How does a filter really work and what can it do for you?

[Interceptor121]

I thought that it would have been useful to all gopro users to understand how a filter really works and what it can or not do for you

Underwater Video Tips: Using filters with your GoPro or Compact Camera « Interceptor121 Underwater Video

Next issue we look at the polarpro filter vs SRP and why is on better than the other

 

[reebok]

Thanks very much for this. I am actually about to get one of the filters you mentioned, do you have any preview of the upcoming article?

 

[toozler]

I'm really curious to see what you have to say about those snap-ons - they, and the whole advertising done behind them, including the comparison video, seem too shady.

How do you correlate color temperature (expressed in thousands) with wavelenghts (expressed in nm)?

My understanding on how filters works is based purely on the physical properties of a material in absorbing certain wavelengths. I have contacted acrylic manufacturers in the past and got a hold of some light transmission charts. There are magenta materials with good transmission properties that will match the f-stop loss of red/orange filters - the downside is that they offer pretty much the same absorption profile as the red/orange materials for the greens, with the disadvantage of being less capable of absorbing the blue wavelenghts. In that point you are correct - if the water is really green, get ready to sacrifice the image quality for as you will most likely have a lower overall light transmission.

 

[RockyHeap]

DOUBLE PLUS REP for Toozler................dang us engineers, actually wanting facts and data.....freq/spectrum/absorbtion/wavelength/transmission..............dood you rock! sigh, so many other "you don't want lights with a gopro posts/topics


.........but wait, Polar Pro is $19.99-29.99 or what-ever with little or no factual youtube/vimeo, verses SRP that has a ton of supporters and hands on results?

this'll get interesting....

as an engineer, it's hard to beat facts and data, and SRP has done their homework.

 

[Interceptor121]

I think my post is already fairly hard to digest and didn't want to digress into how the wavelengths are filtered but in short each colour has an opposite color that absorbs it so to get rid of blue you need yellow and to eliminate green you need magenta this is why blue water filter are orange in fact they filter blue and green
Green water filter only filter green
For what concerns srp there is never an issue with the material they use UrPRO have been in the market since long time and know their stuff
However srp themselves have already made some questionable design decisions like their tray
I have a polarpro filter so I will give you the low down of how it compares to URPRO and also compare the design of the two filters

 

[yarik83]

I would love to hear your thoughts and possibly comparison videos (if you have any) of the polar pro. So far on this board I have been quite vocal about not seeing advantages of high priced filters while also acknowledging that polar pro video is probably digitally mastered to look better. Pending some concrete evidence I would love to be the person to retract my comments and apologize. So far I have yet to see concrete evidence of how $80 filter works better than my $30 for my bonica camera.

Looking forward to reading your review. Thank you.

 

[toozler]

...each colour has an opposite color that absorbs it so to get rid of blue you need yellow and to eliminate green you need magenta this is why blue water filter are orange in fact they filter blue and green
Green water filter only filter green

That phenomena is a consequence, not the cause.

I'm sharing here my plot on the visible wavelength absorption profile of two materials, I can not disclose the manufacturer. The horizontal axis are the wavelengths (I added a color scale on top) and the vertical axis is the % absorbed.



The red line is a very light MAGENTA filter, the blue line a darker ORANGE/RED filter. Just to clarify, these are not IDEAL color profiles for UW color correction, and that's why I'm sharing it

The "dip" is proportional to the amount of coloring agent added to the resin, so the more magenta or red, it will INCREASE that absorption and consequently DECREASE the amount of light reaching the sensor (proportional to the area below the line, if you integrate the % and wavelengths). These are just two materials, different color combinations will produce different profiles, some will "dip" deeper, some will sacrifice some of the oranges and so on. I prefer not to mix color temperature (kelvin) on discussing filters.

Now for the reason why one color "cancels the other" is that our eyes (and the camera sensor) see what is NOT absorbed by the material when the light bounces of it. We see a leaf green because it absorbed most of all other colors leaving only green. This red filter, for example, is red/orange because it absorbs most of the green/blue wavelenghts and reflects (or lets through) the red/orange that we see.

