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Old 10th October 2019, 12:36 PM
Anon01 Anon01 is offline
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Default What is a 6 stop increase

Do you have a moment when you know you know the answer but do you.............

HELP

I am out soon IR film in hand with IR 760 Nm filter

Someone said "
I believe you will find 760 nm cutoff is awfully high for SFX200, you might do better with a 720.

A 760 could require another five or six stops of exposure over a 720 on today's "IR" films which do not extend very far into the IR range."

Am I right that this is. eg. Normal exposure 1/125th so 5 stop increase is 1/4 second ??? or 1/2

Thanks
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Old 10th October 2019, 05:08 PM
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From 1/125 to 1/4 is 5 stops. Or, to put it another way, meter ISO 200 SFX at ISO 6 using a 720nm IR filter. In practice, that is probably a little high and ISO 3 will probably get you a better neg. By all means, feel free to bracket a couple of stops either side of ISO3 - you never really know how much IR is about.

As I mentioned before, I have serious doubts about using a 760nm filter with SFX. A quick look at the graph on the Ilford datasheet (such as it is) suggests there is no response much above 730-750nm.

Hopefully, the filter's response is less than sharp and it lets through anough light away from its centre band of 760nm to provide an image, but I would error on the side of safety and use something like ISO 1 as my base and bracket 2 or 3 stops either side.
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Old 10th October 2019, 05:30 PM
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720nm is typically the value I would expect - the usual filter IIRC is an R72 for example
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Old 10th October 2019, 06:50 PM
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The Ilford chart shows a good response at 720nm, but it then falls off a cliff. There is some response up to 760, but it is very weak.

A 760 could require another five or six stops of exposure over a 720 on today's "IR" films

That quote is suggesting you need 5 more stops even after rating the film at ISO 6. To put some numbers to that:

Full sunlight, SFX200 in camera, 720nm filter in place, exposure would be something like 1/4 seond at F16

Same situation, 760nm filter in place, you'd add 5 more stops, That is, full sunlight becomes 1/4 second at F4. If you need more depth of field that could be 4 seconds at F16. Unfortunately you then hit a thing called reciprocity failure. Basically, the film becomes more sluggish and needs a whack more exposure. Realistically you could need 10 seconds or more in full sunlight.

I'd go with the 720 filter

Last edited by skellum; 10th October 2019 at 06:52 PM. Reason: Sticky keys on my keyboard!
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