At 18:31 5/7/01, Joel Wilcox wrote:
Hi John,
Do you happen to know if your working distance using 135/2.8 with
extension is similar to using a 135/4.5?
Joel W.
Joel,
Minimum "working distance" of the 135/4.5 Macro is 37cm. Its "minimum
focus" is 60cm. I presume the delta here is the "focus distance" from
subject to film plane (??) versus "working distance" of subject to front
element (??).
---------- Short Answer ----------
Working distance would have to be approximately the same for the same
magnification because the focal length is the same. I don't believe the
size of the optical "cell" (the compound lens) is that different between
them either.
I have two sets of tubes. One is a pair of Oly auto-tubes (7mm and 25mm)
and the other is a complete set of Vivitar AT-21 auto-tubes
(12/20/36mm). Usually I use only one tube with the 135/2.8, typically the
25mm or 36mm. With just the 36mm Vivitar tube, the working distance is
farther (about 50cm+) but the magnification isn't as much. However, if I
stack tubes (the Vivitar 36+20 = 56mm) and rack the 135/2.8 focus out
(~15mm) I can get 0.5X magnification and have a slightly longer working
distance between the front element and subject, but only by about a cm or
two. You could also stack a pair of Oly 25mm tubes with an Oly 7mm tube to
get nearly same extension (57mm).
Don't forget about the "bellows extension factor!" Using tubes to get a
0.5X magnification, you lose about 1-1/6 stop from the lens aperture
markings. This makes an f/2.8 lens effectively 1/6th stop slower than an
f/4 lens. An f/4.5 lens is only 1/3rd stop slower than f/4, making it
1/6th stop slower than the f/2.8 lens we started with! In short, if you
were looking for more speed by using a 135/2.8, you lose almost all of it
with the lens extension required for 0.5X magnification.
---------- Long Answer ----------
Working Distance:
For a "working distance" of approximately 37cm from the front of a 135/2.8,
you need three Oly auto-tubes: a pair of 25mm plus a 7mm (57mm total tube
extension). However, this is more than the 0.5X magnification touted by
135/4.5 Macro data sheet (with Telescoping Auto Tube). This will get you
to about 38cm working distance with the 135/2.8 also at minimum focus. A
set of the three different tubes isn't enough (7+14+25 = 46mm).
If you happen to have the Vivitar AT-21 12/20/36mm set of auto tubes, you
can stack the 20+36 = 56mm extension and get to 39cm working distance at
minimum focus. These two get you slightly greater than 0.5X magnification
(at minimum focus). With all three stacked (68mm total) you get down to
about a 35cm working distance at minimum focus and ~0.6X
magnification. Based on lens length at closest focus plus extension, the
minimum focus _must_ be something shorter than 60cm. At inifinity focus,
all three Vivitar tubes stacked is almost exactly 0.5X magnification with a
working distance probably no more than a cm longer than the 135/4.5 Macro.
Magnification:
For 0.5X magnification, the lens must be extended half the focal length
past infinity focus. This includes using the focus extension (helical)
that's built into the lens if necessary. The 135/2.8 racks about 15mm from
infinity to closest focus. You need 67.5mm total for 0.5X magnification
(half the focal length) which is any tube combination between 52.5mm and
67.5mm. Following is logical stackings within this range using Oly tubes
(no more than three):
25+14+14 = 53mm
25+25+ 7 = 57mm
25+25+14 = 64mm
Using more than three tubes is not recommended. For auto tubes, all the
linkages stacked up could create a sluggish stop-down. For both auto and
manual tubes, you can eventually have lens sag with all the bayonet
interfaces. The Vivitar tubes are sturdy, but not quite as sturdy as the
Oly tubes.
Notes (for benefit of list and archives):
1. Magnification is lens extension beyond its infinity position divided by
the focal length. "Extension beyond infinity position" _includes_ any
distance the lens is racked out by its own focusing mechanism. In some
texts "extension" includes the lens focal length also (lens rear principle
point [or nodal point] to film distance). This is not how I'm using it here.
2. Based on the math your question forced me to do, I suspect the Vivitar
AT-21 tube lengths were designed for use with a 100mm or 135mm lens. The
Oly 7/12/25 set was made for use with the 50mm lens.
-- John
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