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Disable Auto Knee


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#1 Alvin Pingol

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Posted 01 July 2004 - 03:34 PM

When auto knee is enabled on my camera, I'll point it at an off-white wall (with zebras set to 95+ IRE) and see stripes. The auto knee quickly kicks in and the zebras disappear, so I know it's working fine (yes, the camera is set to full manual exposure).

However, on extremely bright areas that are several stops overexposed (white), the auto knee does nothing but give them a purple hue. I know this is a normal occurence, but I don't like the way it looks.

If I disable auto-knee, whites are white. My question is, how much highlight detail will I be losing in areas that are around 95IRE? Like the wall I mentioned earlier, does auto knee prevent it from going completely white?

In other words, am I gaining a lot of extra highlight detail/color information by having it on? Or is the difference trivial?

Thank you for your time.
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#2 Michael Nash

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Posted 01 July 2004 - 04:42 PM

It depends on the camera, but in general, yes you are gaining a LOT of extra detail with the knee turned on. More accurately, you're losing a lot without it.

How much this affects your image depends on what you're shooting and how you want it to look. The biggest advantage of the knee isn't so much how much highlight detail you're recovering, but how well it smooths out or rounds off the transitions into white. Without the knee you'll see abrupt burned out patches in overexposed areas, whereas with the knee those areas will still be hot but with a softer edge as the luminance tapers off. So don't think of the knee function as a way to capture more highlight detail, but instead as a way to take the "curse" off of clipped highlights.

If the knee doesn't look good to you, you can instead shoot with it off and try a mild diffusion filter to "blur" the transitons into clipped highlights. Whites will still clip, but at least the clipped areas will blend into adjacent areas a little more smoothly (actually obscuring even more detail in the near-highlights).

I remember you asking about the purple hue to the knee before, but I don't recall what the conclusion was. I've honestly never seen that myself. It sounds like uneven RGB levels within the knee processing. There's also a "knee saturation" control on some cameras, although I don't think you can adjust the RGB levels within the knee via menus.
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#3 Laurent Andrieux

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Posted 01 July 2004 - 07:15 PM

it would be usefull to know what camera you're using. On recent digibeta, you can adjust RGB in the highlights.

In most digital camera, you can set the knee point and the slope that occurs after that point.

would it be usefull me to tell you more about both knee point and slope ?

I wuold say, regarding your main question, that if you have a good monitor with you, with video, "what you see is what you get"

considering this chroma problem, it is known that to strong signal affects chroma, generally in a blueish/ magenta dominante. If cou can't reduce that, make the foreground lighter to reduce contrast, or try to save the shot in post

i like michael'suggestion. Did you do that michael ? I would think diffusion has a stronger effect in low lights, so I would think you'd light the shades as well doing that and losing your blacks... though you can set them a little lower for this purpose. Did you lower the black level using the diff filter ?
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#4 Alessandro Machi

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Posted 02 July 2004 - 11:57 PM

I recommend studying your results on a waveform. I have never been happy with a Hitachi-Z-ONE-C that I purchased used a few years ago. The idiots at Hitachi designed the camera to have an auto knee THAT INCREASED CONTRAST as you opened Iris.

The dimwits actually thought by driving down set-up as you iris'd open they were doing you a favor. I had to fight the Hitachi Service technician to ignore what he had been told by Hitachi, I mean really fight him on this. I mean I really really had to fight him. I think he was concerned because I was the first one to ever tell him he was wrong for supporting Hitachi idiots at headquarters on this issue. He actually suggested that I throw more light on my backgrounds to make up for the camera's contrast shortcomings.

I'm doing ENG, and to appease the idiots at Hitachi, I should light my handheld ENG shot with a HMI 10K???

My JVC KY-19 and KY 27B run rings around the Hitachi from a contrast point of view. What bugs me the most is the technician who repairs the cameras is absolutely brilliant, but he's not a camera guy so he doesn't understand that just following orders is not in the best interest of the camera guy who has invested in a Hitachi Z-One C and just wants to get make back his investment and then some.

I still lost my investment in the camera because I can't use the camera in low light situations because dark hair completely disappears and turns completely matted. I get angry just posting about it. The technician also insists on making camera adjustments with light flooding a chip chart at f 8.0. What a crock!

I don't need the camera to perform flawlessly at F 8.0, I need it to subtly differentiate black tones at f2.0. Grrrrrrr. To this day I don't understand how the Hitachi Z-One could run rings around a Z-One C which came out a few years later and should have been a newer and better camera.
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#5 Laurent Andrieux

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Posted 03 July 2004 - 01:45 AM

what is exactly this hitachi camera ? broadcast ?

