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  1. #1
    Barry Kirsten's Avatar
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    IR 760 filter - rating film speed

    I recently bought an IR 760 filter and notice it's almost black, allowing little or no visible wavelengths to pass. Compared to the IR 720 filter I have of the same brand, the comparison is quite stark. I have had excellent results with the 720, rating Rollei 400 IR rollfilm at ASA 6 and developing in D76 for 6 min, but am looking for suggestions for a starting point with the 760. Just as a rough guess I'd say ASA 3 for 400 ASA film as a starting point; it all depends I guess on how much visible is transmitted.

    Just as an aside, I found these filters, Green.L from China on eBay and decided to give them a try. They seem to have a narrow product range and make filters for photography and scientific applications, apparently. They claim to use optical glass and high grade aluminium. I now have several of their products, IR and ND, and am impressed with the quality. https://translate.google.com/transla...m/&prev=search The eBay shop is called Galacticos.

    Thanks for any ideas.

  2. #2

    Re: IR 760 filter - rating film speed

    I’d say your rough guess is a good starting point. All you can do from there is test. I have some IR filters from China as well and they do seem to be quality.
    --- Steve from Missouri ---

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    Re: IR 760 filter - rating film speed

    Quote Originally Posted by Barry Kirsten View Post
    I have had excellent results with the 720, rating Rollei 400 IR rollfilm at ASA 6 and developing in D76 for 6 min, but am looking for suggestions for a starting point with the 760. Just as a rough guess I'd say ASA 3 for 400 ASA film as a starting point; it all depends I guess on how much visible is transmitted.
    Click image for larger version. 

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    See the graph in the datasheet...

    The vertical scale is logarithmic... this film is around 40 times less sensitive for 760nm photons than for 720 photons, if your filter blocks well shorter than 760nm it would be hard to expose an image because the film has a very low sensitivity by 760.

    Going that deep in the IR would require a film like the defunct Efke IR820.

    You may also hack your film by a pre-exposure bath in a deeper infrared sensitizing dye dilution, but this may be difficult because it not always work with any film (Cryptocyanine, 3,3'-diethylthiatricarbocyanine etc... Several recipes around)

    https://www.largeformatphotography.i...ed-Photography

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    Les
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    Re: IR 760 filter - rating film speed

    I looked up 850nm filter and it's described as 200X limiting factor (7.6 f-stops). Not really clear on linearity here, but in my silly brain that would have been 1.52 f-stop difference (1/5th). Someone might suggest that I'm ingesting the 5th on this, but that would be too much proof for me.

    Hmmm, however close or far away it is, I'd explore it with digital camera before I'd use a sheet of film. Just looked up 760 filter on B&H and the store indicate 1000X limiting factor. Ouch!!!! They call it 10-stopper. So there is that.

    Les

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    Re: IR 760 filter - rating film speed

    Quote Originally Posted by Leszek Vogt View Post
    I'd explore it with digital camera before I'd use a sheet of film.
    A (converted) digital sensor has a lot of sensitivity by 760nm, compared to Rollei 400 IR

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    Les
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    Re: IR 760 filter - rating film speed

    Quote Originally Posted by Pere Casals View Post
    A (converted) digital sensor has a lot of sensitivity by 760nm, compared to Rollei 400 IR
    Pere, perhaps you were thinking about a 'converted' rig....I did not or I'd have mentioned it.

    Les

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    Re: IR 760 filter - rating film speed

    Quote Originally Posted by Leszek Vogt View Post
    Pere, perhaps you were thinking about a 'converted' rig....
    In theory a DSLR sensor cannot see infrared because a NIR blocking filter is installed, the right exposure with a DSLR and a 760 filter won't give a clue about what exposure is required with Rollei 400 IR film because different practical factors are involved, with DSLR exposure depends on how good/bad is the NIR blocking, with Rollei 400 IR it depends on remaining sensitivity at +760nm.

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    Re: IR 760 filter - rating film speed

    Quote Originally Posted by Pere Casals View Post
    In theory a DSLR sensor cannot see infrared because a NIR blocking filter is installed, the right exposure with a DSLR and a 760 filter won't give a clue about what exposure is required with Rollei 400 IR film because different practical factors are involved, with DSLR exposure depends on how good/bad is the NIR blocking, with Rollei 400 IR it depends on remaining sensitivity at +760nm.
    And how much IR is present under the shooting conditions.

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    Re: IR 760 filter - rating film speed

    Quote Originally Posted by Bob Salomon View Post
    And how much IR is present under the shooting conditions.
    Bob, let me add that also what IR reflectance our subject has...

    Vegetation reflects around 8 times more the IR than the visible

    Click image for larger version. 

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    http://gsp.humboldt.edu/olm_2016/cou...flectance.html

  10. #10

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    Re: IR 760 filter - rating film speed

    Quote Originally Posted by Pere Casals View Post
    Bob, let me add that also what IR reflectance our subject has...

    Vegetation reflects around 8 times more the IR than the visible

    Click image for larger version. 

Name:	spec-reflectance.jpg 
Views:	11 
Size:	82.9 KB 
ID:	197597
    http://gsp.humboldt.edu/olm_2016/cou...flectance.html
    Pere, I was in the 363rd Rec Tec squadron. We did most of the Cuban Crisis pictures that you have seen. Many were IR color and B&W. These shots in color easily showed camouflage vs live leaf color as the IR reflection created different colors. In B&W you could easily see where vehicles HAD been parked and how large they were and make educated guesses as to what they might have been carrying. This was because the temperature where they had been parked was different then the surrounding area.

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