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Wednesday, May 07, 2008
More RED Res Testing: The Mystery(ium) Resolved
Adam Wilt | 05/07
A new chart shows what this camera (and several lenses) can really do.
I obtained a new, 4K resolution test chart at NAB, and aimed RED ONEs running build 15 version 2.2.5 at it using four different lenses: a 50mm Super Speed, a 50mm Ultra Prime, a 18-50mm RED zoom, and a 24-290mm Optimo. Cutting to the chase: I’m pleased to report that I see detail extinction at about 3.2K, confirming the numbers RED and others have claimed.
In a couple of previous tests (including the unfair 3-camera comparison, I’d come to the tentative conclusion that I could see detail out to about 2.4K - 2.5K, measured as distinct lines of detail across the image width (in video cameras, it’s normal to talk about “TV lines per picture height”, not lines per picture width; in film and in traditional optical engineering, one normally discusses “cycles” or “line pairs”, where two discernible lines, or a full alternation from dark to light, are taken together. Film scanning and digital cine cameras measure resolvable detail by the number of discrete lines across the image width, so I’ll use this metric for digital cine cameras like RED’s). These previous tests were done with a discrete-frequency chart, the DSC Labs Cam-Align MBSQ-SW v.4.1, showing blocks of lines at 50, 100, 200, 400, 600, 800, 1000, and 1200 TVl/ph. Shot full-height with RED at 4k 2:1, the chart could show me detail at 2000 and 2400 lines per picture width (at 2:1 aspect ratio, the width numbers are twice the height numbers), but couldn’t say anything about higher resolutions.
I tried reframing at half the picture height, thus effectively doubling the resolution of the chart; the chart’s 600 and 800 TVl/ph blocks translated to 2400 and 3200 lines per picture width (or 2.4K and 3.2K in digital cine terms; I’m ignoring the 2.4% difference between 1K=1000 and 1K=1024 in this and in the discussion that follows). I saw a bit of detail at 2.4k and undifferentiated gray at 3.2k, so I conservatively pegged the RED ONE’s resolution at “2.4k+, probably between 2.4k and 2.5k.”
But there’s a big, unexplored gap between 2.4K and 3.2K, and one test with one lens (an 18mm SuperSpeed at a fairly wide aperture) isn’t conclusive, especially when RED’s Graeme Nattress and cinematographer Guy-Louis Mier both claim to have seen the camera reach out to 3.2K. I get annoyed when numbers don’t match, so I resolved (pun intended, and no, it won’t be the last time) to test things further: I’d find a chart that unambiguously swept the frequencies of interest out to at least 3.2k, and I’d try several lenses and run through a range of T-stops on each one, to be sure I found each one’s “sweet spot” for maximum detail.
The Chart
At NAB, I went to the DSC Labs booth to see what they had. As it turns out, DSC had just produced their first digital cinema resolution chart, of which they had four copies, and they retrieved one of them from the Iconix booth for me.
the DSC Labs RB-SRW v8.1
This chart has several notable features:
- 29-line (15 black lines separated by 14 white lines) resolution trumpets: enough lines to easily see when aliasing comes into play.
- Trumpets range from 500-2000 TVl/ph, enough to measure out to 4k when framed full height at 2:1 aspect ratio.
- 16 trumpets around the center circle: one every 22.5 degrees, so one can see horizontal, vertical, and several diagonal resolutions.
- Quarter-circles in the corners, to measure edge sharpness as well as center sharpness.
(No, you won’t find the RB-SRW on the DSC Labs site just yet. Part of my intent on visiting DSC at NAB was to complain about their website, especially when it came to choosing resolution charts: I had really wanted a frequency sweep chart instead of the discrete-frequency chart I wound up with for the Texas Shootout. But the DSC folks, being calm, affable Canadians, simply agreed, saying, “well, the website’s not so good, eh?” DSC’s Michael Weigand then pulled out an MB-SW v4.1 chart—the frequency-sweep counterpart to the discrete-frequency MBSQ-SW chart I already have—and sold to to me with a nice show discount, in addition to the RB-SRW chart. Moral of the story? See the DSC Labs folks at trade shows or call ‘em up if you have questions; they are friendly and approachable. According to Michael, the RB-SRW appears on their price list as the DCR ["Digital Cinema Resolution"], and the SRW size is US$792.)
The Lenses
At Videofax in San Francisco, I used the Zeiss 50mm Super Speed and the Zeiss 50mm Ultra Prime.
At Chater Camera in Berkely I tested the RED 18-50mm and the 12x Angeneiux Optimo 24-290mm.
Now that’s a lens… and they say size doesn’t matter! (Photo: Tim Blackmore)
Many thanks to both rental houses for the opportunity.
Next: lens specifics, RED resolution tests...
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As usual, an excellent technical article that even I can understand.
The more I know, the less I know what I don’t know… 
Posted by Art Adams on 05/08 at 11:39 AM
Hi Adam,
Great article, good to see some independent results that confirm what is being said.
Just wondering, for someone like me who’s not the most technically minded individual, what exactly does 3.2k mean? I’m used to hearing about these things expressed as line-pairs per mm and was wondering how the Red One holds up on that score.
Posted by on 05/09 at 01:04 AM
Extremely interesting and I am grateful to have this test verify the information, your unbiased response is appreciated and I am sure there are many that will feel the same way.