My grudge with these PolarPro filters is that the people that make them apparently are NOT versed in the science behind it or have the diving/UW photography background - they just jumped on the boat and decided on colors just by looking at them, seemed like a profitable business. Judging by the color on their snap-ons and the color of materials and their absorption curves (as above), they seem to be TOO aggressive in absorbing color, that is, the curve "dip" is too deep, leaving little light left for a camera that we know does not perform well in low light (thus the murky look). The fact that they sell color filters for the Hero/Hero2 STOCK housing is a proof - if you ever used the stock housing in water, you would know that it does not focus so why bother having filters? Even for the Hero Dive Housing and the Hero3, having a filter that requires unscrewing the lens does not look like a smart decision, not one someone that actually dives would do. I will take all back once I'm proved wrong.

I do not own any URPro filters but have look at materials that are pretty close to their colors and can tell (visually) that they got it right. These here are might be also worth checking out, they both seem to have done their research: Amazon.com: GoPro HERO3 Red Filter (BG) - Dive Diving Underwater Color Correction: Camera & Photo

 

[Interceptor121]

That phenomena is a consequence, not the cause.

I'm sharing here my plot on the visible wavelength absorption profile of two materials, I can not disclose the manufacturer. The horizontal axis are the wavelengths (I added a color scale on top) and the vertical axis is the % absorbed.

View attachment 143967

The red line is a very light MAGENTA filter, the blue line a darker ORANGE/RED filter. Just to clarify, these are not IDEAL color profiles for UW color correction, and that's why I'm sharing it

The "dip" is proportional to the amount of coloring agent added to the resin, so the more magenta or red, it will INCREASE that absorption and consequently DECREASE the amount of light reaching the sensor (proportional to the area below the line, if you integrate the % and wavelengths). These are just two materials, different color combinations will produce different profiles, some will "dip" deeper, some will sacrifice some of the oranges and so on. I prefer not to mix color temperature (kelvin) on discussing filters.

Now for the reason why one color "cancels the other" is that our eyes (and the camera sensor) see what is NOT absorbed by the material when the light bounces of it. We see a leaf green because it absorbed most of all other colors leaving only green. This red filter, for example, is red/orange because it absorbs most of the green/blue wavelenghts and reflects (or lets through) the red/orange that we see.

My grudge with these PolarPro filters is that the people that make them apparently are NOT versed in the science behind it or have the diving/UW photography background - they just jumped on the boat and decided on colors just by looking at them, seemed like a profitable business. Judging by the color on their snap-ons and the color of materials and their absorption curves (as above), they seem to be TOO aggressive in absorbing color, that is, the curve "dip" is too deep, leaving little light left for a camera that we know does not perform well in low light (thus the murky look). The fact that they sell color filters for the Hero/Hero2 STOCK housing is a proof - if you ever used the stock housing in water, you would know that it does not focus so why bother having filters? Even for the Hero Dive Housing and the Hero3, having a filter that requires unscrewing the lens does not look like a smart decision, not one someone that actually dives would do. I will take all back once I'm proved wrong.

I do not own any URPro filters but have look at materials that are pretty close to their colors and can tell (visually) that they got it right. These here are might be also worth checking out, they both seem to have done their research: Amazon.com: GoPro HERO3 Red Filter (BG) - Dive Diving Underwater Color Correction: Camera & Photo

