Gentlemen, whatever temperature strobes are being used, can we lower please the temperature of this discussion?
 

We ask that you keep it civil. If you disagree, fine. Agree to disagree and leave it at that. Other members can read conflicting views and will come to their own conclusions. 
 

Moving right on along…..

On 5/30/2024 at 12:07 AM, Adventurer said:

 

 

This is confirmed by my recent S220 experience. This strobe is rated quite conservative by INON when looking at the tech specs. It outperformed the higher Guide numbered D200 and equally spec Backscatter MF2. It is not to be mistaken as a macro strobe, like the similar sized red S-2000 was. If you want to shrink travel size and weight or have to live on a small budget, this is the goto strobe in 2024.

 

Why do you say the S-220 is rated conservatively? Sea & Sea rates it as GN22, vs GN20 for the S-2000 and GN 16 for the Backscatter MF-2. This seems about right.

In practice, I can use it (with the 4600k diffuser and a 1/4 CTO gel, which rob about 2/3 of a stop of light) fine for wide angle on the full power setting. Here are a couple of shots, at 1/125, F8 ISO100 and 1/125, F13, ISO200. Based on these

results, I think it's pretty ok for most general wide angle usage, even in bright water. I'd happily shoot it at F13, ISO 400 and crank up shutter speed to 1/250th. 

 

 

On 5/30/2024 at 12:07 AM, Adventurer said:

 

About the color temp discussions, I would like to point out the physics aspect involved. If you use gels or filters, or paying attention to the manufacturers golden tube coating you are holding it wrong. If you look at energy traveling in water you want your photons to travel in the colder spectrum. You can fix this with the camera by setting the white balance manually (in the camera, not in LR). If you leave it in auto and buy a warm strobe you are doing it wrong. I‘d like to compare this with Astro photography where the use of Anti-LightPollution filters just costs you some fstops. But many companies make money on the filter myth. In uw photography many companies make money on the warm color temp myth.

 

All that is absorbed by water.

 

 

This would be true if what we cared about was illuminating the subject, not restoring color. But in most cases, we don't need to illuminate the subject -- the sun does a plenty good enough job of that. What we need to do is restore color, i.e. get more red and yellow light on the subject. Cooler wavelengths of light might help the light penetrate further through the water, but they don't help with restoring color.

 

On 5/30/2024 at 1:50 AM, Interceptor121 said:

Wrong idea

The water in the background is not affected by the strobe. By making the subject warmer the water looks cooler

This is difficult to replicate with masks if your background is messy

however warmer than 4900-5200 is not needed really

 

This seems pretty accurate. In my experience, 4800k produces aesthetically pleasing results, especially with skin tones on subjects.  (The Inon S-220 + 4600k diffuser + 1/4 CTO gets me to 4800k, which is what the above images were globally whitebalanced to). So I think I could live just fine with a strobe in the 4800k-5200k range). 

 

On 5/30/2024 at 4:58 AM, Chris Ross said:

Back to the original question, regarding strobe selection, The small INONs are nice strobes, however my experience is with Z-240  I find that shooting at f8 on m43 lenses I am at 1/2 power or the next level  which is 1/2 stop less than full.  at ISO200 I find lighting big scenes a struggle at times.  On full frame you will be stopping down to F11-13 most likely.  You can bring your ISO up to 400 but might find you are running into your sync speed if shooting sunballs or even bright surface waters.  1/250 @ f8 ISO 200 is the same as 1/250 @ f11 ISO400.  

 

I think in practice F11-13, ISO 400 1/250th is sufficient for even shallow shots in tropical seas. And the S-220s are able to manage that (just barely) with decent color temperature. So.. unless shooting heavily backlit scenes or sunballs, they're 'good enough' for wide angle, even with full frame cameras.  Those limitations come up infrequently enough for me not to matter much.

 

Even if you need to shoot a rectilinear lens at F16, you can simply up the ISO to 800 to compensate keeping all else equal.

 

On 5/30/2024 at 4:58 AM, Chris Ross said:

 

I've just finished diving at Walindi in PNG shooting Barracuda schools etc.  I didn't really want to go full power as I was ready to shoot again quite quickly at the powers I used.  I kind of felt I'd like a little more strobe power.  You could probably get away with the S220 if shooting at less than 1 m distance and using full power, but might find them wanting when shooting big scenes.

 

Agree.. the S-220s seem to be.. 90% of the way there, and the size, weight and price are hard to beat. It would be nice to have another stop of light to play with though.

