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  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

HDR+0

Robert Hofmeyr

Robert Hofmeyr

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This may well have come up before but I'm not sure what to search for. I was shooting in full sunlight without NDs last week and it got me thinking: would it be possible to add an HDRx+0 setting to give us the digital equivalent of a 0.3 ND filter? For example, if I want to shoot 25fps with an 180deg shutter, I set the shutter to 90deg and turn on HDRx+0. The combined HDR frame will have the same exposure as the non-HDR frame but half the shutter speed. This would also be great for pulling sharper stills at the same exposure as the motion.

Thinking further, how about HDRx+0X4 (may have to think of a better name): the desired shutter speed divided into four consecutive frames, merged in post. That would darken the exposure by 2 stops. If the merging could be done realtime in camera, we could just dial in "Digital ND: -2" (that's a better name). One of the 4 frames could be saved for use as a still or for motion tracking etc.

Does this make sense, or is there something I don't understand?
 
Last edited:
Hi Robert, here is how the HDRx stops effect shutter speed. I think +1 does what you are asking for:

HDRx - degrees - fraction

off = 180•, 1/48
+1 = 90•, 1/96p
+2 = 45•, 1/192
+3 = 22.5•, 1/384
+4 = 11.25•, 1/768
+5 = 5.625•, 1/1,536
+6 = 2.8125•, 1/3,072
 
Thanks Justin. Using 25p for simplicity: If the shutter is set to 1/100th and you turn on HDRx +1, you get 2 consecutive images per frame: one at 1/100th (A-track) and one at 1/200th. What I was asking about is 2 consecutive images with the same exposure. i.e. both images are 1/100th and the combined frame is 1/50th (but one stop darker than if it was shot normally). Am I wrong about how this works?
 
HDRx gets its darker exposure from the shorter shutter speed. Took me a while for it to sink in as to why 1/24th lets in more light than 1/30th. Not being a film native camera tech, the whole shutter angle thing confused me for a while. But I'm much better now :)

RED220 on fxphd explained this very well for me, they shot a dolphin jumping just for this purpose. The A-track had a great exposure at 320iso,@1/50th shitter- nice motion blur, looked great in motion. The X track they set to +2 which effectively gave them 1/200th shutter. He pulled a still and opened in lightroom and boosted the exposure to acceptable,and did a bunch of cool color corrections on it. No blur on the dolphin, made a great still. Remember that the iso is metadata, so setting the iso lower doesn't change anything for post. And there was so much latitude in the shot, there was no problem pulling it up.
 
Robert Hofmeyr said:
Thanks Justin. Using 25p for simplicity: If the shutter is set to 1/100th and you turn on HDRx +1, you get 2 consecutive images per frame: one at 1/100th (A-track) and one at 1/200th. What I was asking about is 2 consecutive images with the same exposure. i.e. both images are 1/100th and the combined frame is 1/50th (but one stop darker than if it was shot normally). Am I wrong about how this works?
Click to expand...

It is important to understand: HDRx does NOT take "consecutive" exposures. The shorter exposure is recorded DURING the longer exposure - think of it as one being a "subset" of the other. An HDRx setting of 0 (if such were possible) would give you two images that are exactly the same - the exposures would be identical.
 
Robert, Matt is right. The X track is darker BECAUSE of the higher shutter speed. If both tracks were 1/50 they would be the same brightness. As far as I understand it Terry is spot on with his description.

Using HDRx +1 the photosites start collecting light and write off the X track after 1/96th of a second. They continue collecting light and write off the A track after The full 1/48th of a second.

With HDRx +2 the X track is written off after 1/192nd of a second, etc.
 
Is there definitive info from RED about whether the X exposure is actually a subset of the A track or happens immediately after the read/reset interval (8ms last I heard)?

Based on the HDRx testing I did with Leandro Marini at Local Hero for Assimilate, it looked like you could improve the blend by making slight temporal adjustments to the X track. Perhaps it was an optical illusion - like a number of things in the visual arts, perception is often more important than numbers. What I can say, based on the opinion of a half dozen viewers, is that we were able to make the mismatch of the blur characteristic less noticeable by slipping the tracks.

Cheers - #19
 
My understanding, unconfirmed, is that if the A track is based on a 1/48th of a second shutter then starting from the beginning of a second:

shutter opens for 1/48th of a second, it closes and completes the read/reset period of 6-10 milliseconds, re-opens for 1/384th of a second (based on 3 stops of HDRx), closes for slightly less than 1/48th of a second, then opens again for frame #2 of the A track, etc. This results in a temporal lag of less then 10ms which is perceptually invisible in most cases.

To give HDRx the toughest situation we could cook up, we had high contrast vertical elements moving horizontally across the frame to see if the blur characteristic mismatch would read. We added some motion blur to the X track, Leo created some luma keys and pre-graded the tracks prior to grading the combined image, etc and then we experimented with slipping the X track and in our torture test it made a noticeable difference. It should be noted we did the testing on one of the earliest firmware builds so things may have changed since then.

