Double image when shooting flames at HDR with an Epic

[David (dudi) Namir]

Hi All
Just wonder if anyone having such a problem of Double image when shooting flames at HDR with an Epic

 

[Jason Diamond]

Hi All
Just wonder if anyone having such a problem of Double image when shooting flames at HDR with an Epic

It's a matter of time differential between the A and X track. there's not really a way around it with organic random shapes like flame. the only way to mitigate it to the least possible is use the least number of HDR-X stops you can and that will shorten the exposure distance between the 2 tracks.

 

[David (dudi) Namir]

froze on flames

froze on flames

Thanks Jason
We shot the flames without HDR and eventually camera froze.
We couldn't really "wake it up" the menu was slow to appear on the monitor in fact it didn't appear in one piece but was slowly wiping on the monitor like when you wipe an image with a switcher and each time we tried to record it froze again. We change to Phantom which also gave us some strong "interlaced look" image on some shots.
The whole thing is still under investigation

 

[Terry VerHaar]

It's a matter of time differential between the A and X track. there's not really a way around it with organic random shapes like flame. the only way to mitigate it to the least possible is use the least number of HDR-X stops you can and that will shorten the exposure distance between the 2 tracks.

But there is no temporal difference between exposures; the "short" exposure (X) is recorded during the longer A exposure. What is probably happening is the flame motion is exceeding the speed of the overall capture (as Jason points out); much the same as if you were shooting stills of flames with a long exposure - things would get blurry. You might try setting the shutter speed to a very short duration as well as limiting the number of HDRX stops.

BTW - would like to see a capture. It might be easier to see what's going on.

 

[Les Dittert]

Terry, where did you hear that the short exposure is recorded during the long ?

I find HDRX ruining high contrast edges even with a simple pan, let alone fire like complexity.
Using motion compensation does fix simple motions, but for fire, where each frame is often unique, I don't think it would work well.

Can someone cite where HDRX was used in a feature film 'as is' without special motion processing for motion ? Static shots don't count, I'm talking even simple motion.

 

[Craig Parkes]

Terry, where did you hear that the short exposure is recorded during the long ?

I find HDRX ruining high contrast edges even with a simple pan, let alone fire like complexity.
Using motion compensation does fix simple motions, but for fire, where each frame is often unique, I don't think it would work well.

Can someone cite where HDRX was used in a feature film 'as is' without special motion processing for motion ? Static shots don't count, I'm talking even simple motion.

Les, it's obvious just from looking at the two tracks of HDRX shots that each X track frame exposure is a simultaneously started, but shorter exposure, version of each A track exposure. This exhibits the very issues you describe on shots with high motion blur, so it's not really pertinent to the discussion other than understanding how HDRX an be compensated for.

 

[Terry VerHaar]

Terry, where did you hear that the short exposure is recorded during the long ?

I find HDRX ruining high contrast edges even with a simple pan, let alone fire like complexity.
Using motion compensation does fix simple motions, but for fire, where each frame is often unique, I don't think it would work well.

Can someone cite where HDRX was used in a feature film 'as is' without special motion processing for motion ? Static shots don't count, I'm talking even simple motion.

I can't cite one particular post but the RED folks have said it over and over that there is no time lag between the two exposures. The X frame is simply an interim "save" of the frame while the A frame is still being exposed. Logically speaking, to make them separate exposures would introduce all sorts of issues, both technical and visual.

 

[David (dudi) Namir]

We didn't shoot the flames with HDR. My first post on August 7th was before shooting as we new Epic and especially with HDR has a problem shooting flames. We shoot the flames at regular mode and we got a frozen camera. This happened when we shot tight shot of flames and the flames had a strong presence in the frame.
We switched to Phantom and we also had a few problems with some shots because of the fire. It looks to me that digital processors still can't take the fast changes of colors, contrast, position and motion that are happening in the frame. This is like with the first build of Red One, camera froze when shooting trees.

 

[Les Dittert]

I did a little test to see what is happening. 24FPS, HDRX 2 stops, 120 degree shutter.

I used a red laser pointer for my motion , making quick circles on the computer screen.
You can see almost no overlap between X and A frames. The exposure do not happen at same time.
Fire or other complex motion picture would have all kinds of artifacts from this, as the flame would be in a new position for the HDRX exposures.

Top picture is X frame.
Middle picture is A frame.
Bottom picture is cross dissolved A and X to show any overlap.
( frames are color corrected to make easier to view here )

 

[Errol Afed]

Hi David. What speed were you shooting the flames with the Phantom? I do DIT work with the Phantom Flex and have shot flames at 1000fps and have never had any problems with the footage.

 

[David (dudi) Namir]

Hi David. What speed were you shooting the flames with the Phantom? I do DIT work with the Phantom Flex and have shot flames at 1000fps and have never had any problems with the footage.

Usually we shoot at 1000 FPS but since we changed from Epic to Phantom as the Epic froze and we didn't need 1000 fps, we shot it at 120 fps and we had a few takes with sort of interlaced affect.