On Thu, 10 Feb 2000 Johnsonpa@xxxxxxx wrote:
> For that matter, how is magnification measured?
>I have the 50mm Macro and get what seems to be lifesize on a 4X6 print.
Traditionally, the definition of magnification is
image size/subject size
Where image size is the size of the image of the subject *on the film* -
not on the print. So if you were to photograph a ruler and 20mm or
ruler comes out 10mm long on the negative/slide then magnification is
1:2
>Next, I left the f-stop the same and
>moved the camera to auto and tripped the sutter. The shutter stayed
>open for well over 1/4 second, probably closer to 1 sec. This happened
>for both my OM-2 and OM-2n. I realize this is pretty inexact, but I
>don't have the electronic background as some of you clearly do. So,
>any thoughts on what might be causing this?
In auto exposure mode, the viewfinder display is a guide to what the
exposure is most likely to be. When you release the shutter, the
metering system measures the light reflected off the shutter curtain as
it moves past, and off the film when the shutter is fully open. When it
decides that enough light has fallen on the film, the second curtain is
released.
For exposures longer than 1/60 s, most of the metering is done off the
film, unless there's no film in the camera, in which case it's done off
the pressure plate.... which is painted black and therefore much less
reflective than unexposed film, so the shutter will stay open longer.
If you were to put a suitably sized piece of white card in the back of
the camera and try the same experiment, you should observe an exposure
time much shorter than indicated by the meter.
--
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