Re: Hoya R72 InfraRed filter

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

On  4 Sep 99 at 22:26, danzig@xxxxxxxxxxxxxx wrote:

> 
> 
> ello mates ! ;)
> 
> 
> Finally after waiting for a full year I've got my first opaque IR filter ( 
> things move slow on this blasted island )

I guess ordering locally isn't the wisest thing to do....8-))
 
> It's the Hoya R72 InfraRed filter, & what I want ask is does anyone 
> have any idea what the filter factor is ?

Based on 400 ASA for bare HIE, #25 is 3 stops (50 ASA), and #87C is 5 
stops (12 ASA). So #89B (the Wratten equivalent of Hoya R72) is 
somewhere inbetween, closer to #25 than to #87C.
 
> If any can be derived from something that cuts all visible light out, 
> that is. 

#89B isn't all that opaque....even on a lens you can still compose, 
if not even focus. Opaque starts with #88A.

> I'm not even sure how to conduct an experiment as to how 
> to posibly expose for images.With a deep red filter you can pretty 
> much use the recommended exposure of the meter.
> 
> But on an opaque filter what assumptions should I make ? What 
> ISO should I set my lightmeter to ? (Minolta 4F ) 
> 
> Another question, in Hoya's catalougue, I saw 2 infrared filters, the 
> R72 & the M90. I'd suppose the M90 lets in less light than the R72. 
> Only question is, what exatcly is it supposed to let in & what is it 
> meant for ? 

It looks like an equivalent of #87A, but IMO it is not (see also 
other message).
 
> Could it possibly be used for solar photography ?

No, you need super strong ND filters for that purpose.
ND 4.0 for photo/camera/binocular use, and ND 5.0 for direct vision.
ND 4.0 means a factor 10^4, or 10,000, or more than 13 stops (2^13 = 
8192). 
I believe B+W even makes an ND6.0, 1,000,000x.
Note that for safe solar vision, it must block all wavelengths 
equally, in particular UV and IR....both can and will damage your 
eye when not filtered enough since your pupil is wide open (for the 
lack of visible light).

--                 
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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