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Re: [OM] ......TTL and OTF.......

Subject: Re: [OM] ......TTL and OTF.......
From: "tmlee" <clairetm@xxxxxxxxxxxxxx>
Date: Wed, 22 Nov 2000 22:43:44 +0800
Dear Ray.....
Excellent reply........thanks for your concerted effort in explaining this
.......

I knew there's something special about OTF in the OM..........

Wonderful feature isn't it  ?

Then why don't other camera makers do this ? (Yawn)

Peace
TMLee


-----Original Message-----
From: Ray Moth <ray_moth@xxxxxxxxx>
To: Olympus Mailing List <olympus@xxxxxxxxxxxxxxx>
Date: Wednesday, November 22, 2000 3:06 PM
Subject: Re: [OM] ......TTL and OTF.......


>TMLee wrote:
>Dear All, Is there a difference between these 2 terms ? I mean , are
>they used interchangeably, or do they mean 2 different things ? Are all
>SLR's that are TTL , assumed to have OTF feature ? (Snip)
>=======================================================================
>No, they are not the same. TTL just means that the light is metered
>'through the lens' - most SLR cameras use a photocell or two that
>monitor the light from the focusing screen under the pentaprism.
>Cameras that use this method are 'blind' once the shutter is released
>and expose (in AE mode) based on the reading taken before the exposure
>starts. If the light changes during the exposure, the camera cannot do
>anything about it. Some cameras that use this method, e.g. your Leica
>R7, are provided with a little trapdoor to close the viewfinder lens,
>so as to avoid incorrect exposure through light entering the viewfinder
>when not shielded by the eye (e.g. when using a tripod but not looking
>through the viewfinder).
>
>OTF means 'off the film' - in OM cameras, a photocell (or two) in the
>floor of the mirror box looks back at the shutter curtain, which is
>patterned with black and white square dots to be similar in reflectance
>to the surface of a typical unexposed film. When the sutter is
>released, the OTF measurement begins. For short exposures, the
>measurement is taken only from the shutter curtain. For longer
>exposures, some measurement is taken from the surface of the film
>whilst the shutter is open. The camera is not 'blind' during exposure
>and can adjust the shutter speed (in AE mode) if the light changes
>during the exposure. There is no need for a trapoor to close off the
>viewfinder, since light is measured off the shutter/film, not the
>focusing screen, during exposure.
>
>OTF enables 'painting with light' and other long exposure exercises,
>such as your example of the car headlights. Only fast-reacting
>photocells can be used effectively for OTF, which explains the use of
>silicon blue cells in OMs. One of the special characteristics of the
>OM-2(n), 2S and 4(T) is a long maximum shutter speed of 2 to 4 minutes,
>enabled by the OTF technique.
>
>Hope this helps ,
>
>Regards
>
>=====
>Ray
>
>"The trouble with resisting temptation is
> you never know when you'll get another chance!"
>
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