Custom white balance and filters

Hello everyone

I'm hoping to benefit from all the expertise out there. I'm relatively new to underwater photography and unfortunately never learned the basics of photography before getting into diving. So I'm learning as I go along.

I've recently upgraded to the Olympus TG-6 and the housing, and also bought a red and magenta filter along with the Weefine Smart Focus 6000 video light, which also as a strobe mode, along with a Weefine blue filter for the video light.

My interest in this kit was generated by this video promoting the more expensive Keldan video lights:

Some of my questions are not answered by the video however. 

So here goes.

1. What is the maximum depth you think it is worthwhile using a red or magenta filter on the Olympus housing? 20m? 30m? (I dive a lot in Fujairah in the UAE and the water is more often green than blue)

2. If I'm using red or magenta filter for still or video shots, should a manual or custom white balance be done with the filter on the housing or not? I've seen a very heated debate elsewhere on this board with a big disagreement on this topic.

3. If the filter should be used when doing the manual white balance, what about the video light? Should that be illuminating the white slate or should it just be ambient light, along with the filter?

Many thanks in advance!

Chris

Moved to the lights and strobes forum,  should get more people looking at the topic. - hopefully some who have practiced using red and blue filters.

On your questions, I'm no video specialist but I would suggest maybe 20m is the maximum depth - you basically start to run out of red light as you get deeper and if the water is green perhaps a magenta filter would be best - depending on just how green it is.  It will depend on how sensitive the camera is to red light.

I would think a custom white balance should be done with  the filter on as that is the light you want to record.

The video light is filtered blue so it matches the ambient light - without the blue filter anything illuminated becomes very red.  The issue you will run into is that the colour returned from the light changes as your subject distance changes as the light has to go to the subject and come back again  so your white balance target should ideally be out at the same distance as your subjects.  If it's not arguably it's better to WB without the lights.   I don't have a feel for how noticable this shift is though.

The last point is that for the type of shots depicted in the video the subject is distant so your lower lumen output lights will have reduced range compared to the 10,000 lumen lights.

One final point for the TG-6 - at its widest it has two apertures f2 and f2.8 you can dial in f8 but that is achieved with an ND filter so it just sucks up light - you probably only want to use f8 to reduce the shutter speed in bright shallow conditions or with a strobe to help give a black background. 

Another point that is often overlooked when it comes to shooting with ambient light underwater is the kelvin range capabilities of various cameras.

Without going into too much detail, the kelvin range is what determines the level of color accuracy you can expect at a base level.
For example, the correct kelvin reading at say 10 meters depth might be about 15.000K. But most compact & action cameras will be capped at 8-15.000K. (Most mirrorless/DSLR will have a range hitting 50.000 or so). 

So for heading down deeper, using a camera with a limited kelvin range, coupled with a smaller sensor, makes for difficulties with color.

That's not to say you can't get better results using filters, but the very base output of the camera will always be a limiting factor the deeper you go.

As for how deep you can take your particular setup at the location you most often dive, depends on many other factors such as average visibility, water hue & light intensity from the sun. 

Best thing to do is to just go out and experiment. White balance at 20 meters and look at the footage you get from your camera and how it matches up with reality. If it looks washed out or colorless, you know the limitations of your setup. But understand that all the above factors plays in, and on a super clear day it might perform differently at the same depth.  

The "official" depth that the Keldan filters work to is 18m in blue water. Beyond that depth, there is practically no red left in the spectrum. 

Keldan's classification of their Spectrum filters is that they offer an effective correction shallower than the actual depth that they are used at.

For example, the Spectrum Filter SF -4 B offers a 4m reduction. So the spectrum at 12 m is corrected by the filter to be the equivalent of shooting at 8m. At 18 m, it is the equivalent of 12m and so forth. The "B" designation refers to it being a version optimized for blue water.

The problem is that all filters are by their definition reductive. Small sensor cameras, like the TGs and GoPros have a limited practical ISO range and so at some point you just run out of light!

You need to manually white balance your camera at the depth you are shooting at, with the filter attached and I would suggest, lights off. The problem now is that one white balanced for a depth, if you add a "white" light source, this will appear very orange and unnatural. You can add your blue filter to your light, which may help correct this. However, the Keldan filter solution is very carefully graded specific to depth, and I have no experience of Weefine's blue filter. Suffice to say, typically, it is not quite as simple as using a "blue" filter.

If the water you are shooting in is green, the blue filter will not work too well either.

Despite a lot of marketing to the contrary, lights are not great for still images. The energy produced by a strobe is measured in Watt/seconds as opposed to lumen. Effectively what this does is reduce the differential between the light source and the ambient light. This limits your exposure option. 

Many thanks everyone for your very helpful comments and advice, much appreciated.

I will get experimenting.