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We have seen plenty of tests like the Abel Cine one with different versions of IR Cut filters to control infra red contamination on the Black Magic Cameras, both the Pocket Cinema Camera (you will find the results of these tests a little later in this thread) and the Cinema Camera. I have read conflicting reports on whether or not Vari ND's have the same issue with IR pollution as do regular ND's. Since I like the Vari ND's for run and gun work, I decided to test out two versions I use for IR pollution and color shift and thought I would share my findings.
Additionally, in this thread:
viewtopic.php?f=2&t=14889
I shared a technical paper sent to me by a sales rep from Tiffen explaining why a combination of IR cut and Hot Mirror were the optimum IR Pollution protection. I wondered how those same findings applied in the case of Vari ND's.
This is what I came up with...
Equipment tested in various combination:
1) Light Craft Workshop Digi Pro Vari ND: This is the newer and much improved version over the old one which gave very poor results. I was impressed with the testing I saw done on this and have been happy with its performance. I use a 77mm version which has 82mm outer threads.
2) A few years ago I found a Tiffen 4x5.65 Rota Pola on ebay for $30. I put it in the outer stage of my Chrosziel matte box and put a 4x4 circular pola on the inner stage . It is really versatile because I can rotate the inner stage to either cut out the polarizing affect if I am shooting talent and want to preserve skin highlights, or dial it in if I want to control reflections. Then I slip in the outer stage and dial down my exposure. I have found this combination to be really handy when I am built into Studio mode either on my BMCC, BMPCC or my Sony F3 and don't mind the bulk of a matte box.
3) Tiffen T1 82mm filter: From what I can gather, T1 was originally specifically developed for cameras like the Sony f35 which had effective sensor hot mirror IR protection above a certain wavelength (750nm) but were exposed in the near red spectrum. This filter apparently is targeted at a very narrow band of wavelength.
4) Tiffen 82mm Hot Mirror.(note the T1 and hot mirror were stacked on the outer stage of the vari nd during the testing, the hot mirror always placed on the outer most stage)
5) Tiffen water white irnd6 & irnd1.2, (both in 77mm screw on versions)
I. So first I set up a color chart, and some black synthetic and cotton cloth to establish the base line. I shot both at f22 (2 stops overexposed but the narrowest setting available on my lens) and at F2 with nine stops of regular ND filter (I stacked ND6,9 &1.2). From the following image you can see how intense the IR contamination is. I shot in video mode and did just some basic color correction to pull in some contrast and balance the levels. Even trying to pull down the red channel you can see how intense the color shift is:
II. Then I shot four versions with the LCW Vari ND. Going clockwise in the image:
1) Just dialing down exposure to an f2.8 with only the Vari ND.
2) Dialing to an f2.8 with Vari ND and T1.
3) Dialing to an f2.8 with Vari ND, T1 and Hot Mirror.
4) I decided to test using an irnd6 on the lens and putting the vari nd on after. The idea here being that in really bright conditions, I could use the irnd to cut a certain amount of light and then fine tune it with a more minimal dialing on the vari nd to see if that created any IR advantage. I added the hot mirror as well on the outer stage.
From the color charts you can see that the combination of vari nd, T1 and Hot Mirror preserved the richest colors and retained the deepest blacks. Interestingly, trying to cut a certain amount of visible light with an irnd first and then just fine tuning with the vari nd seemed to be of no advantage (and in fact really was a disadvantage) as you can see a certain amount of shift in the black synthetic bag on the right that is not so in the case of the vari nd, t1, hot mirror version. You can also see from the reference shots that there is no where near the IR contamination with a vari ND as there is with regular ND's. The difference is dramatic.
III. Next, I shot the Rota Pola ND with my matte box. Since I don't have a three stage matte box nor do I have a 4x5.65 hot mirror or T1, I just tested this (going clockwise) by itself, with and irnd6 and irnd1.2 on the barrel. I also shot a reference of just using the rota pola at achieve an f22 (the least amount of polarization affect) to have as a comparison.