Funny thing, I noticed on one of dvcpro 25 AJD 610 panasonic in our school that when I disable autoknee, it actually reduce highlights, just the contrary of what you would expect ! I guess its the setting of the auto correction wich is less efficient than the manual knee default settings ! (didn't go in to check the values yet).

I mean I don't have to consider it is knee correction on/off but auto/manual knee correction. I guess that if I wouldn't any knee correction at all, I'd have to set the value (may be at 0 somehow) of the manual knee.

May be it's the same on your Hitachi. Can you get in to set values ?
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#6 Michael Nash

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Posted 03 July 2004 - 03:29 AM

Many broadcast and HD cameras have a manually programmable knee in addition to the auto knee. Usually it's accessed by the middle position of the bars/camera/DCC toggle switch on the side of the camera. I'm pretty sure it's this way on the Panasonic 610, since I beleive it's that way on the SDX-900 and the Varicam.

The manual knee may have given better results than the auto knee because the knee point and slope were programmed lower than what the auto knee delivered. But this doesn't mean that a manually programmed knee is always better than the auto knee. The tricky thing about knee functions is that its performance really depends on the highlights in the shot (just like exposure). That's why these cameras have an "auto" knee to track the highlights for you, faster and more adaptably than you could program it with menus or even dial it in through a paintbox.

There are certainly times when a manually programmed knee can produce better results for a specific shot than the auto knee will give you. But in general the only time I take the camera out of auto knee is when it's not giving me what I want, and I have the time and control to dial in a better knee response for that particular shot.
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#7 Michael Nash

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Posted 03 July 2004 - 03:37 AM

i like michael'suggestion. Did you do that michael ? I would think diffusion has a stronger effect in low lights, so I would think you'd light the shades as well doing that and losing your blacks... though you can set them a little lower for this purpose. Did you lower the black level using the diff filter ?

I was talking about using very slight or mild diffusion to add a small amount of halation to the highlights, without affecting the blacks too much. Something like a 1/8 or 1/4 White Pro Mist. The idea is to give just a faint glow to the extreme clipped whites so they don't look so harsh, but not add so much diffusion that it's visible in other parts of the frame.

I sometimes lower the black levels with some types of nets or diffusion filters to counteract any fogging of the shadows from the filter. But it depends on the overall look I'm going for.
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#8 Laurent Andrieux

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Posted 03 July 2004 - 04:41 AM

I was just noticing that there is somehow a perverse side about auto stuff, as if you don't suss things out, say you use auto because you think it's gonna compress the highlights, in most situations, as, i guess, an auto user would think, using the factory settings, with these autoknee systems, you'll get the opposite to what you might be waiting for !

The conclusion to this would be : have a good look in different situations while trying the camera for making your mind up, and don't be afraid to go and have a look in the menus to see the factory settings, without being afraid to change them !

Ther's a funny thing like that with flare correction. I noticed that when flare correction is on, and if you have a light that usually would cause flare, protecting the lens with a flag gives a less contrast image than letting the auto correction work ! it's like if detecting a noise in the blacks makes the system lower the black levels, if the blacks are not affected by flare, they'll be upper than if they were!

As you can't turn off flare correction on some cameras, it becomes impossible to volonteerly cause a soft flare in your image!
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#9 Jo Laurits Aksdal

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Posted 04 July 2004 - 10:06 AM

What's the difference between low, mid and high knee setting in the dvx100a? I guess the knee function kicks in earlier in low than in high. And auto is just eh, auto?
Would you gain more detail with the low setting?
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#10 Alessandro Machi

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Posted 04 July 2004 - 11:00 AM

what is exactly this hitachi camera ? broadcast ?

Can you get in to set values ?

Auto Knee is either on or off. With the auto knee off, the camera is completely useless. With it on, it's useful in daylight situations only, (and only after the technicial made the necessary adjustments per my request).

The camera is a 750 line broadcast quality dockable camera made in the early to mid 90's. I once heard that the camera that replaced it had "better contrast". This could have just been a scam to reduce functionality on the Z-ONE-C so that the new camera would look like it had cutting edge improvement to it, because the Z-ONE, which came out in the late 80's, ran rings around the Z-ONE-C

Anyone else have a Z-ONE-C so I can compare notes?
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#11 Alvin Pingol

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Posted 05 July 2004 - 05:50 PM

Thank you all for the responses.

My camera is a JVC DV300. It is not a professional camera ("prosumer" rather), but I really like the images it puts out. Details. I've been told it's like the DVX100, sans the 24P, as both cameras use the same CCD block and have similar image processing circuitry.

My JVC does not have any Knee controls, it's either on or off.

I was playing with it today and was surprised at how fast things clip with it off. However, with it on, I noticed that knee sort of makes all of the highlights the same color, which can look very weird on skintones.

