Can someone explain "knee" & "slope"

[Paul Kellett]

Can someone explain to me in a not to complicated way what the "knee" and "slope" and "point" settings in the picture profile settings do, ie what effect they have.

Thanks.
Paul.

[Piotr Wozniacki]

Paul,

I'm sure that when you take a look at a typical (best Cine) gamma curve, paying special attention to it's upper-right portion - you will figure it out by yourself :)

[Paul Kellett]

Thanks Piotr.
What is the "best cine" ?
And the upper right hand portion of what ?

Paul.

[Leonard Levy]

I don't have time now but don't feel like your an idiot. Its not totally obvious especially point and slope.

[Paul Kellett]

I'm just a knowledge addict !!

Paul.

[Piotr Wozniacki]

Originally Posted by Paul Kellett

Thanks Piotr.
What is the "best cine" ?
And the upper right hand portion of what ?

Paul.

Paul, I was typing in haste (the Euro Cup match Poland vs Austria is about to start now :)

I should have written "Best if it's a cine gamma, cause it shows the knee point more obviously).

I'm sure you can do it:)

[Dominik Seibold]

"Point" controls the threshold where compression starts. "Slope" controls the amount of compression, so the flatness of the curve after the threshold-point.

[Leonard Levy]

To add to Dominick - The knee ( with its point and slope adjustments) is a circuit that compresses exposure areas near the 100% IRE range so you don't blow out highlights so fast. It looks like a rounded off knee on a graph of exposure - The term comes from film.

It has both positive and negative affects on the image and must be used judiciously but is a part of most video response curves.

[Greg Boston]

Using the knee curve is akin to audio compression at the top end whereby you wish to shove 10 LBS of signal into a 5 LB bag.

But in order to get the excess to fit, you don't squeeze it at all points equally. You leave the bottom part intact and squeeze harder at the top.

-gb-

[Serena Steuart]

Gamma is the slope of the graph relating subject brightness (input) to output signal. A common video problem is that highlights (typically clouds) are too bright compared to mid tones to be recorded within the range of the sensor. Expose to hold the clouds and the foreground subjects are under exposed -- expose for the main subject and clouds are burnt out (appear as white blobs). Professional cameras provide facilities for modifying the gamma curve to increase the recordable range of subject brightness. You can change the slope of the gamma curve and the shape of the curve. The simplest adjustment for coping with highlights is to reduce the slope of the top of the curve, which you do by selecting where this begins (say 80%) -- the "knee point" --- and the slope of the curve above that knee point. Generally the curve above the knee point is linear. There are equivalent operations possible for low light. These adjustments approximate (very roughly) the S-shape of a film gamma curve. You will find brief illustrated explanations at http://www.tapestockonline.com/sovicase.html
In the EX1 the standard gammas (STD1,2,3,4) allow you to set a knee point. The cinegammas do not have a knee point, the curve being continuous. CINE4 provides the greatest dynamic range, but all the CINE gammas roll off the slope (compared to STD gammas) for better image latitude.

EDIT: The EX1 allows adjustments to gamma (within each set of curves): confining my comments to CINE4, plus gamma slightly (very slightly) decreases the slope of the curve in highlights with increase in slope for low lights, whereas negative increments have the opposite effect (with more noticeable change). You didn't ask about black gamma, but just in case: black gamma affects the lower portion of the gamma curve (that below 50% image intensity for CINE4). Positive increments in black gamma bulge the curve out (more "sensitive" to subtle black gradients) and negative values draw it in (less sensitive).

Cameras such as the Sony F23 permit the operator a lot of flexibility in setting up the image processing algorithms, such as loading in custom gamma curves, whereas the EX1 only allows us to make minor adjustments to the in-built curves. One of the sites selling custom gamma curves (not for the EX1) has some nice illustrations of what can be achieved. http://www.digitalpraxis.net/sonycurves.htm
Scroll down the page to the curves and photos. Note that the important issue is recording the data for post processing, not capturing images for direct display.

[Tom Roper]

There is a very easy way to practice using the manual knee adjustments. If you're going to shoot in the STD gammas, you need to learn this because the Auto-knee sucks.

Raise your hand above your head. The height of your shoulder above the floor is the Knee Sat Level. The height of your elbow above the floor is the Point. The amount of bend at your elbow is the Slope.