In this case it seems like the irnd6 combination with the rota pola fine tuning the exposure gave the truest colors in the chart There is still a slight shift in the blacks but nothing I couldn't dial out from a log file in Resolve. It seems the adding the higher irnd1.2 was less effective than using the rota pola, again leading me to believe that vari ND,s have less IR contamination issues than do regular ND's I suspect if I could have added a hot mirror to the outer stage I would have seen the black return to true black.
Final Conclusions:
1) As far as IR pollution goes for the cinema camera's sensor, from the dramatic shift to the red in the 1st reference shot, I suspect the Cinema Camera's sensor has very little IR protection compared with many other camera's (I saw this in Abel Cine's tests as well).
2) If you chose to use a vari nd to control exposure, while it introduces a certain amount of color shift to the green, it does a really nice job of controlling exposure without a huge price of IR contamination. Adding a T1 and Hot mirror seems to be the optimum solution (agreeing with Tiffen's findings) to solve what little issues it has and to my eye and from what I saw on my scopes, at least, it introduces the least color shift.
3) While the Matte Box Rota Pola ND was not as neutral as the LCW Vari ND in controlling exposure, knowing its limitations and how to combine it with some IR cut protection on the barrel I feel quite confident in using it when appropriate, particularly when shooting in log as I feel confident that I could correct in post the relatively minor shift to the green.
4) While shooting in raw would probably mitigate the need for concerns about color shift as you could dial the white balance after , I don't think it would solve the issue of the IR contamination and what it would do to certain fabrics. I would use the same protection.
5) And last, some feel that a Vari ND does a certain amount of degradation to the image, or image softening. I cannot repudiate or verify this, but the image from the BlackMagic is so sharp, I don't mind a certain amount of filmic softening to what it sees. My taste.
Hope this is helpful to some of you out there who have been asking some of the same questions as I have. Of course, for wide lenses (roughly 24mm and under) where vignetting can occur with pola's I will have to switich to conventional irnds in combination with the hot mirror.
Later in this thread you will find my results for another version of this test with the Pocket camera. I plan to shoot these same tests with my Sony F3 just to have as a comparison against a sensor that has more IR blocking filtering. I will share those findings as well when I get some time to post again if people are curious.
Additionally, in this thread:
viewtopic.php?f=2&t=14889
I shared a technical paper sent to me by a sales rep from Tiffen explaining why a combination of IR cut and Hot Mirror were the optimum IR Pollution protection. I wondered how those same findings applied in the case of Vari ND's.
This is what I came up with...
Equipment tested in various combination:
1) Light Craft Workshop Digi Pro Vari ND: This is the newer and much improved version over the old one which gave very poor results. I was impressed with the testing I saw done on this and have been happy with its performance. I use a 77mm version which has 82mm outer threads.
2) A few years ago I found a Tiffen 4x5.65 Rota Pola on ebay for $30. I put it in the outer stage of my Chrosziel matte box and put a 4x4 circular pola on the inner stage . It is really versatile because I can rotate the inner stage to either cut out the polarizing affect if I am shooting talent and want to preserve skin highlights, or dial it in if I want to control reflections. Then I slip in the outer stage and dial down my exposure. I have found this combination to be really handy when I am built into Studio mode either on my BMCC, BMPCC or my Sony F3 and don't mind the bulk of a matte box.
3) Tiffen T1 82mm filter: From what I can gather, T1 was originally specifically developed for cameras like the Sony f35 which had effective sensor hot mirror IR protection above a certain wavelength (750nm) but were exposed in the near red spectrum. This filter apparently is targeted at a very narrow band of wavelength.
4) Tiffen 82mm Hot Mirror.(note the T1 and hot mirror were stacked on the outer stage of the vari nd during the testing, the hot mirror always placed on the outer most stage)
5) Tiffen water white irnd6 & irnd1.2, (both in 77mm screw on versions)
I. So first I set up a color chart, and some black synthetic and cotton cloth to establish the base line. I shot both at f22 (2 stops overexposed but the narrowest setting available on my lens) and at F2 with nine stops of regular ND filter (I stacked ND6,9 &1.2). From the following image you can see how intense the IR contamination is. I shot in video mode and did just some basic color correction to pull in some contrast and balance the levels. Even trying to pull down the red channel you can see how intense the color shift is:
- IR Pollution Reference
- IR Pollution reference.jpg (523.35 KiB) Viewed 13023 times
II. Then I shot four versions with the LCW Vari ND. Going clockwise in the image:
1) Just dialing down exposure to an f2.8 with only the Vari ND.