Of course, when off, contrast goes way up, so there's certainly a tradeoff. I guess it does really depend on what I'm shooting. Knee is something that, for me, I would suspect should either be always on or always off, but now that I see how much of a difference it makes and that it can either ruin or enhance a shot, I will be enabling/disabling it depending on what my subjects are.

On a somewhat related note:
I shot an event last weekend that took place outdoors, and I wanted to keep a shallower DOF and make sure my lens reflections weren't in focus (I had a Pola stacked atop a UV over the lens).

I also had the internal ND filter on. This way I was able to shoot at f/2, 1/60th, in direct California sunlight, which was nice. Knee was on.

When I watched the footage on the computer, color saturation sucked rocks, and I was disappointed. It wasn't so bad that the footage was unuseable, but it was a lot worse than I expected. Of course, I couldn't tell saturation would be bad while shooting, as those LCD screens can't reproduce color too well and therefore everything is highly saturated. For a camera of this caliber, the saturation when shooting in such a brightly lit envornment should be top-notch.

I know that ND, Pola, and UV filters should not affect saturation to a noticeable degree, but do you think my combination (internal ND + glass UV filter + glass Pola) is what affected my satruation so much? I could post a still if anyone wants.
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#12 Alessandro Machi

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Posted 05 July 2004 - 07:44 PM

Shooting into backlight will probably cause desaturation. Also you probably should consider going preset otherwise the camera's auto white balance settings are probably negating your filters by trying to maintain white. Whenever you do special filtering I strongly recommend keeping the camera running and on a tripod and actually afix the filters so that later you can see what changed as soon as you put the filter on.
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#13 Alvin Pingol

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Posted 06 July 2004 - 02:24 AM

>>>Shooting into backlight will probably cause desaturation. Also you probably
>>>should consider going preset otherwise the camera's auto white balance
>>>settings are probably negating your filters by trying to maintain white.

I manually white balanced (point camera at something "white" and hold down a button) before I shot anything else.


>>>Whenever you do special filtering I strongly recommend keeping the camera
>>>running and on a tripod and actually afix the filters so that later you can see
>>>what changed as soon as you put the filter on.

Good idea. ^_^
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#14 Michael Nash

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Posted 06 July 2004 - 12:39 PM

I shot an event last weekend that took place outdoors, and I wanted to keep a shallower DOF and make sure my lens reflections weren't in focus (I had a Pola stacked atop a UV over the lens).

When I watched the footage on the computer, color saturation sucked rocks, and I was disappointed.

Were you using a lens shade or matte box? With filters in front of the lens it's very easy for direct sunlight to hit the glass and wash out the image. With video that's hard to see in the VF or the LCD (especially outdoors).

Polarizers can sometimes affect color in odd ways with video depending on the light and white balancing.
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#15 Alvin Pingol

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Posted 06 July 2004 - 01:00 PM

I was using a lens shade, but there were many times when the sun would hit the camera so as to poke through right under the hood and give me troubles.

Sun washing out the image was not the cause of the desaturation, for the saturation level remained constant throughout the day (even when I faced opposite the sun).

I know polas can do some weird things with color, giving things a green bias, or making the image warm, depending on how you rotate the filter. Maybe it was just a combination of bad pola rotation and incorrect white balancing that gave me my saturation problem.
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#16 Laurent Andrieux

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Posted 08 July 2004 - 12:28 PM

but do you think my combination (internal ND + glass UV filter + glass Pola) is what affected my satruation so much?



I'm not sure we can say no.

I would say that balancing the light (white balance) with all filters on would correct any color dominante but for what's about saturation, I don't know if it would correct anything.

about the white compress always on or always off : if you always shoot outside with high level, you can keep it on, but when you work low light inside, the auto knee generates a little noise...
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#17 Alessandro Machi

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Posted 08 July 2004 - 01:09 PM

I was using a lens shade, but there were many times when the sun would hit the camera so as to poke through right under the hood and give me troubles.

Sun washing out the image was not the cause of the desaturation, for the saturation level remained constant throughout the day (even when I faced opposite the sun).

I know polas can do some weird things with color, giving things a green bias, or making the image warm, depending on how you rotate the filter. Maybe it was just a combination of bad pola rotation and incorrect white balancing that gave me my saturation problem.

One technique used by some DP's is that they wll use those Roscoe/Lee swatch colored filters and put a paler one in front of the lens as they white balance. The idea is you can manipulate your the cameras color balance by using the opposite color from what you want to achieve.

A pale blue filter should warm up the image. However, the idea is once the white balance is done you put the filter away. By keeping filters on the camera all day long you have to make sure the camera was not auto white balancing and in essence overriding the filters you left on the camera.

Also, you can always readjust all of your video levels in post. I've been able to take away light fog from a shot by readjusting the set-up level and add color if I thought the shot was too pale.
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