Re: Can you really use an 87 filter with Konica 750?

[Clive Warren <Clive.Warren@xxxxxxxxxxxxx>]

At 4:38 pm +0100 22/2/00, Willem-Jan Markerink wrote:

>
>On 21 Feb 00 at 16:30, Dan Cardish wrote:
>
>> I have shot Konica with an #87 filter, and the results were excellent, very
>> Kodakish.  Very dark skies, white clouds, and white foilage.  I seem to
>> recall exposures on the order of F8 and 1/4 sec, plus or minus, in full
>> sunlight.
>
>Yup, according to the Sunny-16 rule your exposure equals 1 ASA
>(the Sunny-16 rule says "a shutterspeed of 1/ASA @ f16 on a bright
>sunny day").
>
>Bye,
>
>Willem-Jan Markerink


Konica IR film is called 750 for a good reason, the peak sensitivity is 750
nm hence it's name it is sensitive from 640nm to 820 nm (in the infrared
light spectrum) in addition to the sensitivity range of 400 to 500nm.

An 87 filter to Wratten specifications has the following spectral
transmission of infrared light:

	     Wavelength    Light transmission
		(nm)		   %
 		740   		 0.80
                750  		 3.50
                760  		12.30
                770  		22.10
                780  		33.20
                790  		44.90
                800  		56.90
        	810  		63.20
          	820  		68.00


Given that the sensitivity of Konica 750 to infrared light is dropping from
750nm upwards then there ain't a whole lot of light falling on the film
which is going to be recorded.  This has two effects, it increases the
exposure time required as pointed out by W-J (but probably a stop or so
longer is required) and the negative contrast will also probably be
increased.

Check out the curves in Section 11 "Which Infrared Filters?" of the
Infrared FAQ for a visual comparison of Konica sensitivity to filter
characteristics.

It seems to me that an 88A filter would perhaps be a better compromise in
terms of contrast and effective film speed.  Has anyone tried an 88A filter
with Konica 750?

Will do some comparison shots over the weekend if the weather holds out.



All the best,

             Clive   http://www.cocam.co.uk
                     Photographic Services, Filters and Equipment, Infrared FAQ
                            http://clive.bel-epa.com
                            Adventures in Cyberspace - Digital Art and Photos


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