How strong of an IRND filter is really needed?

[Stephen Pruitt]

What are the popular ND filter cuts? I've got some very fast lenses (almost all 1.2 and 1.4, with two 2.8s) and am curious what strengths of IRND would be recommended.

Thanks.

Stephen

[David Mullen ASC]

In theory, only the heavier ND filters need IR because it's the density of the filter that is causing a greater proportion of IR to pass through relative to visible light.

But I recently used an ND1.2/IR combo on a show, loaned to us... and discovered that IR filters have a slight color cast to them, this one shifted the image a bit from the red to the green.

So even though I didn't need an IR filter when I switched to less-strong ND's in lower light, I found that there was a mild shift in color that I had to fix in post later. Next time, I'll probably get more ND's with the IR combined, just so I don't have to intercut angles shot with the IR and ones without. Or I guess I could get a separate IR filter.

But since the problem gets worse as you use heavier ND's, if your budget is tight, start with getting the heaviest ND's with IR, like the ND1.2 and ND.90.

[James T Mather]

My nickels worth: I tend to shoot a hot mirror all the time when outdoors - even if starting with dawn and no Nds as, inevitably, you are going to need it and then you'll have shots that have acquired the blue/green tint. I prefer to keep it consistent and have grading uniformly work the grade (this is no better or worse than other methods btw - just my preference).

I find that if you ND heavily (like getting 32 down to T4 for example then eventually you get some IR contamination - its impossible to avoid - even with a hot mirror - so I figured when buying again I'd go for the strongest ir cut Hot Mirror available).

[Ryan Hamblin]

What are the issues with compensating the IR contamination in post, through red alert? This may be a dumb question as well, but I have a set of the Zeiss 2.1 in arri standard mount (so the real old ones, from IIc days). Could I get a 49mm screw in Hot Mirror, or is that going to cause reflections with my ND being in a mattebox in front of the hot mirror?

[Steve Murray]

[QUOTE=James T Mather;524798]My nickels worth: I tend to shoot a hot mirror all the time when outdoors - even if starting with dawn and no Nds as, inevitably, you are going to need it and then you'll have shots that have acquired the blue/green tint./QUOTE]

Same for me. I keep a Rosco IR in at all times no matter what I'm shooting. Also it should be the first filter to the scene not the last one on a lens nor in the back of the line in your matte box.

[Jon B.]

Hi Stephen,

Are you using Canon L lenses by chance? If so, I'd like to know what you find as I'm keeping an eye out for a 77mm screw on IR/HM for mine. Would like to know your satisfaction w/ whatever one you choose.

Thanks

[Stephen Pruitt]

Hi there, Jon. . .

I'm all Canon L-series lenses here. We think they're great!

I'm still curious if people think I should be buying 0.9, 1.2, 1.5, 1.8, 2.1, etc. IRND filters, though. My lenses are very fast (1.2 and 1.4), but I'd rather not have more ND than I need, as I'm certain that the color will shift quite a bit using filters, at least judging from the tests posted here on REDUser.

Stephen

[David Mullen ASC]

The fast speed of the lenses doesn't matter when dealing with high light levels since you are working at the opposite end of the scale. What matters is the base sensitivity of your camera or film stock.

The "Sunny 16" rule tells you that in direct sunlight on a clear day near noon, your f-stop is f/16 when the ASA rating number is the same as the shutter speed, i.e. 50 ASA at 1/50th, 100 ASA at 1/100th, etc.

Since 1/50th is more or less the same as 1/48th, the shutter speed at 24 fps with a 180 degree shutter, this means that at 50 ASA, you'll be shooting at f/16 in direct sunlight, more or less the worst case scenario short of pointing the camera into a glaring ocean or into the sky.

Let's say you rate the RED camera at 200 ASA (I'm not saying that it is 200 ASA, or that you should use that rating... just that it's easy to calculate being exactly two-stops faster than 50 ASA.) At 200 ASA, 24 fps, 1/48th shutter, that means you'll need an ND.60 to get down to 50 ASA, just to be able to shoot at f/16 in direct sunlight.

So you see why you need heavy ND filters. Has nothing to do with whether your lenses open to f/1.4 or only open to f/4.

Most people would get a set of ND's starting at .30, as in: ND.30, ND.60, ND.90, ND1.2. It's rare to get heavier than that, some people would rather start to stack filters at that point if they needed more, though I wished I had an ND1.5 on my last show. Oddly enough, ENG cameras have really heavy ND's in their filter wheels, in two-stop jumps, something like ND.60, then ND1.2, then ND1.8. You could perhaps try that to save money, live with ND's only in two-stop increments.

[Sidney L. Plaut]

I saw IR contamination when using nd .9 indoor under tungsten light - purple shadows....

- we shot with master primes and used a lmb 5 - with a streak filter - so we could only stack 2 filters. So my DP asked, purple shadows or no streak filter... I chose purple shadows...

I only had a clear IR filter and ND: .3 .6. 9. 1.2+IR - could really have used nd.9IR...

so my advice is get it combined from .6 or min .9...