2) Dialing to an f2.8 with Vari ND and T1.
3) Dialing to an f2.8 with Vari ND, T1 and Hot Mirror.
4) I decided to test using an irnd6 on the lens and putting the vari nd on after. The idea here being that in really bright conditions, I could use the irnd to cut a certain amount of light and then fine tune it with a more minimal dialing on the vari nd to see if that created any IR advantage. I added the hot mirror as well on the outer stage.
- LCW Vari ND
- LCW Vari ND BMCC tests..jpg (392.27 KiB) Viewed 13023 times
From the color charts you can see that the combination of vari nd, T1 and Hot Mirror preserved the richest colors and retained the deepest blacks. Interestingly, trying to cut a certain amount of visible light with an irnd first and then just fine tuning with the vari nd seemed to be of no advantage (and in fact really was a disadvantage) as you can see a certain amount of shift in the black synthetic bag on the right that is not so in the case of the vari nd, t1, hot mirror version. You can also see from the reference shots that there is no where near the IR contamination with a vari ND as there is with regular ND's. The difference is dramatic.
III. Next, I shot the Rota Pola ND with my matte box. Since I don't have a three stage matte box nor do I have a 4x5.65 hot mirror or T1, I just tested this (going clockwise) by itself, with and irnd6 and irnd1.2 on the barrel. I also shot a reference of just using the rota pola at achieve an f22 (the least amount of polarization affect) to have as a comparison.
- Rota Pola Tests
- Rota Pola ND BMCC tests..jpg (355.66 KiB) Viewed 13023 times
In this case it seems like the irnd6 combination with the rota pola fine tuning the exposure gave the truest colors in the chart There is still a slight shift in the blacks but nothing I couldn't dial out from a log file in Resolve. It seems the adding the higher irnd1.2 was less effective than using the rota pola, again leading me to believe that vari ND,s have less IR contamination issues than do regular ND's I suspect if I could have added a hot mirror to the outer stage I would have seen the black return to true black.
Final Conclusions:
1) As far as IR pollution goes for the cinema camera's sensor, from the dramatic shift to the red in the 1st reference shot, I suspect the Cinema Camera's sensor has very little IR protection compared with many other camera's (I saw this in Abel Cine's tests as well).
2) If you chose to use a vari nd to control exposure, while it introduces a certain amount of color shift to the green, it does a really nice job of controlling exposure without a huge price of IR contamination. Adding a T1 and Hot mirror seems to be the optimum solution (agreeing with Tiffen's findings) to solve what little issues it has and to my eye and from what I saw on my scopes, at least, it introduces the least color shift.
3) While the Matte Box Rota Pola ND was not as neutral as the LCW Vari ND in controlling exposure, knowing its limitations and how to combine it with some IR cut protection on the barrel I feel quite confident in using it when appropriate, particularly when shooting in log as I feel confident that I could correct in post the relatively minor shift to the green.
4) While shooting in raw would probably mitigate the need for concerns about color shift as you could dial the white balance after , I don't think it would solve the issue of the IR contamination and what it would do to certain fabrics. I would use the same protection.
5) And last, some feel that a Vari ND does a certain amount of degradation to the image, or image softening. I cannot repudiate or verify this, but the image from the BlackMagic is so sharp, I don't mind a certain amount of filmic softening to what it sees. My taste.
Hope this is helpful to some of you out there who have been asking some of the same questions as I have. Of course, for wide lenses (roughly 24mm and under) where vignetting can occur with pola's I will have to switich to conventional irnds in combination with the hot mirror.
Later in this thread you will find my results for another version of this test with the Pocket camera. I plan to shoot these same tests with my Sony F3 just to have as a comparison against a sensor that has more IR blocking filtering. I will share those findings as well when I get some time to post again if people are curious.
Last edited by Patrick Finnegan on Tue Nov 12, 2013 3:18 pm, edited 3 times in total.
Finnegan's Wake Films
Director/ Cameraman
Director/ Cameraman