Re: I.R. Metering - Step up to the challenge

[Colin McKie <colinmckie@xxxxxxxxxx>]

danzig@xxxxxxxxxxxxxx wrote:
> 
> Hi :)
> 
> The title must be confusing some of you. Let me explain it as
> clearly as can be, it quite simply means,
> 
> that I intend to make an IR sensitive meter.
> 
Big snip!

Ket,

The big unknown in IR metering is the proportion of IR light to visible
light. The nominal speed of 400 ISO for HIE and whatever we use
ourselves, 200 to 800 usually, are based on the proportions in open
sunlight. In other situations the ratios can change in either direction.
For instance, if you meter normally in fog you will probably get
bullet-proof negatives, 3 stops overexposed.

The approach I was considering was to have two similar photodiodes, one
with an 87 filter and one with a blue filter. Without getting too
technical on-list, in photovoltaic connection the diodes give a log
response, so that the difference in outputs is costant in terms of
stops. Once calibrated using ND filters and set to zero indication in
open sun, such a meter could give a plus or minus correction factor for
a visible light reading.

This would reduce the need for excessive bracketing in non-sunny
conditions - often the most interesting for HIE. If you don't believe
me, see some of Hugh Milsom's work in 'Earthsong' (Creative Monochrome).

If anyone wants to discuss the fine detail of such IR metering, we'd
better take it off list.

Colin

-- 
Mainland Mono
Monochrome & Infra-Red
Oxford, New Zealand

http://www.geocities.com/SoHo/Cafe/8570
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