Filters Test

Following a number of posts about filters in various GoPro forums I decided to check in detail the behavior of the UR/PRO filters and see what is that they really do.

I guess the reason is that people do not really know or want to know the physics behind it and prefer to entertain many discussion that involve trial and error.

One of the most interesting one is this clip here that generated some ferocious comments by some users of other products that of course were not pleased having spent money that their choice seemed not to be the best.

To understand a bit more about what filters do we need to look at light temperature graphs like the one below.

Light colour is measured in K and high value means cold blue light and low values represent warm red light.

As we know underwater red gets absorbed approximately at a rate of 1 f-stop every 2.5 meters we go deeper compared to blue light with the result that red colour disappears pretty quick followed by green to become completely blue at depth.

Some time ago cameras were shooting film not digital and there was no way to correct colours when the film itself got developed. So to perform some decent underwater photography people used strobes and fisheye lenses to get really near and blast the reef with light in order to restore the real colors.

This worked very well for close shots but was useless for wide angle at distance, so color correcting filters that could go underwater were developed for ambient light photography.

So what is a CC filter? A common misconception is that filters add colours to the picture, whilst this is practically true this is technically incorrect.

So what does a CC filter really do? As the word says it filters light, more specifically filters selected part of the color spectrum. This is done at expense of available light so if we use a filter we will have less total light or for our purposes exposure. This is well explained on the URPRO website where they talk about film sensitivity, if you use a filter you will loose 1 f-stop so use a higher ISO film.

http://www.urprofilters.com/content.do?region=FilterInstructions

The filter can be designed for different purposes and depending on its color and opacity will be more or less aggressive. If you read very carefully the urpro website says that there is a maximum distance that the filter will be effective at.

UNDERWATER FILTER DISTANCE

Photographically and visually reds, oranges and other warm colors become dominated by the natural blue-green (cyan) effects of the water when it is more than 8-10 feet deep.

Because all water is a continuous filter, the deeper a subsea photographer goes beneath the surface, the more colors are naturally “filtered out” of the spectrum.

As a result, the depth of the water must be added to the distance between the camera (or flash) and the subject to give the underwater filter distance. Use the following formula to determine the correct underwater filter distance:

+ Depth of water above the photographer
+ Distance from camera (or flash) to subject

= Underwater filter distance

Scuba divers using a still camera or a movie camera must combine the depth of the water plus the distance of the camera from the subject. This sum should not exceed 80 feet for color negative films and video, and should not exceed 30 feet for color slides. In all cases, the best color is achieved when the camera is close to the subject!

What that means is that the working distance of the distance is not the same as depth so you can push a filter say down to 24 meters but then the distance to subject needs to be zero. So if you are shooting something 3 meters or 10 feet away in 21 meters of 70 feet that as much as you can get.

When it comes to underwater use we are mostly interested in eliminating blue in tropical waters or green in cold algae waters.

In order to eliminate blue that has a high colour temperature we need warm colors typically something around orange. This will some of the blue beams and also part of the green beams and will produce filter light that is of a warmer color and will look ‘more red’.

But how much does for example a URPRO orange filter for blue water really warm up the light?

I have done some empirical tests with my Sony RX100 in a day where the Sony would measure a temperature of 6500K which pretty much correspond to cloudy day indoor.

The URPRO orange filter which is the same that is on the SRP filter for GOPRO measured 2800K with a tint of 5 Magenta. It also measured an overall absorption of 1 and 1/3 fstop of total light. I measure this filter instead of the one I have (which is not SRP and we can cover why in another place) because I think the SRP is the market lead at this time and it uses UR/PRO filters.

So the URPRO orange filter has approximately a 3700K warming effect with a strong magenta tint. This means that a camera with auto white balance and an average lens like a GoPro can restore some color in the image until the overall color of the light is around 10,000K in water. After that the filter essentially stops working and the light absorption is so much that it just becomes plain noise. I think this website gives a good idea

http://www.seos-project.eu/modules/oceancolour/oceancolour-c01-p07.html

So the reason why the picture stops having any red and starts looking green is that there is no red left in the light!

The breaking point seems to be around 18-21 meters or 60-70 feet and corresponds to our personal experiences I would say.

So what about custom white balance? We are pretty much in the same situation we can re-balance the camera up to 10,000K and add magenta or orange tint to the picture but you reach a point where there is too much red color noise.

The other advantage of custom white balance is that there is no light absorption so the picture ends up being less noisy as at the same aperture a loss of 1 and 1/3 fstop is like saying doubling the ISO from 800 to 1600 to capture the same scene. This is the reason a camera with custom white balance will always outperform the same camera with a filter at depth in terms of picture noise.

And what about Magenta filters? This is an interesting one as magenta filter absorb little light around 1/3 of fstops for URPRO and warm up the colour very little around 700-800K with a magenta tinge more prominent. A magenta filter will therefore absorb less light in total and introduce just a tint to the image.

From my tests a magenta filter is totally worthless on a camera that can white balance and frankly not that good at depth on camera with auto mode only. The reason is that light will get cold very soon at depth and the camera will be outside the AUTO white balance operating range (2700-7000  on average) pretty soon.

So what it means for users of GoPro cameras? The summary is pretty much like this:

• In blue water filters are worth down to around 18 meters or 60 feet

• In green water filters are worth even less as the color temperature very soon reaches a temperature where the auto white balance of the gopro is out of range

Although I have not yet measured I would think probably max 10 meters and that is it as I had similar results with my Sanyo that has same sensor size of the gopro and actually a faster lens.

Hope gopro users find this post useful as well as other users of compact camera to shoot video in ambient light.

In the next post I will compare the optical quality of the SRP dome port with the PolarPro snap on filter that generated such a debate on youtube and we will see why this filter could perform better than the SRP dome.