>converter. That yields a 100mm lens and 8" working distance but also
>costs 4 stops and quite a long lens/tube configuration.
IOW, for a certain magnification _and_ entrance pupil (aperture), the
shorter the working distance, the lower the light loss.
From: usher99@xxxxxxx
>I read a post on Pnet claiming that if one takes a eg. Canon 100mm 2.8
>lens with no IF and adds extension to reach 1:2 keeping it at infinity
>and use a macro lens of the same type with IF, the IF lens will lose
>slighly less light.
Absolutely. The simply extended 100/2.8 at 1:2 will become effectively a
150mm lens, where the constant 35.7mm aperture will give an effective
F-number of f/4.2 -- a one-stop-and-a-sixth light loss. The IF lens would
depend on the particular design, but surely will be closer to the nominal
f/2.8.
IIRC, the catadioptric Tamron SP 500/8 was advertised with 'no light loss at
close range', maybe it kept the effective FL constant, by focusing thru
reducing 'optical' FL... or was it just marketing hype? :-)
>Given that N[eff] = N (1+M)---there are no other variables in this, now
>I think magnification may be the only relevant factor and the pupilary
>magnification is a side issue .
Probably some variables compensate the others and, all things considered, we
get the same formula -- if we think on _effective_ FL terms.
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxxxxxxx>
>It's interesting that the Canon 100/2.8 macro loses 1.3 stops instead of
>1 stop at 1:2. There must be some internal inefficiency that is not
>evidenced at 1:1.
Anyway, the measurement is somewhat coarse, just the typical 1/3-stop
resolution... it could be actually on the expected 1 1/6 stops. Could be a
lot of things, actually.
From: Moose <olymoose@xxxxxxxxx>
>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.
Even with the same sizes, distances may depend on particular designs... but,
yes, there are some other loss causes.
>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,
In fact, the aperture (D) for f-number computations is the diameter of the
entrance pupil... that is, the iris as seen from the front, thru all
elements in the front group. Thet _could_ change magnification on IF lenses.
>Similarly, other restrictions in the light path as components are mixed
>and matched may restrict the passage of some of the light.
Absolutely, but it's more likely to be an issue with very fast lenses --
macros are comparatively slow, so they are less likely to suffer from
mechanical diameter restrictions.
>Oly 2X-A teleconverter entrance lens diameter = 18.8 mm
>Kiron 7 element teleconverter entrance lens diameter = 19.8 mm
Now that you mention it... some (if not most) teleconverters can't use all
of the aperture of fastest lenses! This is clearly documented in the case of
Nikon's TC-16A 1.6x teleconverter -- the maximum "tolerated" aperture is
f/1.8, which after conversion would become f/2.9. The extra rays from faster
lenses won't reach the film/sensor and may lead to metering errors.
Cheers,
--
Carlos J. Santisteban Salinas
IES Turaniana (Roquetas de Mar, Almeria)
<http://cjss.sytes.net/>
--
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