on 11/01/2005 05:08, Moose wrote:
> It sounds like you may have assumed the worst, that f2.8 is less than
> that.
on 11/01/2005 07:55, Gérard Meunier wrote:
> There is nothing wrong with your lens. All Zuiko lenses are the same:
> the difference between full aperture and the the first stop is less
> that what you would expect.
> The reason for that is that all markings on the lenses (except wide
> open) correspond to optical f/stops (light transmitted through the
> lens).
> At full aperture, the marking corresponds to the geometrical opening of
> the lens. The difference (geometrical vs. optical) can be very low, but
> can be as much as 1/2 f/stop on some lenses.
on 11/01/2005 13:23, John Hermanson wrote:
> Normal! Try many lenses going to the maximum f stop and you'll see that
> meter movement between the last 2 clicks is oftem just a half stop instead
> of one. I heard about this back in the 80's. Just use the lens, it's okay.
Yes, I must agree it is normal. But I don't understand why is it so. I was
amazed to discover this 'feature' in 1981, and tried to measure the speed of
my 28/2.8 using a T-32 in 1985. The slides obtained were discussed in a post
within Chuck's thread on the T-32.
The slides and some questions travelled to Tokyo during mid 1986, and
yesterday I finally found the reply I received on September 1986, together
with the returned slides. Unfortunately I cannot find my own questions at
that moment. I'm reading this over and over and must confess I don't fully
understand it. So, I will copy the typed letter and the attached document.
Hope you will be patient enough.
Tokyo, August 20, 1986
Dr. Fernando Gonzalez Gentile
Av. Julio Mª Sosa 2331
Montevideo,
Uruguay.
Dear Dr. Gentile:
This is in reply to your letter of June 21. We are sorry for the delay in
answering your technical questions on the metering system of OM camera and
lenses. The delay was caused due to our examination of the problem and our
summer holidays.
With reference to the aperture of 28mm F2.8 lens, the TTL Number System for
Pen FT camera is applied to the OM-System lenses. According the the [sic]
Pen FT system, the aperture numbers on the aperture ring are inscribed
unevenly so that the light intensity can be reduced by half by stepping down
one stop.
The OM-System lenses are manufactured based on the same concept of this TTL
number system. But the aperture ring of the OM lense engages at each f
number intervals. Therefore, the light passing through the lens is not cut
exactly in half by stepping one stop from full-open aperture to next value.
Because of this, the space between the full-aperture f number and the next f
number is smaller than the distance between other f number. The same thing
can be said with the 28mm f2.8 lens. For your information, we are enclosing
a copy of explanation of the TTL number system.
The exposure value between the F2.8 lens set at f2.8 and the F1.4 lens set
at f2.8 is not the same.
Hope the above information will be helpful.
Sincerely yours,
OLYMPUS OPTICAL CO., LTD.
Hésharu Nozawa (black ink signature)
H. Nozawa
Camera Section
MARKETING DEPARTMENT
CONSUMER PRODUCTS
HNO/st
-----------------
(Attached document)
page 1-6
Olympus TTL number system and how it differs from other light measuring
systems
Conventional coupled or built-in exposure meters are usually designed to
read the light falling on a field roughly equal to that of the normal lens
of the camera. When a telephoto or wide angle is used with a system such as
this, it is easy to see that the resultant reading would not necessarily
indicate the correct exposure for the field of that lens.
TTL (through-the -lens) meters, sometimes referred to as BTL (behind the
lens) metrs [sic], are designed are designed so that they read the light
which actually passes through the lens on its way to the film. A CdS cell,
placed between the lens and the film plane, measures only the light
transmitted by the lens actually in use. With this system,
--------------------
page 1-7
using a telephoto lense would result in a reading which covered only the
subject area this lens would photograph. Using a wide angle lens the TTL
meter would read a larger area than with a normal lens. Thus, the
desirability of a TTL meter is easily established. More accurate readings
are certain to result from the use of this type of meter.
Most TTL metering systems are calibrated in f-stops, which are based upon
the intensity of the light passing through the center of a lens. The
mathematical calculations upon which this system is based should result
theoretically in the light being cut exactly in half each time the diaphragm
is stopped down one f-stop. However, this does not necessarily occur,
especially at wider apertures. This is true because f-stop calibrations are
mathematically formulated and apply only to the center area of the lens. If
the f-stop system were correct, a lens with a maximum aperture of f2, when
used wide open, would transmit the same amount of light as an f1.4 lens
stopped down to f2. Tests prove that the latter transmits more light than
the former.
In order to correct the errors found in the f-stop system, Olympus designed
a special numbering system similar to the "T" system many professional
photographers prefer. In the Olympus system, the calibrations begin with "0"
and end with "7". These numbers are relative aperture values based upon a
wide open aperture, and measuring the average brightness of the entire
picture area.
Why are the TTL numbers spaced unevenly ?
Each time the lens opening is closed by one stop on the Olympus TTL system,
e.g., 0 to 1, or 1 to 2, the the light transmitted to the film is reduced by
exacltly one half. This accomplished by spacing the TTL numbers unevenly so
that the light measurments are averaged at both the center and the edge of
the film. The uneven spacing of the TTL numbers also correct for the errors
caused by the curvature at the edges of the lens
[illustrations of 60, 70 an 100mm Zuiko showing unevenly spaced aperture
rings and 0 ~ 7 calibration on each one]
Since the CdS cell of the TTL metering system is placed behind the
viewfinder prism, the light which reaches the cell is influenced by the
fresnel lens as well as by the camera lens itself. The
-------------------------
page 1-8
fresnel lens centers the light on the CdS cell, and the longer the focal
length of the camera lens, the greater the exaggeration of this centering
effect. Therefore, readings taken with telephoto lenses differ from those
taken with shorter lenses. A subject that reads "0" with a 60mm lens would
indicate a reading of "1" when taken with a 100mm lens. On Olympus Pen FT
lenses, corrections have been made so that the maximum aperture of each
telephoto lens reflects a different TTL number for the maximum opening.
How to use non- Olympus lenses with Pen FT = Close Aparture Method
(omitted)
This is The End of my info.
I still don't understand why there is not a full stop difference between 2.8
and 4, both in two 28mm f2.8 samples and in a 24/2.8 H.Zuiko I mounted on my
2n this afternoon. And both had a slight uneven distance between 2.8 and 4.
As John suggested, I tried my 21/3.5 which shows a full stop difference
between 3.5 and 5.6, as does the 200/4 between 4 and 5.6. Still studying the
85~250 between 5 and 8 at different focal lenghts.
It was somehow a weird coincidence that Chuck discussed and tested the wide
coverage of the T-32, which lead me to remember this old unresolved
question.
It was also very weird of an acquaintance of mine who has two plain 4 bodies
and two 28/2.8 for sale. He would be happy if I bought him one, so he told
me on the phone that his 28s on his 4s showed a full stop difference between
2.8 and 4. I believed him, so I disassembled the back of mine just to see if
some adjustment was possible. I reassembled the lens and left no marks
whatsoever, but went to bed at 3 a.m. Doesn't matter, it was a hot night and
I learned a lot about this guy. Or is he telling the truth?
Thanks all who answered,
Fernando.
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