Johan,
I wrote the below linked post in response to a question about adapters but
it took me until today to find it in the archives. Hopefully it should help
your understanding of the mechanics in lens mounts.
http://lists.tako.de/Olympus-OM/2007-02/msg01611.html
To expand on what Jez wrote regarding how the OM's tabs work:
The OM lens mount has two tabs, one for stopping down the aperture and one
for communication between the lens and body. The stop down tab is activated
by a lever in the body and does its job after the shutter has been fired.
This tab is also linked inside the lens to the depth of focus preview
button. The communication tab tells the meter how many stops the fully open
reading must adjusted by to give a reading as if you had taken one through
the stopped down lens. Since all readings are taken through the lens there
is no need for the meter to know the lenses actual aperture value.
In the OM design there is no facility for the lens to tell the body what the
actual aperture is. Depending upon your personal position this could be seen
as a small failing of the OM system as it follows that there is no way for a
viewfinder display of the aperture setting in use. It is possible that the
OM mount would have evolved in the way that the Nikon F mount did to add
this feature but alas that was not to be.
HTH, Dan S.
-----Original Message-----
From: olympus-owner@xxxxxxxxxx [mailto:olympus-owner@xxxxxxxxxx] On Behalf
Of Johan Malmström
Subject: [OM] Re: OM and aperture
While taking a shower and after that staring at ceiling for a while, this
actually came to me as the way I would have done it :)
Thanks Jez, now I'm waiting for a extended version also. Would be great!
/ Johan
25 mar 2008 kl. 20.19 skrev Jez Cunningham:
>
> For sure others will provide a more accurate explanation, but in my
> understanding the little sticky out thing tells the body how many
> stops difference there is between the fully-open aperture (eg f/1.8)
> at which metering will take place, and the dialled-in setting (eg
> f/11) at which the exposure will be made.
>
> The body just has to compute the necessary exposure (speed) setting
> for that amount of light taking the 'difference' into account.
>
> Ignore all the above if you're considering how OTF TTL does it...
>
> Hope you're feeling a bit more illuminated.
>
> br
> jez
> On Tue, Mar 25, 2008 at 8:04 PM, Johan Malmstrom <jmalmstrom@xxxxxxx>
> wrote:
>
>>
>> But how did the OM's do it?
>>
>> What information is actually "sent" with that little thing sticking
>> out of the lens. When comparing two lenses (35-70/3.6 and 50/1.2) I
>> can't see any difference in them?
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