Internal ND filters

[talazar]

Hello,

This subject has been discussed in other places before but I'd like to try and get some official information if you'd like to share it and perhaps insight from others.
I am purchasing an Alexa Mini which should arrive in the next month or so, and I am currently purchasing the needed accessories and equipment. I am looking to purchase ND filters to be able to reach all the ranges of exposure possible in combination with the internal ND filters. The question of the necessity of IRND and / or the type has come up. Therefore -

1. Is the IR filter in front of the sensor and the internal ND filters used in the Alexa Mini identical to the ones found in the Amira and older Alexa models? This information will allow me to use or ignore previous tests done with these sensors.

2. I am aware of the difficulty in having a 'one size fits all' solution when it comes to IR filteration on the sensor level and rendition of skin-tone. Previous tests have shown that the cutoff wavelength is around 700nm, so filters cutting light at a higher wavelength (such as the Schneider Platinum IRND which I believe are at 750nm) will have no effect (when not using dyes etc). Is that something you are willing to confirm or comment on? This will greatly shorten the 'candidate list' for IRND filters with this camera.

3. Lastly, I am curious about other people's experience with this. I know that many people decide to use non-IR ND filters with Alexa, while others swear by some Tiffen filters. The direction I am leaning towards now is to carry non-IR ND filters, however I am curious about the effect of such filters as the Schneider True-Cut 680 IR filter, if anyone decided to reduce the near-red even more.

Thanks,

Tal Lazar
Director of Photography
Los Angeles, CA

[Jan Heugel]

Dear Tal,

yes the IR filter is similar to those used in ALEXA and AMIRA.

ad 1:
We are using Full Spectrum Neutral Density Filters (FSND) filters in Mini, AMIRA and ALEXA SXT/XT.
These special filters have an even reduction of light over the whole spectrum (see red line in attached image). So it is better than a IRND (and difficult to manufacture in bigger sizes/sensitive to scratches).

ad 2:
Since you have a 0.6, 1.2 and 2.1 filter inside the general rule of thumb remains: Up to ND 0.9 no additional IR is needed, other than that (1.2 and above) we recommend using a combined IRND. Last time I checked we use Tiffen IRNDs or Schneider Platinum at our Rental depending on the client's requests.

ad3:
That's a question to the folks out there. What's the word on the street?

I just learned that Formatt produces normal size filters similar to our FSNDs, more here.

Best,
Jan

[Stout Film]

Jan,
I will be using a .9 Schneider Platinum IRND in addition to the internal ND's on the Mini (sometimes with no internal ND to get .9, sometimes with the .6 to get 1.5 and sometimes with 2.1 to get 3.0). It's right on the border you mention there. Do you expect this to do unwanted color shifts to my image in any of these cases, or is it just that IRND isn't needed and one could use either IRND or regular ND and all be the same for .9 and below? Thanks.

[Brant]

I am a little confused reading this. Jan are you saying Alexa Mini operators should use an external IR filtration when using the Mini's internal filters 1.2 or 2.1? Your comment in "ad2" leads me to believe this, but "ad1" suggests the internal filters do it all.

[Stout Film]

Jan is saying (please correct me if I'm wrong) the internal ND's are good to go as is and you won't need irnd until you hit 1.2 being added to the front of your lens. My question is if it is a problem to add a .9 irnd to the front or if either would be ok.

[Jan Heugel]

Hey guys,

to clear it up, generally speaking:

- when using ND filters above 0.9, it should not be regular NDs (color shift)
- Amira and Mini (and ALEXA XT, SXT with filter module) have Full Spectrum Neutral Density filters built-in. Meaning all of the filters reduce the full light spectrum by their density (yes visible and not visible light). An additional IR filter is not needed.
- when combining the built-in filters with NDs in front of the lens, "the game is the same": add 0.9 ND and you are still good. Add a 1.2, 1.5 or 2.1 ND in front of the lens you will end up with funny colors.

Which additional IRND filters you should choose is up to your testing. But since the introduction of ALEXA this is a very well explored test subject, so ask your googly friend. If time is at stake: ask your local camera rental, they are happy to help!

Best,
Jan