John,
I had some time to go over your equations this weekend. Specifically
the corrected equation for effective aperture using the markings on the
lens. This is: (Effective A) = A*[(f+x+y)/f]. I am trying to compute the
effective aperture of the 135/4.5 on the telescoping auto tube when
focused at infinity (65mm ext). I'm not coming up with anything
meaningful probably because I'm not sure I understand the definition of
the variable "y". You state, "y=lens extension from focusing helical _if
focused closer than infinity_" (emphasis is mine) Plugging in "0"
obviously is meaningless. I must assume that y must be >0 or if I leave
the focusing ring alone and just use the telescoping tube i can
eliminate the "y" variable altogether (same as choosing a value
approaching 0). This gives an effective aperture of f6.7 at the very
best for the 135/4.5. The 135/2.8 yields f4.1 albeit in a bulky package.
Looking at the 80/4 and 85/2 w/ 65mm ext. I get f7.3 and f3.5
respectively. For the 90/2 I let y=65 and didn't use the x variable. I
don't know if the 90 actually extends that far but in pictures it looks
like it might. Anyway @ 65mm extension effective A = f3.4.
So now if you look at the magnification factor a few things really
stand out. To compare i looked at a ruler to see how many mm approx. top
to bottom in the viewfinder when in focus. I think about 5mm is a ratio
of 1:2. For the 135/4.5 there are a lot, it's at infinity focus. The
135/2.8 4mm, 80/4 5mm, 85/2 3mm and 50/1.8 2mm (eff app f4.1). So if you
are on a budget the 85/2, 100/2.8 or the 135/2.8 with extension tubes
are winners here. The 50mm puts your lens in the puddle so to speak. Too
short w/ 65mm extension (but the 50/3.5 is a bargain so get one of those
and skip the tubes). Don't have a 90 to check. Maybe the resolution
suffers using these standard lenses as macros but if you are handholding
and poor who cares?
i guess the next thing to do is look at the effective aperture at a
given magnification. i suspect that this is where the big bucks spent on
a 90/2 pay dividends.
OK do these ramblings make any sense? Or is my wife right after all.
mike
--
Latitude 48° 32' North, Longitude 123° 7' West
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|