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.
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.
1 Attachment(s)
Re: IR 760 filter - rating film speed
Quote:
Originally Posted by
Barry Kirsten
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.
Attachment 197582
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
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
Re: IR 760 filter - rating film speed
Quote:
Originally Posted by
Leszek Vogt
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
Re: IR 760 filter - rating film speed
I think Leszek just gave you a good starting point with this filter. That would result in a very long exposure... but then you also have to slap on reciprocity correction. You could be looking at a half hour or longer exposure. Is it really worth it? I'd stick with the 720... but hey, go for it! Let us know how it went, and if you got anything, post them here!
Re: IR 760 filter - rating film speed
Quote:
Originally Posted by
Pere Casals
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
Re: IR 760 filter - rating film speed
Rather than getting a converted camera, I have a Leica M8, it does not have an infrared cut filter. In fact to get good color you need an infrared cut on the lens. With a 720nm filter it is giving me, in sun light, an exposure of f5.6 at 1/30 set at 320 ISO. Have never used the Rollei film, but may try.
With Kodak HIE I would meter and ISO 6 or 12 with a Kodak 87C. I also had interesting result putting an 87C on a flash, using a Speed Graphic.
Re: IR 760 filter - rating film speed
Quote:
Originally Posted by
Leszek Vogt
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.
Re: IR 760 filter - rating film speed
Quote:
Originally Posted by
Pere Casals
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.