Dear Mickey,
<< 1. Please verify to me that I can use a linear polarizer on the OM4T and =
get accurate readings in manual mode, but in auto mode exposures will be =
off quite a bit.
2. Would the auto mode be equally wrong with the polarizer and a T-32 =
flash in TTL? Thought I might be able to use the neutral density quality =
of the filter with the flash for control of aperture and flash/natural =
light balance.
3. Is the exposure difference predictable? >>
Answer:
Hmmm... I don't have the specific experiences others have had with this
phenomenon the OM-4Ti etc., even though I am an OM-4Ti owner. I don't recall
ever having used a polarising filter with that camera. Previous messages on
the list drew my attention to it so I might now avoid any mistakes! Thanks
guys. :-)
It is also dealt with on the Olympus FAQ at:
http://brashear.phys.appstate.edu/lhawkins/photo/olympus.faq.html
Question B28 (I hope the author will forgive the quote!) :
<< B28) What OM cameras can use a linear polarizer? Which must have a
circular polarizer?
Due to the semi-silvered mirrors in the OM-{2S, 3(Ti), 4(Ti),
and PC} you must use a circular polarizer with them, otherwise
your exposures will be off by 2-3 stops. The OM-{1(N), 2(N),
10, G, and F) do not have a semi-silvered mirror, and can thus
use linear polarizers without any problem. This problem
reportedly only affects the spot metering modes of the OM-{2S,
3(Ti), 4(Ti), PC and probably OM-2000}. OTF (normal auto)
exposures don't technically need the circular polarizer, but
why go to all the trouble of carrying two different polarizers
and risk using the wrong one? >>
This problem is I think caused by polarisation effects of
reflection/transmission of light at dielectric surfaces (e.g. GLASS), which
are governed by Brewster's Law and are dealt with in university/college level
optics textbooks.
I would therefore guess that manual exposures are likely to be less accurate
than auto exposures, for the following reasons:
The source of the problem may be the semi-silvered main mirror and/or the
Fresnel lens mirror behind it. Basically their reflectance varies with the
orientation of linear polarization of the incoming light, because of the way
the electromagnetic light waves interact with the electrons in the glass at
its surface(s).
If the metering system is calibrated for 'unpolarised' light, it may then be
quite a way off if you use a linear polariser and have it turned to certain
angles.
The metering system uses both mirrors before you take the picture, while you
are viewing, so I would guess that the displayed settings in the viewfinder
in auto or manual will be inaccurate. Light is transmitted through the main
mirror and reflected by the Fresnel mirror into the silicon photo cell in the
base of the mirror box. (Of course some of the incoming light is also
reflected by the main mirror into the viewfinder so you can see through the
lens!). Of course, in Manual Mode, you will set the camera bythis data.
However in Auto mode, the camera takes its actual exposure reading from the
light reflected off the shutter blind in to the silicon photocell, once you
have pressed the shutter button and both mirrors are raised out of the way.
The fact that this is only one reflection may give a more accurate exposure
setting in Auto mode. Also I don't know what is used as a reflective coating
on the patterned shutter blind - if it is vapour deposited metal for
instance, it might not be subject to this variation with polarisation
orientation.
At longer exposures than 1/30 second, the meter is then modifying the
exposure by reading light levels reflected from the film which is plastic
basically - (or gelatin, isn't it?) and again, inaccuracies due to light
polarisation may be insignificant.
This effect of polarisation on reflection/transmission of light is also
greatest at fairly high angles. The semi-silvered main mirror is tilted at 45
degrees when framing the picture, and the at this angle the ratio of light
reflected by the mirror to that transmitted may change depending on whether
you are using vertical polarised light, horizontal polarisation, or
unpolarised light.
I don't know exactly the pattern on the Fresnel lens, but it looks like there
are lots of little prisms to direct the reflection onto the small photocell,
and there may be high angle reflections which will be greatly affected by the
initial polarisation of the light, thereby affecting the exposure reading
significantly.
So, having a go at answering your specific questions:
1) My guess would be that manual readings will be the least accurate, and
auto exposures might be less affected. (It has been said that there is no
problem with the OM-1(N) and OM-2(N), which have a fully silvered mirror.
Presumably this is silvered or alumisised with 100% reflectance and no
transmittance, so it should be unaffected by these factors. However the
OM-2(N) works similarly to the OM-2S/3/4/4Ti in auto mode I think (shutter
blind reading, off the film reading), and if the OM2 is OK then maybe the
others are!).
2) I would guess that TTL auto with/without flash will be more accurate than
manual exposure mode (see above), but I couldn't absolutely guarantee it! Do
you mean placing the polariser on the lens or on the flash? I would think
that any effect would be less if you mean on the flash, because polarised
light from the flash will be scattered from the subject and the polarisation
will be somewhat randomised by that.
But to avoid the possibility of error, a neutral density filter would be
safer I think!
3) If you worked for Olympus and were on the design team and knew all the
parameters involved, the shape of the Fresnel reflector, the design of the
photocell, the ratio of transmittance/reflectance of the main mirror etc.,
then it could be predicted I would say. Different components could be tried,
and the finished design could be calibrated.
What we need is some kind of technical sheet from Olympus giving exposure
compensation factors. Or a keen list member to do some experiments and
measure the exposure errors. Maybe someone has done these already!
Your best bet is to go for a circular polariser instead! These employ an
additional 'quarter wave plate,' which results in circular polarized light in
which the polarisation angle constantly rotates along the light waves with
every oscillation of the wave, and presumably this must allow the above
factors to canel out, as for unpolarised light! Or something!
Good luck!
Dave Bellamy.
http://members.aol.com/synthchap/
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