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[OM] Re: 28/2.8; f-stops; effective speed -- some measurements

Subject: [OM] Re: 28/2.8; f-stops; effective speed -- some measurements
From: Andrew L Wendelborn <andrew@xxxxxxxxxxxxxxxxxx>
Date: Thu, 20 Jan 2005 21:26:59 +1030

Having been intrigued by this thread from last week, I tried to do some
measurements of different lenses to confirm how often there was a difference
of less than one stop between two consecutive f-numbers, and how great
the difference was.

I wanted to do it without using the camera, to eliminate any effects of the 
TTL system.

After a bit of fiddling around I set up things thus:

. bellows, with lens under test attached, but no camera;
. slide copier, attached to bellows in usual way via filter ring
     i.e. only 49mm lenses can be tested;
. slide projector acting as light source;
. glass diffuser plate (from Durst Laborator 1000 enlarger) placed
  upright in front of projector, so as to be fully illuminated and provide 
  a bright, even diffuse light source;
. copier / bellows unit mounted on tripod and positioned so that rear end
  of lens abuts diffuser plate;
. the ground glass plate in the slide copier is then evenly illuminated;
. Minolta Spotmeter F, mounted on second tripod and positioned against slide 
  copier plate;
. spot is positioned approximately at centre of illuminated area, and light
  meter readings can be taken as lens aperture is varied.

I tested a few lenses, taking several readings at each stop, and running
up and down the range several times.

Here are summarized results, recording ERRORS i.e. deviations from expected
f-number differences (usually one, but not always, as in going from eg f5 to 
f8, the expected difference is 1.3 stops). If the error is 0.1 or less,
it isn't recorded below.


50/1.8    f1.8 -> f2.8   error 0.5 stops

28/3.5    f3.5 -> f5.6   no error
          f11  -> f16    error 0.3

50/1.4    f1.4 -> f2     error 0.6
          f11  -> f16    error 0.3 

50/3.5    NO ERRORS

200/5     f5  -> f8      error 0.7
          f22 -> f32     error 0.3


Note errors are big for the 50/1.4 and 200/5.

I've started to do some timing tests on a 2n.
For the 50/1.4, I get 1.5 increase from f1.4 to f2
(exactly as Wayne H did), which is an error of 0.4 stops,
a bit less than the 0.6 observed above. Not sure why.

For the 200/5, I get exactly the same error as above. A bit disturbing.


Couldn't test a 28/2.8 as I don't have one  :(

Can't test my 24/2 until I find a 55 to 49 step-down adaptor. :(


As I said, this is very much summarized, so happy to answer questions if
anyone is actually interested.


regards
  Andrew











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