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Filed under: Cameras •Production •Tips •Training •

F35 IR Filter Shoot-Out

FIRST UP: SCHNEIDER OPTICS HOT MIRRORS

I’ve been informed by Sony engineers that the F35 has a 700nm cut filter built onto the sensor, which would mean that the red contamination we’re seeing is far red, not IR. Far red is at the edge of the visible spectrum and is a color of light that our eyes can’t see very well, if at all, but it is not yet pure infrared heat. The far red “leak” typically occurs between where ND filters start passing non-visible light (around 680nm) and where the IR filter on the sensor cuts (700nm).

We’re about to find out whether that’s true or not. Schneider Optics kindly provided me with three filters to test: a 680nm cut filter, a 720nm+ND 1.2 combination filter, and a 750nm cut filter. If there really is a built-in 700nm cut filter on the chip then we should see some results from the 680nm filter, but not the others.

First, here’s the 680nm filter:

This is the Schneider Tru-Cut IR 680 filter, in combination with a Schneider ND .9 and ND 1.2. It has a very strong dichroic coating, which gives it a cyan cast and costs between a half stop and a full stop of light. As we’ve seen, an overall color cast isn’t an issue at all: what’s important is what’s left once we white balance through it.

The Tru-Cut 680 does an excellent job of removing all far red or IR contamination from this shot.

BUT…

Here’s the flat field test, with the Tru-Cut 680 on a 16mm Ultra Prime:

Dichroic coatings tend to show up most when viewed at an angle other than perpendicular, and in this case the lens angle of view is too extreme around the edges of the filter. Let’s look at a 24mm lens:

Better, but there’s still an obvious cyan cast to the edges of the frame. Let’s see if the 32mm works:

At this point it appears safe to say that the cyan edges have given way to an overall cyan cast that can be easily white balanced away. Let’s take one more look, this time at a 50mm lens:

This image looks virtually the same as the 32mm, which means that 32mm is the widest lens one could safely use with the Tru-Cut 680 to avoid color vignetting.

The next filter in the Schneider series is the Tru-Cut IR 720 + ND 1.2 combo:

It seems that Sony’s engineers were correct. The 720 filter shows virtually no effect. The final Schneider filter is the Tru-Cut 750:

This filter has no effect at all, and when we compared the two filters we discovered that the 720 had some effect but not nearly enough. This can be explained by the sensor’s IR filter having a gradual roll-off effect that starts at 700nm, instead of a hard cut of everything above 700nm.

Just for the record, here are the flat field tests on the Tru-Cut 720 and 750 filters. First, the 720 as viewed through a 16mm lens:

And then the 750, viewed through the same lens:

We saw no evidence of a color vignette, only an overall color shift that is easily removed by white balancing.

While these filters don’t work so well with the F35 they do have some positive results on the RED.

Let’s move on to Formatt’s offerings…

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Will there be a Sony EX shootout? There would be a tremendous amount of interest in one.

Posted by DR  on  02/20  at  10:01 AM

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I’m working on that right now. I’m hoping to have it up in the next week. I’ve got one more set of filters to test, and so far the results have been very surprising—in a good way!

Posted by Art Adams  on  02/20  at  01:01 PM

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Excellent! I can’t wait for the results. (along with a couple thousand anxious EX users)

Posted by DR  on  02/20  at  01:23 PM

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Agree with DR. I am really interested in the results too.

Posted by Marcis Gasuns  on  02/20  at  04:37 PM

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ANOTHER good article!

Posted by billS  on  02/21  at  05:24 AM

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