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Filed under: Cameras •Tips •Training •

Next Stop: The Last Stop! RED MX Latitude Tests

When DSC Labs offered to send me a 102db (translation: 18-stop) exposure latitude chart, I just couldn’t bring myself to say no. And, naturally, the first camera I wanted to test was the RED ONE MX. How much better is the new sensor? Let’s take a look…

The Ambi-2 102db test chart, by DSC Labs, is a backlit chart. Due to the limitations of print technology it’s currently impossible to create a printed test chart with a range of more than five or six stops, and today’s cameras regularly offer ten or more stops of latitude. A backlit chart, where light is passed through a series of progressively denser strips printed on a piece of clear plastic, is the only way to create a chart that meets or exceeds the latitude of today’s cameras.

The Ambi-2, with a full-color rear-lit chart. This is not the chart I’ve been using, but shows the overall setup. My configuration does not have the light source mounted on the stand, as this one does, as DSC sent a prototype LED light that clips on the back.

This is the rear-lit gray scale used for these tests. When backlit there are 18 steps, so with the first (far left) step exposed such that it just barely clips, the next 17 steps show the actual dynamic range of the camera. While it is an 18-step chart it is intended to show a maximum of 17 stops of latitude. The image above is a representation as there isn’t a camera today that can resolve the entire scale.

I performed these tests with the aid of Adam Wilt, in his office at Meets the Eye Studios in San Carlos, CA. This space was perfect but for one thing: it wasn’t light-tight. Backlit charts only work properly if there is NO spill light on the front of it, as that may have the effect of “flashing” the shadows and showing more latitude than the camera can see under normal conditions.

So Adam made a tunnel out of black showcard:

I bought some black plastic ground cover cloth at Home Depot, and a few yards later we had both the front and the back sealed against outside light. The only light source used was inside the back of the Ambi-2 itself and was an LED source that looked a lot like a Rosco LightPad, although I don’t know who makes it. It was designed to be a perfectly even light source, as unevenness could result in misleading test results, and its only drawback is its spectrum: it’s a very blue-green light. I hope to repeat this test at some point in the future using a full-spectrum tungsten light, both raw and through full CTB for daylight testing.

Adam is an extremely thorough tester, which is nice as we double-check each other to make sure we don’t inadvertently get into trouble. In this case Adam solved two very tricky problems that I hadn’t thought of before the test.

The first problem was eliminating the effect of lens vignetting. While we started off using a RED zoom we switched over to a Zeiss Ultra Prime to eliminate the effects of vignetting. RED zooms tend not to vignette if one stops down far enough (about T5.6) but the chart wasn’t bright enough to allow for that stop. A 65mm Ultra Prime showed no vignetting and allowed us to clip the clear wedge at around T2.8. (Adam determined that there was no vignetting by panning the camera left and right and watching a waveform monitor to make sure the edges didn’t dip.)

The second problem was finding a way to expose the clear wedge at 100% exactly, considering that the RED’s monitoring output is only meant as a “video tap.” Adam’s solution was to start with the aperture wide open, and then close it down as the camera rolled. In Apple Color it was quite easy to scrub through the frames and find the one with the right exposure for the tests I wanted to perform.

On to the next page, where we can see some test results…

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Hey Art, very neat chart!

I wanted to point out that you can turn on the spot meter on the camera, which will bring up a small box on the EVF or LCD, and you can aim that box at anything you want and it will tell you the IRE value at the bottom of the screen. You can also use the joystick on the back of the RED to make the box bigger or smaller by spinning it clockwise or counter clockwise. I’ve found the spot meter to be very useful, but also more tedious than zebra’s, or the predator vision (which is questionable now in B.30).

Posted by .(JavaScript must be enabled to view this email address)  on  08/03  at  01:09 AM

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In just looking at the images and the waveforms provided I am seeing a tremendous amount of noise. I was wondering at what level of noise you wrote the image off as unusable? I have often found with the RED 5 or so stops of usable latitude and this has held true with my experiences with MX.

Posted by .(JavaScript must be enabled to view this email address)  on  10/14  at  10:23 PM

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