1.) Practice shooting high contrast scenes into the sun. Put a tree or some foliage between the camera and the sun. You will be attempting to retain shadow detail and color of the foliage in strong backlight, and at the same time have a bright sun or sky with cloud details but no zebra stripes.

2.) Turn on Zebra 2 (100%).

3.) Expose for the mid tones. The backlit areas will be badly blown, zebra stripes everywhere.

4.) Start with the default manual settings, Point 90, Slope 0, Knee Sat Level 50.

5.) Bring down the Point, 90, 80, 70.... See how easy it was to make the zebras go away?

6.) Now raise the point back up to 90, zebras galore again. Raise the Knee Sat Level from 50 to 75 and increase the slope (+) until the zebras go away. See how easy that was?

What was different between 5 and 6 above? In 5 you just lowered the clipping point. In 6, you changed the rate (delta y/delta x) at which the clipping point is approached. Either can be a correct course to take depending on the situation. You are almost always safe just doing 5. Doing step 6 can additionally make things possible like retaining cloud details, contrast and texture. Or you can take flat lighting and go the other way (negative) on the Slope and brighten up a flat sky to add punch to a dull scene, without affecting the mid-tones.

Just remember the default manual settings are very good for typical situations, so commit the numbers to memory, 90, 0, 50 so you can always return to them. And even just switching off the Auto-Knee function (but keeping the Knee circuit enabled) with the default manual settings will cure or greatly reduce a lot of overblown highlight situations that occur in Auto-Knee mode. And when you do make adjustments to Point, Knee Sat level and slope, you can go pretty bold without too much worry. The Zebra stripes are your friend.

This may come as a surprise, you can shoot straight into the sun, and create beautiful effects, while retaining shadow detail, color and sharpness, even in STD-3!!! You just have to be willing to try new things!

And don't limit your line of thinking to just sun and sky. Using a combination of Knee Sat Level, Slope and Point, you can for example, tone down a blown out sky, put back detail in the clouds, and add or take away highlight details like the glinting reflections on water, or the reflective highlights of wind blowing through tree leaves and foliage. Totally cool. Try it and have fun with it.

[Piotr Wozniacki]

Serena & Tom,

We all thank you for these excellent explanations - I couldn't have done it better.

And Paul, once again sorry I wasn't very helpful last night - was very preoccupied and excited about the "last chance" match of the Polish team at Euro-Cup in Vienna. Unfortunately we didn't win, after all - in spite of being so damn close!

[Paul Kellett]

Thanks everyone for all your answers.
I think i get it now.

My dilema is i usually shoot with Bills PP, however, recently i've been working with another operator who uses a Z1 so i'm using the EX1 standard PP, for matching footage purposes.
I've been shooting weddings with this guy and find it hard to control the highlighs using std PP. A shoot i did recently was an outside ceremony on a very sunny day, so bright white dress, a few white clouds in the sky etc, you get the picture.
So i just wanted to find a safe way to remove the 100% zebras. So i'll play around with Tom's suggestion, Numbers 5 & 6.

Thanks very much guys.

Paul.

[Dominik Seibold]

One information to add is that compression-algorithms lower the maximal output. To compensate this audio-compressors have a parameter often called "makeup-gain". The ex1 also has such a gain, but you can't adjust it - it's fixed. Turning knee off doesn't disable that gain because it's "hard-coded" into the std-gammas, so turning knee off leads to earlier clipping and not using the whole dynamic range of the sensors respectively.
So if you don't need the knee-compression but want the whole dynamic range of the sensors for getting lowest noise I recommend to use cine1, because it's most similar to no-knee-std-gammas, but of course uses the whole dynamic range because cine-gammas don't have/need makup-gain because they don't have that knee-feature.

[Paul Kellett]

Ok this is what i did.
My willing participant (the missus) stands with her back to the window, i expose correclty for her with 65% zebras on the skin, window behind has 100% all over it, so i lower the knee point till the 100% zebras disappear, which is about 76 (originally 90).
Did i do this correctly ?

When i flick between std and this new PP i can definately see the difference when the zebras are off/out of the way.
With the std PP,from my same original position, looking out of the window is almost white,nothing there, however with the modified std PP i can see detail such as house roofs,hills in the distance etc.

Thanks guys.

Paul.