RE: IR response of meters

["Brewer, Kevan" <kevan.brewer@xxxxxxxxxxxxxx>]

I've been following the IR metering threads with interest
since my first attempts with HIE were rather disappointing,
getting very overexposed negatives. Part of the reason I
believe was due to over-developing but not the whole reason.

I have a Canon EOS 10 which I use and metered TTL with one of
two filters in place. I rated the film at ISO 400. The filters
I used were a Cokin 003 - like a Wratten #25 and used by
several other list members - and an Ilford SFX filter - near
enough opaque. I guess its response is similar to an 89B since
the response of Ilford SFX 200 cuts off about 740nm.

When I metered through the 003 I got an exposure of roughly 3
stops under that with no filter. With the SFX filter the
exposure was probably another 3 stops under that.

I have subsequently obtained a #70 filter which, on paper
should give 4 stops under an unfiltered exposure reading, the
readings with that were about 5 stops under.

All of this goes to show that my meter has a poor IR response.
I have since exposed HIE rating the film at ISO 50 and metering
without the filter in place, this produced much more satisfactory
results.

Going back to the Nikon thread, it would seem that the variations
in meters are not just model-dependent but also camera-dependent
to a degree. Does anyone else use an EOS 10 and if so, what have
your experiences been with HIE?

The thought has crossed my mind to build an IR light meter. If
anyone knows of where I can find a design for a visible light
meter I'd be interested (analogue electronics was never my forte!)

Kevan

> -----Original Message-----
> From: phillip geller [mailto:filje@xxxxxxxxxxx]
> Sent: Tuesday, October 03, 2000 3:23 AM
> To: infrared@xxxxx%INTERNET
> Subject: Re: IR response of meters
> 
> 
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> 
> > On the other hand, when you slap an IR filter on the 
> camera, the TTL meter
> > is responding to something... perhaps the little bit of red, or some
> fringe
> > IR which you cannot see, but to which the meter has a bit 
> of residual
> > sensitivity?
> I would agree
> 
> 
> > If you think about it, no camera manufacturer is going to 
> leave much IR
> > sensitivity in the TTL meter, nor even a hand held. 
> Otherwise, how could
> > you ever hope to correctly meter a scene for color? (In an 
> ideal case, the
> > meter response would be exactly the same as the film... of 
> course, this is
> > only true in an approximate sense.) Any light source which 
> is for some
> > reason "rich" in IR is going to upset the total meter 
> reading, and cause
> > you to underexpose the visible light portion.
> 
> 
> indeed, if the ir blocker isn't on over a ccd cell the red 
> tones in a video
> picture
> are over saturated. cds cells are yellow green sensitive 
> mostly as I think
> are selenium.
> another thing to consider about ttl metering is how that 
> light gets to the
> meter,
> on canon, it goes through a beam splitter prism in the 
> focusing screen, and
> if
> your eye isn't right up in the eyepiece as it wouldn't be 
> with eyeglasses,
> then light coming in through the eyepiece will inflate your 
> meter reading.
> 
> phil
> 
> 
> 
> 
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