Toozler am not sure what is the point you want to make but filter have two characteristics colour (temperature) and opacity. Usually the more colour you throw in the darker the filter more the absorption and stronger the effect. Usually diving filters are CTO (colour temperature orange) with some tweaks, depending on where you dive they will suit better or worse and absorb more or less light, if they are stronger they will have better results but less working depth.
In both cases SRP and Polarpro the company itself has internally no expertise on how to make a filter nor are the expert in light absorption. URPRO instead do they have been around for a while and know their stuff starting from film photography. If you look at the tray you would think that also SRP when it comes to photography or video know very little.
The two products actually declare different working depth with polarpro declaring less as it is the case of backscatter that also did a filter with A mustard that knows very well about filters but still designed something different from URPRO
The filter polarpro sell are for the gopro hero2 dive housing not for the stock housing, this is the same housing that can mount the SRP dome. SRP had the blurfix but not everybody wants to modify the housing for warranty or practical reasons.
So why don't you wait until next saturday when I post the results of the comparison? You don't need that chart to work out which filter does what if you have another camera that measures colour temperature and as a matter of fact I do

---------- Post added January 5th, 2013 at 06:47 PM ----------

By the way it does not look that bad?
[youtubehq]2qD7KuGkaag[/youtubehq]

Also looking at that link you post those are filters for inside the housing, the whole idea is that you remove the filter at surface so you have footage you can use
There are plenty of solution including the original magic filter if you want to put the filter inside the housing

 

[Marty Cerven]

I really dont see why you think so poorly of the SRP tray, it works great has the ability to be very stable when placed on the ground for stationary shots either in regular position thanks to the v shaped bottom or slightly higher up when rested on the handles and camera tilted down. Has so many ways of mounting thing to it and is very versatile all round and well designed and built. In use the shape has no negative impacts but adds much more so maybe you should give them credit for trying a different approach that adds more functionality to a tray rather then every other one that looks and acts exactly the same, just some are crappier then others due to poor quality materials.

I guess you think great design is just copying everyone else? a traditional tray is just a flat piece of aluminum with holes drilled out. There are hundreds of those already who needs one more flat piece of metal so bash someone for thinking outside of the box and giving you an alternative.

I think SRP know the best way to make a filter, that is use the best materials available thanks to the many years of URPro experience, not go out and try make a generic clone trying to copy the filter whilst also using the cheapest materials in order to maximize profits as many others do.

The thing with a filter comparison using a fully auto randomly shifting white balance is quite pointless, as I found with my earlier filter tests. The results are so random as the hero2 struggles and so the shifting wb doesn't really give you an indication of what the filter does in side by side tests due to these random issues effecting all cameras and filters.

I would have tried a polar pro filter as they offered to send me one in a thread but thats as far as it went after not replying to the PM sent when they asked for my address lol. Its cheap but I have no interest in buying one as I already had all others and am not using the 2's much anymore anyway as the 3 does much better underwater thanks to its wb improvements. Seeing I found it pretty pointless to do a direct side by side comparison due to the shortcomings of the hero2 good luck with your definitive results

 

[Interceptor121]

The tray is a classic example where they seems like they have tried so hard to come up with something original. The position of the handles is simply wrong and does not allow the lights to be in the right position, considering the gopro does already bad in low light putting them further away seems pure genius, and you can make a tripod in many other ways that do not cost that amount of money??

In some posts you go advocating the cheapest possible solution and then you try to justify SRP products at premium price, can I ask have you paid for the ones you have or have you been given free samples?

The Gopro accessory market is a lucrative segment where people that have little history or idea about taking items underwater feel they can come up with something innovative that is not a problem but then you can't just reinvent the wheel at any occasion

A filter is a piece of colured plastic or glass as long as it does not introduce defects there is no magic other than knowing which color to make it and doing some trial and error on opacity, nobody has said anything about or against URPRO as you may notice from my previous posts in fact I have this equipment myself and is well documented.

The polar filter costs less than half than other solutions and there is interest on the market place as the gopro users try always to go low cost I do not see why this should not be investigated

Also some of your early filter tests were done in conditions that were not suited to the equipment you were meant to test now I see you are mostly in shallow water no wonder everything work wonders as that is where the filters really work?

Anyway this is pointless as everybody is entitled to an opinion and I have not said anything yet about the SRP filter so you are just making assumptions I guess