Re: BTFR question

["Willem-Jan Markerink" <w.j.markerink@xxxxx>]

On 26 Apr 00 at 23:46, David J. Romano wrote:

> I have a question regarding the between the film rails placement of the
> IR filter. Don't TTL metering systems measure the light reflecting off
> the film plane (OTF)? If so, wouldn't the meter reading be different
> with the filter placed between the rails as opposed to in front of the
> lens because the light would then pass through the filter twice? Or, am
> I wrong, and the camera's meter does not read light reflected off the
> film plane?

As others stated, the filter sits behind the shutter curtain, and 
only a few cameras measure off the 'filmplane' directly, where 
'filmplane' can be further distinguished in the shutter itself 
(ambient light), or the actual film (applies to most flash TTL 
systems).
Direct-looking sensors also exist in two varieties btw, those in the 
bottom of the mirrorbox, and those up in the prism. In modern 
cameras, like EOS, the one down in the mirror-box is less foolproof, 
since it requires exposure-compensation for each different lens, and 
this info is only available with EOS/EF lenses, not with all manual 
lenses (like M42 and T2 adaptations). Luckily this 'bug' is only 
present with the (fine)spotmeter on the EOS-1(n)/3/1v, all other EOS 
cameras and metering modes are robust enough.

And, as with any use of a #87C filter, effective exposure index 
becomes 12 ASA....but unlike a TTF-reading (ordinary filter mounted 
in front of the lens), the light sensor is now unaffected by the 
filter, so no recalibration is needed....simply set camera's exposure 
index to 12 ASA, and off you go.
Or, if you like routine, you can apply the Sunny-Sixteen-Rule (1/ASA 
seconds @ f16 on a sunny day), and end up with a typical bracket of 
1/60s @ f5.6-11. This routine is in particular recommended if you shoot with 
a wide arsenal of all-manual cameras, without auto-exposure. Over 
time, this gives you a better 'feel' for the film IMHO.

One such #87C filter has been sitting in my EOS-1 for several years
now....and on and off a #12 medium yellow for Kodak EIR is sitting
in a second body.....wouldn't hate to give up this solution, as it
makes hectic lens changes in awkward & hostile environments far more
easy....especially with HIE and EIR spread over two bodies, and
juggling lenses on a steep skislope, with a murder arctic wind, you
basically eliminate Murphy....;-))

(btw, I recently lost two normal color films because I forgot to remove a 
#87C from a Noblex panorama camera....when in use, one can't see the 
magnetic filter disk sitting inside....8-))

Still planned is a BTFR solution for Mamiya RB (offers 70mm backs, 
even apart from Rolland Elliot's 120/220 solution)....I think the
rotating adapter allows even the best type of foil (0.4mm thick) to
be mounted....so I bought a spare adapter for this
project....probably one of the weirdest 'filter-accessories'
possible....;-))

--                 
Bye,

Willem-Jan Markerink


      The desire to understand 
is sometimes far less intelligent than
     the inability to understand


<w.j.markerink@xxxxx>
[note: 'a-one' & 'en-el'!]
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