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03-22-2017, 11:10 AM   #1
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Metering Infrared with the Pentax MX

Since the MX's GPD meter is completely insensitive to infrared light, how would one meter with an R72 (720nm cut off) filter on front of the lens? I know with different films have a different characteristic curve, but I already know the relative EI (exposure index) of some of the film I wish to try (for example, getting the wood effect with AEU 200 rated as EI 6).

Would sticking the filter first in front of my gossen digisix accomplish the starting point (either as incident or reflective), or would there be too much light leaking in from the side to get even close an accurate reading?

Or do you know if there's a specific stop loss when it comes to R72?

03-22-2017, 01:07 PM   #2
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I shot a roll of infrared film last year. Metering (and focusing) is definitely a problem. I came across a couple of rules of thumb, somewhere online, that suggest you meter it as an ISO 12 film or that you start with f8 and 1/60 on a bright sunny day, and bracket... I tried the f8 and 1/60, and some were underexposed, particularly in late afternoon. I guess I was not compensating enough...
03-22-2017, 04:19 PM   #3
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QuoteOriginally posted by IgorZ Quote
I shot a roll of infrared film last year. Metering (and focusing) is definitely a problem. I came across a couple of rules of thumb, somewhere online, that suggest you meter it as an ISO 12 film or that you start with f8 and 1/60 on a bright sunny day, and bracket... I tried the f8 and 1/60, and some were underexposed, particularly in late afternoon. I guess I was not compensating enough...
Focusing usually isn't a problem for me, since I just need to shift to the IR mark on the lens. On other cameras I can meter somewhat since the meter cell in them are somewhat sensitive to IR light.
03-22-2017, 04:41 PM   #4
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The TTL meter on your MX should be close to right, but you will still need to bracket your shots. Check the data sheet for the IR film you are using and it will list the various IR filters and exposure compensation using a TTL meter..

If you are worried about the MX meter being out of range and want to use your Goosen, you will need to apply some sort of filter factor. The B+W IR filters that I have list the filter factor on the filter and I set the filter factor dial on my Goosen Luna Pro to match.

You most likely are also going to need a tripod, as the exposures can be very long. Focusing is difficult, but you can do this with the filter removed and then adjust with the lens IR mark.

Phil.

03-22-2017, 06:12 PM   #5
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QuoteOriginally posted by gofour3 Quote
The TTL meter on your MX should be close to right, but you will still need to bracket your shots. Check the data sheet for the IR film you are using and it will list the various IR filters and exposure compensation using a TTL meter..

If you are worried about the MX meter being out of range and want to use your Goosen, you will need to apply some sort of filter factor. The B+W IR filters that I have list the filter factor on the filter and I set the filter factor dial on my Goosen Luna Pro to match.

You most likely are also going to need a tripod, as the exposures can be very long. Focusing is difficult, but you can do this with the filter removed and then adjust with the lens IR mark.

Phil.
The MX isn't just out of range it's as I said completely insensitive to infrared, which is actually a "feature" of the GPD meter in the brochure, ie it'll show under exposure red led 100% of the time with the R72 filter on it.

I am well aware of the needs around longer exposure or need of tripod etc as I've shot ir, nd, long exposure etc in the past (mostly with digital, which will meter the ir as it sees TTL.)

I don't know the filter factor of the Hoya R72 hence asking if anyone knew what stop loss it has.
03-22-2017, 09:43 PM   #6
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While I know the emulsions are going to have various characteristic curves (Ilford SFX being my best bet for a panchromatic film that's known to be IR sensitive, down to say Arista EDU Ultra/Foma 200 that isn't listed as having it, but can create a wood effect when exposed as IE 6).

I tested my R72 on my little Pentax Q10, which I haven't tried the R72 filter on before so I don't know how sensitive it is compared to my Olympus E-M5 (I know my Panasonic GH1 is less sensitive than the E-M5, so much so it's impractical to use for it). Course this is off of some CFLs in the house, both my ceiling lights and desk lamp are using 100W equivalent, 6500K balanced CFLs, so actual daylight infrared could be stronger.

A metered exposure at 30 seconds, f/2, ISO 400 with the Hoya R72 filter on front.



Then reduced the shutter until it showed the same metered results, everything else remaining same including focus (white balance switched to daylight).

1/500, f/2.0, ISO 400


So, 14 stop difference between with and without the Hoya R72 filter on front for this indoor scenario with daylight balanced CFLs. (so a filter factor of... 16,384 in this case)

I'll have to wait til next week to test it on film because right now I got FP4+ loaded in the MX (which drops off on the spectral sensitivity just before 650nm), and because it's supposed to be thunderstorming all weekend.

