The spot reading is determined by a smaller area of the silicon
photo diode from light reflected off the sub-mirror. The readings
are digitized and exposure computed from the readings. That is
why the EV sensitivity is only EV0 for spot mode. The EV-5 in auto
mode is because the sensor has time to integrate the light reflected
off the film. There is no OTF spot mode. Joel is correct.
There are no multiple sensors, only two. The shape of the SPD
(silicon photo diode) is a smaller rectangle surrounded by a larger
rectangle. During auto OTF exposure, both diodes are used to
detect average weighted exposure. In Spot mode, the central SPD
is used to measure the light from the sub-mirror, then digitized,
essential taking a spot reading of the point where you are aiming
the central area of the display at the time you take a reading.
We're talking 1980's technology. How can it remember where
you aimed the camera when you pressed the spot button?
When you press the spot button, you just took the reading and it
is stored, with the shutter speed determined from the aperture
and asa/compensation dial. Taking multiple spot readings just
adds more readings to compute the shutter speed from. The
speed is exactly what you see in the display. Unlike auto mode,
which averages the reading off the film (OTF) and whose result
can be determined with the MEMO function - and may be different
from the pre-exposure display due to difference in film reflectance
and difference in the sub-mirror pattern versus OTF pattern - A
memo'ed spot reading does not change after the shutter is released,
because it is a predetermined exposure based on the spot readings
you took before pressing the shutter button.
Auto-OTF exposure is continuos. However a spot reading , being
digitized, is only in 1/3 EV increments. It is not continuous.
If I take a spot reading, then turn 180 degrees and snap the picture,
the exposure was determined by a reading I took behind me. There is
no way the camera can determine such an exposure OTF. It has
to be a stored reading.
Wayne
At 11:08 PM 8/25/2004, you wrote:
>At 01:23 PM 8/25/2004 -0700, Jim wrote:
>
>>The spots are just defined points in the frame while you're composing your
>>shot. The cpu determines the 'correct exposure' for the average of your
>>selected spots, then lets the OTF sensors shut the second curtain when that
>>predetermined 'best average' exposure is achieved.
>
>Jim,
>Are you sure this is correct? I thought that with the multi-spot metering,
>the exposure is calculated entirely by the forward meter, the OTF circuit
>not being involved at all.
>
>Joel W.
==============================================
List usage info: http://www.zuikoholic.com
List nannies: olympusadmin@xxxxxxxxxx
==============================================
|