An interesting experiment. It's been a long time since I did any
experimental design. I do remember one constant problem is to measure
what is intended to be measured and not the characteristicss of the
experiment. Careful definition of what one desires to measure is also
important. Asking about hidden assumptions can help in resolving these
issues.
Andrew L Wendelborn wrote:
>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.
>
Generally a good idea, but see below.
>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;
>
So, as I understand it, the lens being tested is being tested backwards,
with the light coming through the rear and out the front, where it is
measured. Unexamined assumption 1 is that this will give the same
results as using the lens in the direction in which it was designed to
be used. My assumption based on experience and intuition, but not direst
knowledge, is that this will tend to be true of the rather symmetrical
designs of 'normal' focal length lenses and may become less true s one
goes farther toward WA and.or tele.
Again as I understand it, no effort is made to use the lens in anything
like a normal distance relationship to subject and film plane.
Unexamined assumption 2 is that this will give the same results as using
the lens 'in focus'. I realize that the subject is diffuse and
measurement is through a diffuse medium, but still don't know that
address the assumption. A quick test with the 200/5 shows that the light
level shown by the meter of an OM-1 varies as a blank white subject is
brought into and out of focus. So there is at least a potential effect
to be considered.
>. 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.
>
Another interesting assumption, that only light at the center need be
measured. Here, the experiment runs up against the issue of what is to
be measured and what the designers may have measured when setting the
stop detents. Simple brightness at the center is a possible criterion,
but not the only one. Another measure could be total light delivered to
the film plane, summed over the whole area. Here we encounter the issue
of vignetting, at least for tests from wide open to 1 stop down, but to
some extent with lower stops. In a lens with significant vignetting,
measures of the light delivered at the center at the 2 stops will differ
by a different amount than will measures of unweighted average
brightness over the whole film frame at the same 2 stops. And the
direction of the difference will be in the direction of the 'error'
under discussion and that you have measured.
>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.
>
Not quite sure what you mean by this. If using actual shutter speed of
the TTL system, (using very slow film speed settings and a stop watch?),
this should give much broader averaging of light over the whole frame,
depending on the evenness of coverage of the little lens on the mirror
box floor. It may vignette too. I always thought TTL metering on the 2n
was slightly center weighted, but have no info or proof either way. If
the lens being tested is at least roughly focused on the light source,
the first 2 assumptions are also eliminated as possible sources of error.
>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.
>
That bothered me too. I did a much less elaborate test. Using an OM-1
fairly recently CLAed, I measured exposure through a 200/5. I used an
OM-1 for several reasons. First, it is easy to see small variations in
the analog meter, giving what feels like finer graduation than on a 4,
whether true or not. Second, its light metering is rather mildly center
weighted. Third, the CdS cells respond slowliy enough that I could use
my computer screen for the first test without the meter jumping around
as on the later models. A fourth, non-logical reason is that it had the
roll of film I was using at the time in it so I could also observe
reaction with the 200/5 while out shooting.
Aiming at an almost blank, white subject, my computer screen, and with
the lens not really in focus, but as close as I could get without
getting out of my chair (This IS the meaning of armchair science, isn't
it?). Moving from f5 to f8 and back, I observed a 1 stop difference in
the meter. As the difference between those apertures is slightly less
than 1 1/3 stops, that would indicate a difference of no more than 1/3
stop, quite a bit less than you got. Out in the world taking pictures, I
observed the same difference.
>As I said, this is very much summarized, so happy to answer questions if
>anyone is actually interested.
>
I'm actually interested, less than because I think it makes a lot of
practical difference, but as an intellectual exercise in definition and
measurement. I'm not trying to put down your tests either, just
exercising my own curiosity on the subject.
Moose
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