But I'm thinking when I do get a roll in there such as the AEU200, I can cut a short roll since I'm bulk loading, and shoot in 2 stop increments, aiming at an EI of 6~12 under broad daylight. But I'll have to factor in reciprocity failure, and lengthen development times if the exposure times meter a second or longer (AEU/Foma 200 's schwarzschild adjustments are 1s = 3x exposure, or -1.5 stops in aperture, 10s = 9x/-3, 100s = 18x/-4)

Last edited by kb244; 03-22-2017 at 11:37 PM.
03-23-2017, 06:17 AM   #7
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QuoteOriginally posted by kb244 Quote
I don't know the filter factor of the Hoya R72 hence asking if anyone knew what stop loss it has.
According to the Hoya web site the R72 FF varies depending on the lighting:

Due to the nature of infrared light, filter factor and exposure compensation vary widely from visible light and depends largely on lighting conditions.



Phil.

03-24-2017, 06:40 AM   #8
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Bracket and keep notes if you are using a filter deep enough for white leaf syndrome.
Filter factor is dependent on light spectrum, and film spectral sensitivity.
Wood effect tree and light dependent
Foma400 is cheap and has some IR sensitivity
03-24-2017, 03:54 PM   #9
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QuoteOriginally posted by Xmas Quote
Bracket and keep notes if you are using a filter deep enough for white leaf syndrome.
Filter factor is dependent on light spectrum, and film spectral sensitivity.
Wood effect tree and light dependent
Foma400 is cheap and has some IR sensitivity
I already got Foma 200 (rebranded as Arista EDU Ultra) bulk roll, which is said to give the "wood effect" when rated as EI 6. I'm guessing if I set my gossen to ISO 6, and bracket 2 stops around that (giving me a wide latitude to check) I can get an idea of what direct sunlight gives me. Shame it's going to be thunderstorming all weekend.
03-24-2017, 03:55 PM   #10
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IR film photography is something I never tried. IIRC a deep red filter was required.
It must be a real PITA to view, focus and compose through an SLR lens using such.
Doubtless I'd choose a viewfinder-type film camera or IR-converted digital instead.

Chris
03-24-2017, 06:07 PM   #11
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QuoteOriginally posted by ChrisPlatt Quote
IR film photography is something I never tried. IIRC a deep red filter was required.
It must be a real PITA to view, focus and compose through an SLR lens using such.
Doubtless I'd choose a viewfinder-type film camera or IR-converted digital instead.

Chris
That's why it's pretty useless if your lens doesn't have the infrared focusing Mark. All of mine do fortunately.

Basically you focus without the filter, put the filter on and turn the focused point so it's at the red infrared mark.
03-24-2017, 07:45 PM   #12
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QuoteOriginally posted by kb244 Quote
Since the MX's GPD meter is completely insensitive to infrared light,
Hi KB, Can you cite a ref for that?
Here is a spreadsheet of my measurements with an F.D. filter that may be useful.
https://app.box.com/s/vzvkaze7mdgqbdykjz7ex5dyv57dyyrv
Regards
03-24-2017, 08:08 PM   #13
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QuoteOriginally posted by wombat2go Quote
Hi KB, Can you cite a ref for that?
Here is a spreadsheet of my measurements with an F.D. filter that may be useful.
https://app.box.com/s/vzvkaze7mdgqbdykjz7ex5dyv57dyyrv
Regards
From this thread : Pentax MX Achieve the Impossible brochure - PentaxForums.com

03-25-2017, 06:52 AM   #14
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QuoteOriginally posted by kb244 Quote
I already got Foma 200 (rebranded as Arista EDU Ultra) bulk roll, which is said to give the "wood effect" when rated as EI 6. I'm guessing if I set my gossen to ISO 6, and bracket 2 stops around that (giving me a wide latitude to check) I can get an idea of what direct sunlight gives me. Shame it's going to be thunderstorming all weekend.
http://www.foma.cz/en/fomapan-200
http://www.foma.cz/en/fomapan-400
Seem same spectrally?
I'd try sequence of 2 stop brackets!
We would like to see any wood effects BTW
03-25-2017, 04:45 PM   #15
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Yea I noticed on paper they seem the same, just 200 has an older looking tabular grain versus the 100/400 which seems closer to what you'd get from most modern film. I'll try to get results soon, just all dreary and rainy this weekend.
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