BTW Justin, the way I understand the operation of a CMOS sensor there wouldn't be a way to pull an exposure sub-set from the same read-reset cycle but perhaps the magic pixies found a way ;-)

Cheers - #19
 
Blair - I believe if was JJ himself that confirmed the two exposures happen simultaneously.

If that weren't the case, you could never shoot HDRx with a 360 degree shutter because the two exposures would interfere with each other.
 
From the Scarlet Ops Guide -

"When enabled HDRx simultaneously shoots two images of identical resolution and frame rate- a normally exposed primary track (A-track), and an underexposed secondary track (X-track) whoseexposure value reflects the additional stops of highlight protection desired. i.e. if you select an HDR value of +2 and your primary track exposure is 1/48th sec, the secondary trackexposure will be 2 stops under, or 1/192 sec. The ISO and Aperture remain the same for both exposures.
During acquisition and recording, the two tracks are "motion-conjoined", meaning there is no gap in timebetween the two exposures. This is very important to note because if they were traditional alternatingexposures, there would be a time gap between the two tracks that would show up as an undesirablemotion artifact. Both tracks (A & X) are stored in a single .R3D® file. "
 
I believe the only way to accomplish what you want is to shoot 100fps with a 360 degree shutter and then drop frames. You would keep two and then drop two. Keep two and then drop two etc.
 
Thanks Terry. I always understood "no gap in time between the two exposures" to mean they were consecutive not concurrent. If HDRx can be enabled with a 360 degree shutter angle it would indeed prove they are not consecutive. Can someone with an Epic or Scarlet handy confirm this?

Gavin: yes, that is kind of what I was thinking. Bit of a hassle for 1 stop though. The ideal would be if the camera could shoot 4 consecutive frames at 200fps, pause for 1/50th sec, then shoot another 4 frames at 200fps etc. then save one of the 4 frames for stills/motion tracking and merge all 4 frames as the motion track. Then you would have a normal 1/50th sec motion track and a 1/200th sec stills track, with the same exposure (you'd lose 2 stops of sensitivity, but this would be useful in many scenarios - especially when more sensitive sensors are released).
 
Blair S. Paulsen said:
it looked like you could improve the blend by making slight temporal adjustments to the X track
Click to expand...

Interesting observation Blair....

Just a thought, but who says the simultaneous exposures START at the same time? Perhaps they end at the same time...so the X track is using an exposure differential that puts its measurements in the higher total cumulative values instead of starting in the noise floor and taking a quicker 'snap' that is close to the noise threshold of the sensor?
 
Terry, I have found this to be untrue.
I took a laser pointer and shot it doing rapid circles in HDRX mode.
If you look at the A and X track of a particular frame, the motion streaked trail that the laser pointer makes on the surface I was photographing is clearly happening at different times, as the streak from one exposure follows the other exposure.
I pointed this out in another thread, which promptly ended with no further comments.

Perhaps some people can't relate the streaked laser pointer trail withg exposure time and just plain don't understand what is shown. I have no idea.
Here are the images again. ( A track, X track, and a thirdr frame that disolves the two to better see there is no overlap and therefore not simultaneous exposure.

A track:
a_frame.0000102.jpg



X track:
x_frame.0000102.jpg



Dissolved to see NO OVERLAP OF EXPOSURE TIME :

AX_frame.0000102.jpg



Terry VerHaar said:
It is important to understand: HDRx does NOT take "consecutive" exposures. The shorter exposure is recorded DURING the longer exposure - think of it as one being a "subset" of the other. An HDRx setting of 0 (if such were possible) would give you two images that are exactly the same - the exposures would be identical.
Click to expand...
 
Robert Hofmeyr said:
Brilliant Les. So what happens if you enable HDRx when shooting with a 360 degree shutter?
Click to expand...

I haven't tested that, but I'd guess that it would reduce the 360 enough to 'fit' the other shorter exposure time.

To help people understand my above laser pointer images, think of the red streak as a car traveling very fast , and the length of the red streak as the actual time the exposure was.
Motion blur is directly related to exposure time. The long streak is a long exposure. The short streak is the short exposure. See how they don't occupy the same time ?
Sheesh , I don't want to sound like I'm explaining this to a child.
 
HDRx and 360 shutter can't co-exist, the camera auto-limits your a track shutter angle to 281.5 degrees.

(I get some monitoring anomolies (split screen image) requesting 1 stop of HDRx and 360 shutter)
 
Hey Blair, this is just a guess on my part, as I don't own a RED, but I would guess what you're seeing is entirely correct.

If the X-track comprises the first 50% or 25% or 12.5%, etc of the A-tracks exposure time (depending on how many stops are needed), then the sharp action captured in the X-track frame will temporally represent the earliest part of the A-track like we see with the laser pointer example. A temporal offset forward of 25% for 1 stop, 37.5% for 2 stops, 43.75% for 3 stops etc, where the percent is in terms of the frame-rate, might give a better starting point for the frame blend. This is a pretty big 'might' though.

If this improves things, it's likely having the temporal nudge happening at capture time is a better bet. If the X-track was captured during the middle of the X-track exposure, instead of the beginning (which is how the laser pointer example appears), I wonder how that might look.

Edit: I misread Les' example. It looks like they don't overlap in his footage, weird.
 
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