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[OM] Re: ?simple? problem driving me nuts... calculating magni

Subject: [OM] Re: ?simple? problem driving me nuts... calculating magnification from focal length and focusing distance
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxx>
Date: Thu, 03 Nov 2005 07:40:39 -0500
As Shaw says, modern lenses don't necessarily behave as one would 
expect.  Glad to see you've forsaken the math and science and joined the 
engineers.

It reminds me of a career talk given by a Ford Motors truck engineer 
whilst I was a student in a mechanical drawing class in high school some 
45 years ago.  I commented to him that it must be extremely complex, 
using a truck's chassis drawing, to accurately compute the length of the 
copper fuel line running from the fuel tank to the fuel pump.  "Oh, no, 
my lad!" says he.  "That's not the way you do it.  You wait until the 
prototype chassis is available, walk down to the shop floor, cut a long 
piece of tubing for the fuel line, install it, remove it and then 
stretch it out and measure it."  I have well remembered that lesson to 
this day.

Another case in point.  Many years ago my wife and I volunteered to help 
her brother move from a second floor apartment to a first floor 
apartment in an adjacent building.  There were a few bulky but not 
terribly heavy items that we decided might be more easily lowered on a 
rope from his balcony rather than maneuvered down the stairs.  My wife 
suddenly commented: "The problem is that we'll have to throw up a rope." 
  My reply was: "Why don't we just carry the rope up the stairs and 
lower the rope from the top?"  Ever since then "throwing up a rope" has 
become synonymous with trying to do something the hard way.

Chuck Norcutt


jking@xxxxxxxxxxxxxx wrote:

>>I guess I should have posted the bits from Shaw about why you don't want
>>to bother with the math.  As Walt mentioned: "With lenses like the 90/2
>>Zuiko and its ilk, your guess is as good as mine about where to measure
>>from."  you need too much data about some modern lenses to be able to
>>make the math work for you.
> 
> 
> well I spent a few hours with a sekonic l-358 in reflective mode showing
> raw EV values and my bronica etrsi with the lenses set to T i.e. shutter
> open all the time and the sekonic where the film back would be. I measured
> the light loss vs infinity focus for all 6 of my lenses at every interval
> marked on the distance scale. The results are STRANGE..
>
   <big snip>
> 
> CONCLUSION
> The maths may work for bellows extension but they do not work for focusing
> distance at all!



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