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[OM] Av with chipped adapters [was How Ken invented aperture priority (a

Subject: [OM] Av with chipped adapters [was How Ken invented aperture priority (and the internet? (was "Re: Fungal Friday")]
From: Moose <olymoose@xxxxxxxxx>
Date: Tue, 08 May 2012 21:51:14 -0700
Wow, all that made my head hurt. Confusing, possibly impenetrable comments on 
exposure with chipped adapter, suddenly 
interleaved with focus confirmation/accuracy comments.

I think this post of Chuck's comes closest to showing the problem understanding 
Av behavior everyone seems to be having.

First, those who can't understand why it would make any difference to the 
exposure system whether greater or lesser 
light coming through the lens was due to aperture, filters or simple ambient 
light would be right - are right in 
principle - except for one camera exposure system problem that chipped adapters 
solve.

Second, Chuck is simply wrong in assuming that exposure non-linearity should 
persist with a chipped adapter.

Third, getting the right aperture in the EXIF, while nice, isn't the most 
important reason for having the adapter tell 
the camera the maximum aperture of the lens.

The issue is that the exposure sensing cells in the viewfinder housing, peering 
at the viewfinder screen, at angles, are 
affected by the maximum aperture of the lens. I could speculate as to the 
combination of reasons, but the point is that 
it is simply so. (Does not, of course apply to live view.)

Way back in 2006, I did this simple test of exposure with manual adapter with 
50/1.4 attached. 
<http://www.moosemystic.net/Gallery/tech/300D/300Dexp.jpg>

Note that Manual exposure is largely consistent across aperture, slight 
differences probably due to imperfect diaphragm 
precision.
Then note that Auto exposure is quite inconsistent at large apertures, settling 
down at smaller ones. I believe this is 
the effect that Oly was concerned about in use of OM lenses on the E-1, 
although the details of their system will give 
different incorrect results than Canon's.

THUS - These exposure systems are dependent on knowing the full aperture of any 
lens in order to calculate the correct 
exposure!

SO - When a chipped adapter reports the correct maximum aperture of the lens, 
the exposure system works correctly.

THEN - Exposure tracks correctly throughout the aperture range.
-------------------------------------------------------------------------------

This aspect of a chipped adapter is the most important to me. Shooting with 
Sigma 600/8, it often took me several trials 
with chimping to get exposure correct. I even had to go to Manual because the 
correction is greater than the range of EV 
adjustment.

With the Big-IS adapter, and I would think, any others that are working, I 
usually get correct auto exposure first shot.

At least with Big-IS and Canons, once the correct max aperture is set, one may 
set the Av the actual shooting setting of 
the lens. This has no effect whatsoever on the actual exposure, per "First", 
above. But it does record that aperture in 
the EXIF, which is useful to me.

Full Aperture Moose

On 5/8/2012 12:42 PM, Chuck Norcutt wrote:
> OK, I hear you but:
>    1) why does that work and
>    2) how did you figure that out
>
> It seems to me that, with the camera set to f/2.8 but the lens set at,
> say, f/5.6 the camera would sense the ambient lighting at two stops less
> than the actual.  But, believing the camera is measuring at f/2.8 it
> would underexpose on the shutter speed.
>
> That apparently doesn't happen.  What's wrong with my thinking?  Is it
> that the camera sees only the stopped down reading at the time of
> exposure and makes a correct exposure because of it.  If so, what's the
> relevance of telling the camera the maximum aperture?

-- 
What if the Hokey Pokey *IS* what it's all about?
-- 
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