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Thread: IR exposure with my Phase One back

  1. #1
    mikeinlondon
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    IR exposure with my Phase One back

    I recently got a Phase One P21+ back converted to IR but something very strange seems to happen when exposing..

    The back will expose pretty much as normal without an R72 filter on the lens, but when I add the R72 (virtually opaque) it massively OVER exposes by consistently 6 stops!

    How can this be? Can someone explain?

    I created a support case but they seemed to think its normal.

    I'm at a loss at how this can increase it by so much!

    Regards

    Mike

  2. #2
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    Re: IR exposure with my Phase One back

    So, you had it converted to IR, or full-spectrum? If it's IR converted, I don't understand why you're placing an R72 filter on the lens.

    Also, I'm speculative that the TTL metering might not be metering "non-visible" parts of the spectrum. Thus, when you place the IR filter over the lens, it just treats it as if it were a thick ND filter, and adjusts the exposure as if it were only dealing with visible light.

    I've done both full-spectrum, and fully converted IR, so I'm under the impression that you're running full-spectrum. Metering never worked very well with full-spectrum. Actually, it doesn't work extremely well in IR, in general.

    Don't worry about the metering. Just pay attention to the histogram and you'll be fine.

  3. #3
    mikeinlondon
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    Re: IR exposure with my Phase One back

    I asked for IR, I didnt know full spectrum was an option. But I was specifically told I would still need to use an infra red filter on the lens as all that is done with an IR conversion is to remove the IR cut filter, and re calibrate the focus for IR - or so i believe

    thus to get the B/W IR effect I was after you would still need to use and IR filter.

    "Also, I'm speculative that the TTL metering might not be metering "non-visible" parts of the spectrum. Thus, when you place the IR filter over the lens, it just treats it as if it were a thick ND filter, and adjusts the exposure as if it were only dealing with visible light."

    Do you think that is what has happened?, but surely a thick ND filter should dramatically reduce exposure time? I use a 10 stop ND on my P25+ and yes the metering is off but the exposure time is reduced not increased

    "I've done both full-spectrum, and fully converted IR,"

    with P1, or another brand?

    Im not really worried about the metering, just that the P21+ base iso is 100 so if you like to shoot at f8 or f11 on a bright sunny day you might soon run out of shutter speed options....dont really want to add ND filters too!

    Mike

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    Re: IR exposure with my Phase One back

    Using the same exposure or using auto metering?

    Meters are almost always built to measure visible light. So by blocking visible light (R72 filter) the meter would push the camera to expose many stops more than it should if you're target is only IR light. You should expose manually and adjust based on the red channel of the histogram.

    Your Phase One Dealer should be able to guide you through issues like this. Good ones will have spent the time (as we have) to learn niche applications like IR inside-and-out.

    Full Spectrum is not an option through Phase One and having any third party do it would void the warranty.

    Doug Peterson (e-mail Me)
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  5. #5
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    Re: IR exposure with my Phase One back

    Quote Originally Posted by mikeinlondon View Post
    I asked for IR, I didnt know full spectrum was an option. But I was specifically told I would still need to use an infra red filter on the lens as all that is done with an IR conversion is to remove the IR cut filter, and re calibrate the focus for IR - or so i believe

    thus to get the B/W IR effect I was after you would still need to use and IR filter.
    Not really "IR." Your camera is now full-spectrum, and capable of IR, but it's not quite the same. My IR cameras are all fully modified so that I do not need to use a filter; the IR filter is installed atop the CMOS/CCD itself, allowing for me to compose the scene without constantly removing/reinstalling a filter.

    "Also, I'm speculative that the TTL metering might not be metering "non-visible" parts of the spectrum. Thus, when you place the IR filter over the lens, it just treats it as if it were a thick ND filter, and adjusts the exposure as if it were only dealing with visible light."

    Do you think that is what has happened?, but surely a thick ND filter should dramatically reduce exposure time? I use a 10 stop ND on my P25+ and yes the metering is off but the exposure time is reduced not increased
    Yep, I think that's what's happened.

    Your camera is now sensitive to IR light, while it wasn't before. The R72 will behave similarly to an ND filter on a camera with an IR-blocking filter over the CCD/CMOS. However, now that your camera's IR-blocking filter has been removed, the R72 filter is only filtering wavelengths less than 720nm.

    "I've done both full-spectrum, and fully converted IR,"

    with P1, or another brand?
    Canon, predominantly. However, the science behind this is essentially universally applicable.

    Im not really worried about the metering, just that the P21+ base iso is 100 so if you like to shoot at f8 or f11 on a bright sunny day you might soon run out of shutter speed options....dont really want to add ND filters too!
    Mike
    Yeah, it's not really like that. The ND filter will actually not affect your IR exposure. It will, however, correct your camera's metering. Thus, if you're six stops off in metering, throw a six-stop filter in front of your R72, and you'll meter correctly. The pictures will be the same, either way, in IR, as the IR light will pass right through the ND filter. That said, the varying amount of IR makes it difficult to correctly calibrate scene-to-scene; as to say, "+ six stops of ND" will not always be correct. Watching the histogram is the only reliable method of shooting in IR, from my experience.

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    Re: IR exposure with my Phase One back

    Quote Originally Posted by coulombic View Post
    Yeah, it's not really like that. The ND filter will actually not affect your IR exposure. It will, however, correct your camera's metering. Thus, if you're six stops off in metering, throw a six-stop filter in front of your R72, and you'll meter correctly. The pictures will be the same, either way, in IR, as the IR light will pass right through the ND filter. That said, the varying amount of IR makes it difficult to correctly calibrate scene-to-scene; as to say, "+ six stops of ND" will not always be correct. Watching the histogram is the only reliable method of shooting in IR, from my experience.
    +1

    Especially since a P21+ is capable of rendering an amazing amount of detail and a 6 stop ND filter on top of a R72 filter would worry me on reducing image quality.

  7. #7
    mikeinlondon
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    Re: IR exposure with my Phase One back

    Hi Doug

    Didnt realise the metering worked like that! That explains it then!

    You know from our email conversations (mike from Greenwich London) the problems I have had getting advice on P1 IR, no-one over here knows their stuff!

    I appreciate the you taking the time to reply!

    Kind regards

    mike

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    Re: IR exposure with my Phase One back

    Quote Originally Posted by dougpetersonci View Post
    Full Spectrum is not an option through Phase One and having any third party do it would void the warranty.
    Wow, careless typo. My apologies. Correct below:

    Full Spectrum IS an option through Phase One (which means if the back is under warranty the warranty is maintained); whereas having any third party do it would void the warranty.

    Doug Peterson (e-mail Me)
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    Head of Technical Services, Capture Integration
    Phase One Partner of the Year
    Leaf, Leica, Cambo, Arca Swiss, Canon, Apple, Profoto, Broncolor, Eizo & More

    National: 877.217.9870 *| *Cell: 740.707.2183
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  9. #9
    Senior Member ondebanks's Avatar
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    Re: IR exposure with my Phase One back

    Mike,

    I'm curious - how much did the IR conversion cost you? And was this done by Phase One themselves, or as Doug says, by a third party?

    Ray

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