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[OM] OTF metering vs Spot/Mem/ESP / OM-2S Program Flash. For Curtis.

Subject: [OM] OTF metering vs Spot/Mem/ESP / OM-2S Program Flash. For Curtis.
From: DBellamy2k@xxxxxxx
Date: Mon, 30 Jul 2001 20:17:34 EDT
From: "Curtis P. Hedman":

 > > As to the Program mode on the 2S and PC, I have
 > > almost no clue, though I think (if I understand the instruction books) I
 > > still get real time OTF exposure control.... >>

1. Yes, you do get OTF in Program mode on the OM-2S / OM-2SP:
In Program mode, the aperture value is calculated from the reading taken just 
as you press the shutter, when the semi-silvered mirror and the Fresnel 
mirror are still down, reflecting the light off the Fresnel mirror onto the 
silicon photocell below.
After the mirror is raised, the shutter blind reading is taken and the 
shutter speed set, compensating for any imperfections in the original 
aperture reading - by altering the shutter speed if needed, I guess.
Presumably for longer exposures than about 1/30 second, the light reflected 
off the film is monitored and the shutter closed when needed, as with the 
OM-2(N), OM-4(T,Ti) and OM-2S OTF Auto modes. How cool!  ;-)
There is a bit about the OM-2S Program mode in Vision Age magazine number 3, 
if you ever see it.

2. It also talks about OTF Auto and OTF Program flash modes on the OM-2S. OTF 
Program flash prevents possible underexposure which can be caused by setting 
too small an aperture. It looks like the program mode selects an aperture 
before the flash fires, depending upon the ambient light reading, and then 
sets it and does the OTF flash routine to get the right flash exposure - this 
looks likely to give you a better rendition of any ambient light level there 
is, like a sunset sky for instance.

3. In spot auto mode on the OM-4Ti, there is no OTF because the light 
readings are from the spot readings and they are not modified once the 
shutter is released. I used to wonder is there was any modification by OTF, 
but it looks like there isn't. From the diagram on page 6 of VisionAge 
magazine 1 on Peter Gilbert's brochures site, the spot system and OTF 
(average metering) system are pretty much separate, using separate sections 
of the silicon photocell.

4. In MEMO mode on the OM-4(Ti), the readings from the previous exposure are 
just held across to the next exposure(s), so if you have MEMO engaged there 
is no OTF for subsequent exposures. You can use MEMO mode to remember an 
exposure which was made/calculated from OTF Auto or Spot/Mulit-Spot mode, but 
it is just fetching that exposure from the memory unmodified when you make 
the subsequent exposures.

I hope that helps!

Dave Bellamy.
http://members.aol.com/synthchap/

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