Tag Archives: balance

Setting The White Balance When Using The Variable ND Filter

It’s no secret that the variable ND fitted to many Sony cameras does introduce a colour shift that changes depending on how much ND you use. But the cameras are setup to add an offset to the WB as you switch the ND in or out and change the amount of ND, so in practice most people are completely unaware of this shift.
 
If you watch carefully when you engage or disengage the ND you can sometimes see a fraction of a second where the cameras electronic offset that corrects for the shift is applied just as the filter comes in. Then once the filter is in place the colours appear completely normal again.
 
So when should you white balance from a white card? With or without the ND filter in place?

You can actually white balance either with or without the ND in place. Because the camera knows exactly what offset to apply for any ND value if you change the ND it will compensate automatically and generally the compensation is very accurate. So in most cases it doesn’t really matter whether the ND filter is in place or not.
 
However, my personal recommendation is where possible to white balance with the camera setup as it will be when you are taking your footage. This should then eliminate any small errors or differences that may creep in if you do change the ND or switch the ND in or out.
 
But I wouldn’t be too concerned if you do have to do a WB at one ND level and then change the ND for whatever reason. The in camera compensation is extremely good and you would only ever really be able to see any difference if you start doing careful like for like, side by side, split screen direct comparisons. It’s certainly highly unlikely that you or your audience would ever notice any difference in normal real world applications.
 
You will often see greater colour shifts if you add external ND filters or swap between different lenses, so treat the internal ND as you would any other ND filter and WB with your lens, filters and everything else as it will be when taking the footage. I think one of the truly remarkable things about the variable ND filter is just how consistent the output of the camera is across such a wide range of ND.

How to get the best White Balance (Push Auto WB).

Getting a good white balance is critical to getting a great image, especially if you are not going to be color correcting or grading your footage. When shooting traditionally ie – not with log or raw – A common way to set the cameras white balance is to use the one push auto white balance combined with a white target. You point the camera at the white target, then press the WB button (normally found at the front of the camera just under the lens).
The white target needs to occupy a good portion of the shot but it doesn’t have to completely fill the shot. It can be a pretty small area, 30% is normally enough. The key is to make sure that the white or middle grey target is obvious enough and at the right brightness that the camera uses the right part of the image for the white balance. For example, you could have a white card filling 50% of the screen, but there might be a large white car filling the rest of the shot. The camera could be confused by the car if the brightness of the car is closer to the brightness the camera wants than the white/grey card.
The way it normally works is that the camera looks for a uniformly bright part of the image with very little saturation (color) somewhere between 45 and 90IRE. The camera will then assume this area to be the white balance target. The camera then adjusts the gain of the red and blue channels so that the saturation in that part of the image becomes zero and as a result there is no color over the white or grey target.
 
If you fill the frame with your white/grey card then there can be no confusion. But that isn’t always possible or practical as the card needs to be in the scene and under the light you are balancing for rather than just directly in front of the lens. The larger your white or grey card is the more likely it is that you will get a successful and accurate white balance – provided it’s correctly exposed and in the right place.
 
The white target needs to be in reasonable focus as if it is out of focus this will create a blurred edge with color from any background objects blending into the blur. This could upset the white balance as the camera uses an average value for the whole of white area, so any color bleed at the edges due to defocussing may result in a small color offset.
 
You can use a white card or grey card (white paper at a push, but most paper is bleached slightly blue to make it look whiter to our eyes and this will offset the white balance). The best white balance is normally achieved by using a good quality photography grey card. As the grey card will be lower down in the brightness range, if there is any color, it will be more saturated. So when the camera offsets the R and B gain to eliminate the color it will be more accurate.
 
The shiny white plastic cards often sold as white balance cards are often not good choices for white balance. They are too bright and shiny. Any reflections off a glossy white card will seriously degrade the cameras ability to perform an accurate white balance as the highlights will be in the cameras knee or roll-off and as a result have much reduced saturation and also reduced R and B gain, making it harder for the camera to get a good white balance. In addition the plastics used tend to yellow with age, so if you do use a plastic white balance card make sure it isn’t past it’s best.
Don’t try to white balance off clouds or white cars, they tend to introduce offsets into the white balance.
 
Don’t read too much into the Kelvin reading the camera might give. These values are only a guide, different lenses and many other factors will introduce inaccuracies. It is not at all unusual to have two identical cameras give two different Kelvin values even though both are perfectly white balance matched. If you are not sure that your white balance is correct, repeat the process. If you keep getting the same kelvin number it’s likely you are doing it correctly.

The relationship between White Balance and the Matrix.

So… you want to change the look of the colour in your pictures but are not sure how to do it. One of the first things that you need to understand is the relationship between white balance and the colour matrix. They are two very different things, with two different jobs. As it’s name applies white balance is designed to ensure that whites with the image are white, even when shooting under lighting of different colour temperatures. When you shoot indoors under tungsten lights (you know, the one the EU have decided you can no longer buy) the light is very orange. When you shoot outside under sunlight the light is very blue. Our eyes adjust for this very well, so we barely notice the difference, but an electronic video camera is very sensitive to these changes. When you point a video camera at a white or grey card and do a manual white balance, what happens is that the camera adjusts the gain of the red, blue and green channels to minimise the amount of colour in areas of white (or grey) so that they do in fact appear white, ie with no colour. So the important thing to remember is that white balance is trying to eliminate colour in whites and greys.

The Matrix however deals purely with saturated parts of the image or areas where there is colour. It works be defining the ratio of how each colour is mixed with it’s complimentary colours. So changing the white balance does not alter the matrix and changing the matrix does not alter the white balance (whites will still be white). What changing the matrix will do is change the hue of the image, so you could make greens look bluer for example or reds more green.

So if you want to make your pictures look warmer (more orange or red) overall, then you would do this by offsetting the white balance, as in a warm picture your whites would appear warmer if they are slightly orange. This could be done electronically by adding an offset to the colour temperature settings or by using a warming card, which is a very slightly blue card. If you want to make the reds richer in your pictures then you would use the matrix as this allows you to make the reds stronger relative to the other colours, while whites stay white.

PDW 700 Native White Balance

The PDW-700 cameras are balanced for daylight optically and then corrected electronically for tungsten etc.

Traditionally cameras were balanced for Tungsten and then added colour correction optical filters to get to daylight. This was done as CC filters absorb light and thus make the camera less sensitive. Normally when shooting outdoors in daylight sensitivity is not an issue while shooting indoors under tungsten light you used to need every bit of sensitivity you could get.

The down side to this approach is that tungsten contains very little blue light so to get a natural picture the blue channel was often running at quite a high level of gain which increases noise in the blue channel and thus overall noise. In addition when you rotated in the CC filters to get to daylight the sensitivity of the camera was reduced, so you did not have constant gain.

With the PDW-700 (and also the F350 I believe) the cameras are essentially balanced for daylight, without the use of any CC filters, which helps reduce noise in the blue channel. Then for tungsten shooting you electronically re balance the camera. By doing this the overall sensitivity of the camera is constant whether shooting at 3.2K or 5.6K and you only get additional blue channel noise while shooting under tungsten. If you are worried by blue channel noise you can always correct from daylight down to tungsten with an optical CC filter (80A) and leave the camera set to daylight, although this will reduce the systems overall sensitivity by around 1 and a half stops.