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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