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Why do my pictures go soft when I pan? Camera Detail Correction in depth.

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Why do my pictures go soft when I pan? Camera Detail Correction in depth.

This article is my Christmas present for my readers. When your trying to set up a camera or brew up a picture profile it really helps if you understand the ramifications of each of the settings. I hope this helps explain how detail correction works and how it effects your image.
I am often asked to explain why someones images are going soft when they pan the camera or when there is a lot of movement in the scene. Well this can be down to many things including poor compression or too low a bit rate for the recording, but the two main issues are shutter speed (which is tied in to your frame rate) and detail correction. I’ll cover frame rates and shutter speeds in the near future, but today I’m going to look at Detail Correction.
First of all what is detail correction for? Well originally it was used to compensate for the low resolution of domestic cathode ray tube TV’s and the limited speed at which a CRT TV could go from light to dark. Modern LCD, Plasma and OLED displays handle this much better, but still detail correction remains important to this day to as a way of adding the appearance of additional sharpness to a video image. You’ll often see extreme examples of it on SD TV shows as a dark halo around objects.

Perfect Greyscale

The image above is of an imaginary perfect greyscale chart. Looking at it you can see on your screen that each grey bar is quite distinct from the next and the edge between the two is sharp and clear. You computer screen should be quite capable of showing an instant switch from one grey level to the next.

Grey Scale with Waveform

Now if we add the waveform that the “perfect” greyscale would give we can see that the transition from each bar to the next is represented by a nice crisp instant step down, the transition from one bar to the next happening over a single pixel.

Grey Scale as seen by camera

The image above represents what a typical video camera might reproduce if it shot the greyscale chart without any form of detail correction or sharpening. Due to the need to avoid aliasing, lens performance and other factors it is impossible to get perfect optical performance so there is some inevitable blurring of the edges between the grey bars. Note that these images are for illustration only, so I have exaggerated the effect. I would expect a good HD camera to still produce a reasonably sharp image.

Camera Waveform

Looking at the cameras waveform you can see that the nice square edges we saw in on the perfect greyscale waveform have gone and instead the transition from bar to bar is more rounded. Now there are two things that camera manufactures commonly do to correct or compensate for this. One is called aperture correction which is a high frequency signal boost (I’ll explain that another time) but the one were going to look at in this case is called detail correction often simply referred to as “Detail”.

Detail Correction sampling

So what happens in the camera? Well the camera constantly compares the video luminance (brightness) levels of the image over a set time period. This time period is incredibly short and in the example given here is the time it takes for the cameras line scan to go left to right from point A to point B. If the  difference in the brightness or luminance of the two samples is greater than the threshold set for the application of detail correction (known as crispening on Sony cameras) then the detail circuit kicks in and adds a light or dark enhancement to the brightness change.

Camera image with Detail Correction added

With an HD video camera the light or dark edges added by the detail correction circuit are typically only a few pixels wide. On an SD camera they are often much wider. On a Sony camera the detail frequency setting will make the edges thicker (negative value) or thinner (positive value). The Black and White limit settings will limit how bright or how dark the added correction will be and the detail level control determines just how much correction is added  to the image overall.

One important thing to consider is that as the amount of detail correction that is applied to the image is dependant on differences in the image luminance  measured over time, so you have to consider what happens when the scene is moving or the camera pans.  Two things happen when you pan the camera, one is that the image will blur a little due to things moving through the frame while the shutter is open and from line to line objects will be in a slightly different position.

Blur due to camera pan softens the image.

So looking at the waveform we can see that the waveform slope from one grey bar to the next becomes shallower due to the blur induced through the motion of the camera. If we now sample the this slightly blurred image using the same timescale as before we can see that the difference in amplitude (brightness) between the new blue samples at A and B is significantly smaller than the difference between the original red sample points.

Smaller Luma difference due to pan blur

What this means in practice is that if the difference between the A and B sample drops below the threshold set for the application of detail correction then it is not applied. So what happens is that as you pan (or there is motion in the scene) the slight image softening due to motion blur will decrease the amount of detail correction being applied to the image so the picture appears to noticeably soften, especially if you are using a high detail correction level.

Detail correction is applied to both horizontal image differences as outlined above and also to vertical differences. As the vertical sampling is taken over 2 or 3 image lines there is much longer time gap between the samples. So when you pan, an object that was in one position on one line may have moved significantly enough by the time the frame scan has progressed 2 more lines that it is in a different position so the detail sampling will be wrong and detail may not be applied at all.

If you are finding that you are seeing an annoying amount of image softening when you pan or move your camera then you may want to consider backing off your detail settings as this will reduce the difference between the detail “on” look and detail “off” look during the pan or movement. If this softens your images too much for your liking then you can compensate by using Aperture Correction (if your camera has this) to boost the sharpness of your image. I’ll explain sharpness in more depth in a later article.

Merry Christmas!

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Brewing up a Scene File: Gamma and Knee

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Brewing up a Scene File: Gamma and Knee

Before anyone complains that I have missed stuff out or that some technical detail is not quite right, one of the things I’m trying to do here is simplify the hows and why’s to try and make it easier for the less technical people out there. Lets face it this is an art form, not a science (well actually a bit of both really).
So what is a gamma curve anyway? Well the good old fashioned cathode ray tube television was a very non-linear device. You put 1 unit of power in and get one unit of light out. You put 2 units in and get 1.5 units out, put 3 in and get 2 out… and so on. So in order to get a natural picture the output of the camera also has to be modified to compensate for this. This compensation is the gamma curve, an artificial modification of the output signal from the camera to make it match TV’s and monitors around the world. See Wikipedia for a fuller explaination:   http://en.wikipedia.org/wiki/Gamma_correction
So, all video cameras will have a gamma curve, whether you can adjust it or not is another matter. Certainly most pro level cameras allow you some form of gamma adjustment.
The PMW-350 has 6 standard gamma curves, these are all pretty similar, they have to be otherwise the pictures wouldn’t look right, but small changes in the curve effect the relationship between dark and bright parts of the pictures. Todays modern cameras have a far greater dynamic range (range of dark to bright) than older cameras. This means that the full dynamic range of the sensor no longer fits within the gamma curves used for TV’s and monitors. In broadcast television any signal that goes over 100% gets clipped off and is discarded, so the cameras entire brightness range has to be squeezed into 0 to 100%. The PMW-350 sensors are capable of far more than this (at least 600%) so what can you do?
The older and simpler solution is called the “Knee”. The knee works because in most cases the brightest parts of a scene contains little detail and is generally ignored by our brains. We humans tend to focus on mid-tone faces, animals and plants rather than the bright sky. Because of this you can compress the highlights (bright) parts of the picture quite heavily without it looking hugely un-natural (most of the time at least). What the knee does is takes a standard gamma curve and up near it’s top, bends it over. This has the effect of compressing the brighter parts of the image, squashing a broad range of highlights (clouds for example) into a narrow range of brightness. While this works fairly well, it does tend to look rather “electronic” as the picture is either natural (below the knee) or compressed (above the knee).
The answer to this electronic video look is to replace the hard knee with gentle bend to the gamma curve. This bend starts some way down the gamma curve, very gentle at first but getting harder and harder as you go up the gamma curve. This has the effect of compressing the image gently at first with the compression getting stronger and stronger as you go up the curve. This looks a lot more natural than a hard knee and is far closer to the way film handles highlights. The downside is that because the compression starts earlier a wider tonal range is compressed. This makes the pictures look flat and uninteresting. You have to watch exposure on faces as these can creep into the compressed part of the curve. The plus point is that it’s possible to squeeze large amounts of latitude into the 100% video range. This video can then be worked on in post production by the editor or colorist who can pull out the tonal range that best suits the production.
These compressed gamma curves are given different names on different products. Panasonic call them “Film Rec”, on the EX1 they are “Cinegammas” on the PMW-350 they are “Hypergammas”. The 350 has four Hypergammas. The first is 3250. this takes a brightness range the equivalent to 325% and compresses it down to 100%. HG 4600 takes 460% and squeezes that down to 100%. Both of these Hypergammas are “broadcast safe” and the recordings made with them can be broadcast straight from the camera without any issues. The next Hypergamma is 3259. This takes a 325% range and squeezes this down to a 109% range, likewise 4609 takes 460% down to 109%. But why 109%? well the extra 9% gives you almost 10% more data to work with in post production compared to broadcast safe 100%. It also gives you the peak white level you need for display on the internet. Of course if you are doing a broadcast show you will need to ensure that the video levels in the finished programme don’t exceed 100%.
My preferred gamma is Hypergamma 4 (4609) as this gives the maximum dynamic range and gives a natural look, however the pictures can look a little flat so if I’m going direct from the camera to finished video without grading I use either a standard gamma or use the Black Gamma function to modify the curve. I’ll explain the Black Gamma in my next post.
There are 6 standard gammas to choose from. I like to stick with gamma 5 which is the ITU-709 HD standard gamma. To increase the dynamic range I use the Knee. The default knee point setting is 90, this is a reasonable setting, but if your shooting with clipping set to 100% you are not getting all the cameras latitude (the Knee at 90 works very well with clipping at 108%). Lowering the knee down to 83 gives you almost another stop of latitude, but you have to be careful as skin tones and faces can creep up towards 83%. It’s very noticeable if skin becomes compressed so you need to watch your exposure. This is also true of the Hypergammas and with them you may need to underexpose faces very slightly. The other option is to set the knee point to 88 and then also adjust the knee slope. The slope is the compression amount. A positive value is more compressed, negative less compressed. With the knee at 88 and slope set to +20 you get good latitude, albeit with quite highly compressed highlights.
If you want to play with the gammas and knee and see how they work one method you can use is to use a paint package on your PC (such as photoshop) to create a full screen left to right graduated image going from Black to white. Then shoot this with the camera (slightly out of focus) while making adjustments to the curves or knee and record the results along with a vocal description of each setting. Import the clips into your favorite editing package and use the waveform monitor or scopes you should be able to see a reasonable representation of the shape of the gamma curve and knee.
So my Gamma Choices are:
For material that will be post produced: Hypergamma 4609 (HG4)
For material that will be used straight from the camera: Standard Gamma 5 Knee at 90 with clip at 108% for non broadcast or Knee at 88 with slope +20 with white clip at 100% for direct to broadcast.

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PMW-350 Aperture Correction what is it doing?

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PMW-350 Aperture Correction

After completing the multi camera shootout at Visual Impact, one thing was bothering me about the pictures from the PMW-350 and that was the way the specular highlights in the tin foil were artificially enhanced. During the test the camera was set to factory defaults, which IMHO are too sharp, but the foil in particular looked nasty. Since then I have been further refining my paint settings for the 350 and looking at detail and aperture. Today I was replicating the tin foil test and looking at the aperture settings (not the knee aperture) and I noticed that turning aperture on and off had a very pronounced effect on highlights but a much smaller effect elsewhere in the image. Normally I would expect the aperture setting to act as a high frequency boost making subtle textures more or less enhanced, which it does, but the amount of enhancement appears to vary with the brightness of the image with specular highlights getting a really big hit of correction. If you look at the images to the left at the top you have aperture correction on at +99. There are big ugly black lines around the highlights on the foil and the texture of the carpet has been enhanced. To some degree this is the expected behaviour although I am surprised by how thick the edges around the highlights are, this looks more like detail correction (it could be “ringing”). The middle images are aperture off, not zero but actually off and you can see that the edges on the foil have gone and the carpet is no longer enhanced. The bottom picture though with aperture on at -99 though is very interesting as the carpet appears slightly softer than OFF, which is not unexpected while the foils is sharper than OFF and this is not expected. I don’t like this behaviour I’m afraid to say as a typical way to get a filmic look from a video camera is to turn the detail correction off to give a natural picture and then use Aperture correction to boost high frequencies to retain a sharp image. On the PMW-350 you can’t do this as this as a high Aperture setting will give you those nasty edges on highlights. So what can you do? Well the 350?s native, un-enhanced resolution is very high anyway so it doesn’t need a lot of correction or boosting. The default Detail and Aperture settings will give some really nasty highlight edges so you need to back things off. If your going for a filmic look I would turn OFF aperture correction altogether, for video work with pictures that have some subtle enhancement I would use Aperture at around -20, certainly never higher than -15 unless you like black lines around specular highlights.

My current prefered detail, aimed at giving a very slight, not obvious enhancement are are as follows:

Detail Level -12, H-V Ratio +15, Crispening 0, Frequency +30, White Limit +30, Black Limit +40 (all other detail settings at default)

Aperture OFF for filmic look, Aperture -20 for video look.

I have also made some changes to the Matrix settings. I have been finding the pictures from Sony cameras to be a little on the Green/Yellow side so I have tweaked things a little to remove the yellow cast and put in a bit of red, this is a subtle change but really helps with skin tones, stopping on screen talent from looking ill! These settings work in the PMW-350, EX1/3 and PDW-700.

On an EX1/EX3 this works best with the Standard Matrix, On a PMW-350 or PDW-700 you can use it on it’s own or mix it with one of the preset matrices as a modifier. User Matrix On, R-G 0, R-B +5, G-R -6, G-B +8, B-R -15, B-G -9

Have Fun!

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Brewing up a scene file: Black Gamma

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In the posts above I looked at how the gamma curves effect the contrast range within the picture and highlight handling. I also noted that while I like the latitude (range) offered by using the Hypergammas that they produce a very flat looking picture. One of the adjustments that you can make to the Gamma curves is the Black Gamma. Adjusting the Black Gamma stretches or compresses the bottom part of the gamma curve, this makes the darker parts of the picture darker (negative setting) or brighter (positive setting). When setting the Black Gamma you will find 4 different ranges to choose from. Low, Low-Mid, Hi-Mid and High. These settings determine the range over which the black gamma works. Low only effects the darkest 10% of the image, L-Mid the bottom 20%(approx), H-Mid the lowest 30%(approx) and Hi the lower 35% (approx). So if you just want to make your deep shadows and blacks darker you would use Low. If you want to make the overall image more contrasty you would use H-Mid or Hi. I like to give my images a bit more impact so I often use H-Mid at -30. If the pictures are to be graded I would not use any negative black gamma.

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Brewing up a Scene File for the PMW-350 (and other cameras)

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I decided to write a more detailed post to continue the discussions on scene file settings for the PMW-350. This is a work in progress. Some of this may also be of interest to other camera users as I hope to give a basic description of what all the various settings do.

First off let me say that there is no “right way” or “wrong way” to set up a scene file. What works for one person may not be to anothers taste, or suit different applications. For me, my requirements are a neutral look, not over corrected or too vivid, but retaining a pleasing contrast range. I hope, as this thread develops to explain a little bit about each of the settings and what they actually do in the hope that it will make it easy for you to adjust the scene files to suit your own needs. I hope others will jump in with their suggestions too!

So first of all I have been looking at the sharpness of the image. The principle settings that affect this are the Detail and Aperture settings.

Detail enhances rapid transitions from light to dark within the pictures by exaggerating the transition with the addition of a black or white edge. So it only really works on object outlines and larger details (low frequency). The circuitry that determines where these edges are uses an electronic delay to compare adjacent pixels to see whether they are brighter or darker compared to each other. Because of this any rapid movement within the frame stops the circuitry from working. If you have picture with a lot of detail correction and you do a pan for example the image will appear to go soft as soon as the camera moves as the detail circuitry can no longer determine where the edges within the image are and thus applies less detail correction. A good way to visually gauge how much detail a camera is applying to a clip is to look for this. With a good high resolution camera, set up well, it should not be all that obvious, but a low resolution camera that uses lots of detail correction to compensate will exhibit lots of softening on pans.

As well as adjusting the amount of detail correction (Detail Level), you can also adjust the ratio of horizontal and vertical correction, the maximum brightness or darkness of the applied edges (white and black limit). The thickness of the edges (frequency), the minimum contrast change that the correction will be applied to (crispening) and you can tell the camera not to apply detail correction to dark areas (level depend).

The other setting that effects picture sharpness is Aperture. Aperture correction is a high frequency boost circuit, it simply, in effect, enhances transitions from dark to light or light to dark in fine detail and textures such as fabrics, skin, hair, grass etc. It’s operation is not as obvious as “Detail” correction, but if overdone it can make textures sparkle with flashes of white or black, all very un-natural.

An important note about image detail is that if you have too much of it for the given image resolution then you get problems such as aliasing and moire which manifest themselves as rainbows of colour or buzzing, jittering areas in the picture. If you want to know more about this look up Nyquist theory. This is one of the reasons why downconverting HD to SD and getting a good picture can be harder than you might think as you are often starting out with too much detail (but that’s another topic on it’s own).

So… on to the PMW-350. Out of the box it’s really sharp. The camera has full 1920×1080 sensors, so even with all detail correction turned off the image is still pretty sharp. However most viewers are used to seeing picture with some detail correction, so if you turn it all off, to many it looks soft. If you were going for a really filmic look, detail off and aperture off would have to be a serious option. For my customers though a little bit of subtle “zing” seems to be what they like.

I found that these settings worked well for general all-round use.

Detail Level -14?H/V Ratio +20 (helps balance horizontal and vertical resolution)?Frequency +35 (makes the edges thinner, if your doing a lot of SD you may want to go the other way to -50 so that the edges can still be seen in SD)?White Limit +35 (limits brightness of white edges)?Black Limit +30 (limits darkness of black edges)

Aperture -20

If you are doing a lot of grading and work with low key scenes (large dark areas) you can use the level depend and crispening settings to help prevent “detail” being added to any picture noise. This makes any noise less apparent.

A starting point for this would be:

Crispening +35?Level depend +20

For normal light levels these are not needed with the 350 IMHO. If you are shooting with more than +6db gain then raising the level depend to +60 will help with noise.

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Brewing up a Scene File: Gamma and Knee

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Before anyone complains that I have missed stuff out or that some technical detail is not quite right, one of the things I’m trying to do here is simplify the hows and why’s to try and make it easier for the less technical people out there. Lets face it this is an art form, not a science (well actually a bit of both really).

So what is a gamma curve anyway? Well the good old fashioned cathode ray tube television was a very non-linear device. You put 1 unit of power in and get one unit of light out. You put 2 units in and get 1.5 units out, put 3 in and get 2 out… and so on. So in order to get a natural picture the output of the camera also has to be modified to compensate for this. This compensation is the gamma curve, an artificial modification of the output signal from the camera to make it match TV’s and monitors around the world. See Wikipedia for a fuller explaination:   http://en.wikipedia.org/wiki/Gamma_correction

So, all video cameras will have a gamma curve, whether you can adjust it or not is another matter. Certainly most pro level cameras allow you some form of gamma adjustment.

The PMW-350 has 6 standard gamma curves, these are all pretty similar, they have to be otherwise the pictures wouldn’t look right, but small changes in the curve effect the relationship between dark and bright parts of the pictures. Todays modern cameras have a far greater dynamic range (range of dark to bright) than older cameras. This means that the full dynamic range of the sensor no longer fits within the gamma curves used for TV’s and monitors. In broadcast television any signal that goes over 100% gets clipped off and is discarded, so the cameras entire brightness range has to be squeezed into 0 to 100%. The PMW-350 sensors are capable of far more than this (at least 600%) so what can you do?

The older and simpler solution is called the “Knee”. The knee works because in most cases the brightest parts of a scene contains little detail and is generally ignored by our brains. We humans tend to focus on mid-tone faces, animals and plants rather than the bright sky. Because of this you can compress the highlights (bright) parts of the picture quite heavily without it looking hugely un-natural (most of the time at least). What the knee does is takes a standard gamma curve and up near it’s top, bends it over. This has the effect of compressing the brighter parts of the image, squashing a broad range of highlights (clouds for example) into a narrow range of brightness. While this works fairly well, it does tend to look rather “electronic” as the picture is either natural (below the knee) or compressed (above the knee).

The answer to this electronic video look is to replace the hard knee with gentle bend to the gamma curve. This bend starts some way down the gamma curve, very gentle at first but getting harder and harder as you go up the gamma curve. This has the effect of compressing the image gently at first with the compression getting stronger and stronger as you go up the curve. This looks a lot more natural than a hard knee and is far closer to the way film handles highlights. The downside is that because the compression starts earlier a wider tonal range is compressed. This makes the pictures look flat and uninteresting. You have to watch exposure on faces as these can creep into the compressed part of the curve. The plus point is that it’s possible to squeeze large amounts of latitude into the 100% video range. This video can then be worked on in post production by the editor or colorist who can pull out the tonal range that best suits the production.

These compressed gamma curves are given different names on different products. Panasonic call them “Film Rec”, on the EX1 they are “Cinegammas” on the PMW-350 they are “Hypergammas”. The 350 has four Hypergammas. The first is 3250. this takes a brightness range the equivalent to 325% and compresses it down to 100%. HG 4600 takes 460% and squeezes that down to 100%. Both of these Hypergammas are “broadcast safe” and the recordings made with them can be broadcast straight from the camera without any issues. The next Hypergamma is 3259. This takes a 325% range and squeezes this down to a 109% range, likewise 4609 takes 460% down to 109%. But why 109%? well the extra 9% gives you almost 10% more data to work with in post production compared to broadcast safe 100%. It also gives you the peak white level you need for display on the internet. Of course if you are doing a broadcast show you will need to ensure that the video levels in the finished programme don’t exceed 100%.

My preferred gamma is Hypergamma 4 (4609) as this gives the maximum dynamic range and gives a natural look, however the pictures can look a little flat so if I’m going direct from the camera to finished video without grading I use either a standard gamma or use the Black Gamma function to modify the curve. I’ll explain the Black Gamma in my next post.

There are 6 standard gammas to choose from. I like to stick with gamma 5 which is the ITU-709 HD standard gamma. To increase the dynamic range I use the Knee. The default knee point setting is 90, this is a reasonable setting, but if your shooting with clipping set to 100% you are not getting all the cameras latitude (the Knee at 90 works very well with clipping at 108%). Lowering the knee down to 83 gives you almost another stop of latitude, but you have to be careful as skin tones and faces can creep up towards 83%. It’s very noticeable if skin becomes compressed so you need to watch your exposure. This is also true of the Hypergammas and with them you may need to underexpose faces very slightly. The other option is to set the knee point to 88 and then also adjust the knee slope. The slope is the compression amount. A positive value is more compressed, negative less compressed. With the knee at 88 and slope set to +20 you get good latitude, albeit with quite highly compressed highlights.

If you want to play with the gammas and knee and see how they work one method you can use is to use a paint package on your PC (such as photoshop) to create a full screen left to right graduated image going from Black to white. Then shoot this with the camera (slightly out of focus) while making adjustments to the curves or knee and record the results along with a vocal description of each setting. Import the clips into your favorite editing package and use the waveform monitor or scopes you should be able to see a reasonable representation of the shape of the gamma curve and knee.

So my Gamma Choices are:

For material that will be post produced: Hypergamma 4609 (HG4)

For material that will be used straight from the camera: Standard Gamma 5 Knee at 90 with clip at 108% for non broadcast or Knee at 88 with slope +20 with white clip at 100% for direct to broadcast.

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PMW-350 Scene Files. Two to get you started.

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With the PMW-350 now starting to ship I thought I would dig out the settings I used when I did my 350 review. I created two scene files and the details are as follows:

File One: Aimed at giving high latitude with deep almost crushed blacks and the image well saturated and slightly warmed up.

Detail: ON    Aperture: ON    Detail Level: -15     Aperture Level: -10     Detail Frequency: +24

Matrix: ON      Matrix (User): ON      Matrix (Preset): ON      Matrix (Prst) Sel: 6

R-G 1, R-B 12, G-R 2, G-B 11, B-R 0, B-G 0

Gamma: ON         Gamma Table: STD           Gam Table (STD): 5

Black Gamma: ON       Black Gamma Range: HIGH        Master BLK Gamma -24

Knee: ON     Knee Point: 85.2      Knee Slope: -14

White Clip: ON     White Clip Level: 109.0 (If will be graded) 104 (if not graded or for broadcast)

Master Black: -2

File Two: Aimed at giving maximum latitude with deep but not crushed blacks and vivid slightly warmed up colours.

Matrix: ON      Matrix (User): ON      Matrix (Preset): OFF      Matrix (Prst) Sel: 1

R-G 8, R-B 10, G-R 0, G-B 15, B-R 5, B-G 6

Gamma: ON         Gamma Table: HG           Gam Table (HG): 4

Black Gamma: ON       Black Gamma Range: H.MID        Master BLK Gamma -28

White Clip: ON     White Clip Level: 109.0 (If will be graded) 104 (if not graded or for broadcast)

Master Black: -3

Detail: ON    Aperture: ON    Detail Level: -15     Aperture Level: -10     Detail Frequency: +24

These settings were created on a pre-production PMW-350 so it would be wise to try them and look at them before using them in anger. All other settings are default or 0.