(Page 1 of 2 pages for this article  1 2 >)

Filed under: Cameras •Production •Tips •Training •

Arri Alexa and Far Red: A Problem That’s Already Been Solved

Last night I had the chance to—finally!—touch an Arri Alexa at Chater Camera’s Alexa preview party. And, being a curious sort of fellow, I decided to run some impromptu tests. IR tests are the easiest to do so I quickly did some—and in the process learned some interesting things about Alexa.

I handhold an Arri Alexa prototype at Chater Camera’s “Welcome to Alexa” event last Monday evening. John Chater puts the camera on my shoulder so I can enjoy the feel of a perfectly balanced camera.

Michael Bravin of Arri brought us an Arri Alexa prototype with a working ProRes recording module, so I happily shot my tests to ProRes422(HQ) and carted them away. I have to say that I have never had an easier time getting footage into Final Cut Pro.

I’ve received some complaints lately that some of my articles are too hard to follow. Several people have requested “executive summaries.” Okay, here it comes: the IR filter that you’ll need when using Alexa is:

The Tiffen T1.

The Arri Alexa seems to respond to non-visible red light in much the same way that the Sony F35 does, and probably for much the same reason: the color red has been, for a long time, the bastard stepchild of television. In the NTSC days we used to issue wardrobe instructions that stated “No white, black or red!” White and black were difficult tones to reproduce accurately, since exposing for flesh tones usually made white too bright and black too dark; and bright red was known to smear disturbingly when broadcast. The red phosphor used in early televisions wasn’t bright enough to compete with the blue and green phosphors, so some green was added to the red phosphor to “pump it up.” This resulted in—you guessed it—orange-ish reds.

Real red was almost never seen on television. And when it was seen, it was feared. The color red, in the old days, never got no love.

The first time I fired up a Sony F35 I was standing next to video engineer Jim Rolin. The camera was aimed down at a Persian carpet and Jim exclaimed, “I’ve never seen so many hues of red from a video camera before!” Sony built the F35 for digital cinema, which didn’t have the color gamut restrictions imposed by earlier analog broadcasting standards, and all the colors were rich and beautiful without being oversaturated—especially red.

Arri has done much the same thing here: they’ve pulled out all the color stops. But when you do that with red there are certain… side effects.

Filter tests with an Arri Alexa prototype at Chater Camera. To the right is my (as yet un-patented) IR/far red test chart.

Far red is not infrared. Infrared is heat, and every silicon sensor ever made is primarily sensitive to heat. It’s just a fact of life: given its druthers, silicon sensors would rather function as IR imagers because heat is what they see best. Humans, on the other hand, don’t see heat at all, so we cut off the portion of the spectrum silicon loves most (above a wavelength of about 750nm) and keep the rest. From the perspective of a piece of silicon it’s a bit like keeping the gravy but throwing out the roast beef, because we can taste the gravy but not the beef.

Far red is a color that is on the very edge of the visible spectrum. It’s just above red and it’s a color that humans don’t see easily, if at all. Most modern cameras, such as the F35 and likely the Arri Alexa, have an IR cut filter that chops off the spectrum starting at around 700nm, where the visible spectrum (and the ITU-709 broadcast standard) ends, and that eliminates infrared contamination that starts at around 750-800nm. But there’s a little sliver of far red, just below that range, that, if amplified, shows up in certain types of synthetic fabrics and clothing dyes.

What’s the best way to amplify a color like far red? Put a filter in front of the lens that blocks visible light, requiring the stop on the lens to be opened. Opening the stop brings the level of visible light back to normal but boosts far red, because far red was never cut to begin with.

That filter is called a neutral density filter. You may have heard of it. Other than a polarizer there is probably no type of filter that is more used in front of lenses.

ND filters block visible light only. They have no effect on non-visible light. The stronger the ND filter, the more visible light is blocked and the more the lens stop is opened to compensate—which lets in more far red.

Let’s look at some actual pictures on the next page…

(Page 1 of 2 pages for this article  1 2 >)

                   

 

So You Want to Buy a RED Scarlet? Matt Jeppsen | 01/01 Do you really need that new camera? An intervention. I got a question on Twitter recently regarding the pros and cons of purchasing a Red Scarlet camera. After responding,…

ARRI Alexa In-Camera Recording Using Avid’s DnxHD MXF File Format PVC News Staff | 11/07 ARRI announces a major new recording feature for its ALEXA camera range, offering in-camera support for Avid®’s open standard and widely adopted Avid DNxHD® codec.…

Tiffen Showcases Award-Winning Line of Visual Imaging Techonlogy at Influential Photo Industry Expo PVC News Staff | 10/12 Tiffen joins the photo industry’s most creative and influential movers and shakers for the annual PDN PhotoPlus International Conference and Expo in NYC The Tiffen Company, a leading manufacturer and distributor of award-winning accessories for the still imaging, motion picture, and broadcast markets, is pleased to announce…

You must be registered to comment. This is an effort to reduce spam. Please REGISTER HERE.

Art, great piece once again.

Michael showed me the originals from these tests while he was on the phone to you this AM.- very nice work.

I told you that you were going to love the camera.

gary adcock

Posted by .(JavaScript must be enabled to view this email address)  on  07/28  at  02:54 PM

---

Hi Gary- Sorry, it didn’t register that he was sitting with you when he called! I would have said hi by proxy.

I do love the Alexa. I can’t wait to see more of it. There’ll be a DCS event locally on August 10th where we’ll get to see it again, and I hope to do a live exposure latitude test using the DSC Ambi-2 chart. Should be fun… for geeks!

Thanks for your kind words. Any idea where I can get an Alexa T-shirt, bumper sticker and tattoo?

Posted by Art Adams  on  07/28  at  04:03 PM

---

By the way, autobalance in Color balances the highlights and the blacks but does nothing to the mid-tones… which makes sense, as it’s easy to figure out where the top and bottom of a signal are, but not so easy to find the middle.

Posted by Art Adams  on  07/28  at  04:04 PM

---

Are the Schneider Platinum IRND filters available yet? The are supposed to work with the EX/F23/35, and filter IR, add ND and have no colour cast as the T1 does. I heard they were delayed though?

Posted by David Williams  on  07/28  at  11:00 PM

---

I need to get in touch with Schneider and ask. They sent me some a while back to test but they were the lighter ones (ND .30) and I couldn’t determine how effective they were before I had to send them back. I’m getting a lot of requests for them, though, so I’ll see what I can find out.

Posted by Art Adams  on  07/29  at  12:42 PM

---

Thanks for this test. I had seen the page in their Alexa brochure where they really touted their OLPF and IR filtering. Thanks to your tests, it seems that, under certain circumstances, a bit more filtering in front of the lens may well be in order.

Excellent work as always fine sir.

Posted by IEBA  on  07/30  at  10:10 AM

---

Sorry to repeat a request has anyone managed to test the ALEXA’s response with Schneider Platinum IRND filters yet?

Posted by Nicholas JS Wise  on  09/03  at  07:43 AM

---

Name:

Email:

Location:

URL:

Smileys

Remember my personal information

Notify me of follow-up comments?

Submit the word you see below: