Re: Filter/Speed/Meter & DON'T meter using a Nikon Camera

[Otto Giesenfeld <otto.giesenfeld@xxxxxxxxxxxx>]

--On den 3 oktober 2000, 12:55 +0000 Rolland Elliott
<rolland_elliott@xxxxxxxxxxx> wrote:

> The exact same analogy holds for IR photography. Except for the fact that 
> the IR block filter is permanently attatched right above the exposure
> meter.  You can try to meter IR light through a filter that BLOCKS IR
>   light, but  it's not very logical to do so.
> 

The intresting question here is to what precise extent the correction
filter over the metering cell blocks IR. If it attenuates IR by, say, 5
stops, but the IR-pass filter over the picture-taking lens attenuates
visible and UV light by 8 stops, the system would still act as a pretty
good IR light meter.

Since strictly IR-blocking filters are supposed to be difficult to make,
this scenario seems quite likely. (Of course, this difficulty may only
exist for filters that are to be inserted into an image-forming light path,
where scattering is not tolerable. The metering cell's filter can very well
be a bit diffuse, which might allow for a larger choice of materials.)

> If anyone is attending college or has access to a spectrometer, I'll
> gladly  ship the small IR blocking filter I removed from the Nikon N90s
> camera I  have for testing.  It would be interesting to get a curve of
> this filter's  spectral response. It is 1/4 the size of a penny. For
> being so small it sure  does cause a lot of problems for us IR
> photographers.

That's the kind of test that would be ideal. Or one could simply meter the
output of an IR LED with and without the filter in front of it.

Best regards,

Otto Giesenfeld
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