What concerns me as a person that will soon to be emailed with the arrival date of a RED order.
If focus is SO difficult due to lack of a LCD with proper resolution.
I fail to see how I am going to be able to pull off such a thing, on a set when in a studio setting it is so evasive?
But it is being done, according to various posts.
I would also like to see DOF tests.
Thanks again
Posted by on 05/09 at 09:31 AM
“...what exactly does 3.2k mean?”
The camera can resolve about 3200 lines across the image, or 1600 line pairs. The sensor is 24.4mm wide, but the tech specs don’t say if that’s the total width, or the width of the active area. Assuming the active area is 24.4mm, then we’d be seeing 1600/24.4 = 65.6 lp/mm. If the active area is smaller, the lp/mm figure would go up proportionally.
“If focus is SO difficult due to lack of a LCD with proper resolution. I fail to see how I am going to be able to pull off such a thing, on a set when in a studio setting it is so evasive?”
An excellent question!
1) In most serious large-format production, focusing is done by tape measure, not by eye. This, of course, requires accurately set flange focal depth, so that the lens focusing scales can be trusted. We found that the flange depth was NOT properly set on the REDs at Chater (more on this in my next article), so you’ll want to check that out very carefully (as will we at Meets The Eye if we choose to take delivery of our REDs when our numbers come up) .
2) The lack of any means to check accurate focus, either optically or electronically, is a major issue with RED. The camera desperately needs a better image-mag function for this purpose. We can only hope that future firmware builds provide this missing feature.
3) If it looks focused at 720p, or at 1080p, and that’s your deliverable, then you’re OK as-is. My blown focus looked fine on the 720p output, and even on 720p magnified. Only at 4K viewing did we see the errors.
4) “The question is: do you feel lucky?” <grin>
Posted by Adam Wilt on 05/09 at 02:55 PM
So Adam… what do you get if you put Red, Green or Blue gels over that chart?
Bruce Allen
http://www.boacinema.com
Posted by Bruce Allen on 05/09 at 03:26 PM
...or on a filter in front of the lens of course… or used a colored chart.
Like in this guy’s tets:
http://www.ddisoftware.com/sd14-5d/
I’m sure the Red would still do well, just interested in how much its resolution decreases relative to the resolution decrease for a 3CCD cam…
Bruce Allen
http://www.boacinema.com
Posted by Bruce Allen on 05/09 at 03:30 PM
“...what do you get if you put Red, Green or Blue gels over that chart?”
I’d expect to see results much like Mike Chaney got on the webpage you referenced: slightly reduced green-only res; more reduced red-only or blue-only res. But I haven’t done such tests, and have no RED testing scheduled in the near future, so all I can do is speculate.
The res loss would be somewhat comparable to the res loss in a pixel-shifted 3-chip camera (especially when considering 4:2:2, 4:1:1, or 4:2:0 outputs, grin), but unless you’re planning to shoot saturated monochromatic images, such comparisons aren’t as useful as you might think. I find such pure-color subjects to be fairly rare, prizewinning flowerbeds, stop signs, and blue-glass bottles aside; the vast majority of things we shoot have surprising amounts of at least two primary colors in them. Outside of CGI, high saturation and color purity are unusual, and a good debayering algorithm (and Graeme writes good algorithms) does quite well with even trace amounts of multiple primaries.
Posted by Adam Wilt on 05/09 at 04:04 PM
Thanks Adam!
Agreed that for that any month-long “black cat against chroma key” marathon shoot I have planned in the future, I’d be prudent to go with greenscreen, not blue… but for most normal things we’re good assuming around 3.2K.
Bruce
Posted by Bruce Allen on 05/09 at 05:39 PM
Adam, what distance did you have from the chart to the camera sensor (focus hook) when you shot the chart above, the shot with 50mm Utra Prime T1.9?
Also what is the horizontal dimension of the chart for 4K area (corner to corner marks)
Posted by on 05/15 at 04:53 AM
Adam,
Lots of good stuff to know, thanks for all your good work.
I second the concerns of Mr. Meisel.
We considered, are still considering the camera.
So what’s the solution as far as focus is concerned;
do it several times, then wait for the files to load,
and hope you’ve got a good take?
Posted by on 05/15 at 03:07 PM
“...what distance did you have from the chart to the camera sensor...?”
Whatever distance it took to fill the image vertically (as the chart is calibrated in lines per picture height, not width). Sorry, I didn’t record the distance; it was on the order of 6 feet (a bit under 2 meters) as I recall, but it might have been a bit more.
“...what is the horizontal dimension of the chart for 4K area (corner to corner marks)”
The chart is 16x9, and I went with height, not width.
Width: 23 1/8 inches / 58.7 cm
Height:13 inches / 33 cm
“So what’s the solution as far as focus is concerned;
do it several times, then wait for the files to load,
and hope you’ve got a good take?”
Well, that’s the only way to be *sure*, but it isn’t usually practical. Practically?
1) Set back-focus very precisely; check it often; check your lens scales. Then focus by tape measure.
3) Rack focus through the point where fine detail in the subject shows the most aliasing in the viewfinder / LCD, and hope for the best.
3) Hope for better image-mag in a future firmware build.
Posted by Adam Wilt on 05/16 at 12:00 AM
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