 

20 hours ago, Adventurer said:

 

 

I can just encourage everyone to burry the old recommendation that you need warm lights or strobes underwater.

 

Once you embraced the idea that you have an absorption related depth of field for underwater color, you will improve your imaging and minimize travel weight.

 

 

I shoot video too. It all comes down to what you're trying to achieve. Blue filters on your lights are great if you want a consistent foreground to background color temperature, but (a) they don't match the color contrast you can get to separate the subject from the background with warm strobes and (b) they're not exactly ideal when shooting models where you want to get natural skintones (not 50 shades of cadaver grey...) 

 

20 hours ago, Architeuthis said:

 

In addition to several other filters, Backscatter offers also blue filters for cooling down the temperature of their HF-1 strobes. Is this the diffuser/dome you would take?

 

Wolfgang

 

Useful in some circumstances. I'm happy to see Backscatter offer the option, and wish other strobe manufacturers would do as well. Of course, you can achieve same result with the right combination of cheap gels.

 

18 hours ago, Interceptor121 said:

The comparison is accurate however if you read carefully the strobes are compared without diffusers

the retra are designed differently they have an opaque front and the element is protruding 

however once you put a diffuser on other strobes the gap closes 

i took the sea and sea ys-d2 diffuser image and overlayed in photoshop with a difference operator and it was essentially identical to the retra strobe 

which is the reason why I will keep using them until they go

inon z240 are considerably weaker despite the specs and the z330 are no longer made

i am interested in a flat front polycarbonate strobe that is powerful and reliable unfortunately I dont see any

 

I think your options are basically Sea & Sea YS-D3 or the new Backscatter HF-1. The HF-1 however is already 1.1kg, so.. not light. The YS-D3 is a much more appealing 730g, but was measured by Backscatter as 6600k. So you probably need at leas 1/4 CTO gel to make it reasonable color temperature. Which makes it ~GN24, not 32. 

 

 

17 hours ago, Interceptor121 said:

Anyway to answer the question of this post GN 32 for strobes with diffusers -0.8 works

or GN22 net

 

Strobes that are GN22 to start with are insufficient for wide angle big scenes based on my experience comparing my old Z240 with the Sea and Sea YS-D2

 

I think that conclusion may be influenced by the fact that the Z240s weren't truly GN24 as advertised? To me, they seemed almost a stop weaker than the YS-D2s at the time when I tried them.  A true GN24 strobe should be more then enough, if it's GN24 underwater at one meter -- when do you ever shoot wide angle at F22 ISO100? The only example I can think of is sunballs. If a strobe produces GN24 underwater with a color temperature between 4300k-5200k, I'd be quite happy. 

 

 

17 hours ago, Interceptor121 said:

With a strobe GN32 and diffuser you can deal pretty much any scenes you want even strongly backlit shots at close range

 

Obviously the issue is that some of the declared power may not be true...

 

If anyone knows of any current strobe with this power with weight less than 750 grams please let me know

 

Agree that GN32 (true, in water) would be plenty enough. Seeing as the Inon S-220s doesn't get you there (but are probably close in power to the old Z240s), the only options I see is to step up to the YS-D3 (730g) or Retra Pro Max (878g w/ 4 batteries). I wonder how the Seacam 60D compares here. It's only 550g, so marginally heavier than the Inon S-220s. Seacam rates it as GN8 in water (which probably is accurate/conservative). That would put it more or less in the same ballpark as my test results for the S-220 in water. Which is to say.. good enough for 90% of use cases.  GN8 gets you F8/ISO 100 or F11/ISO 200 or F16/ISO 400. It's good enough if not shooting sunballs. 

9 hours ago, Interceptor121 said:

 

It does not matter diffuser or not at distance every light is a point source and what matters is pure power

As per backscatter test the retra pro max are gn22 while the sea and sea are 32 insert a diffuser and you are still higher than 22 so the sea and sea have indeed more power which was the same before. 

 

Sometimes spending more money results in the illusion things are better however this is simply due to lack of rigorous test backscatter measures are accurate and bring things in check credit to them

 

Strobes like retra are interesting at near range where however power does not matter is more about not having hot spots

 

At distance bigger strobes like seacam and oneuw will have better results simply because of higher power so if your focus is big scenes wide angle that would be the way to go

 

Agree with this. Also, I think this thread (despite veering towards personal fights) did highlight that what's needed for close, colorful reef shots is not the same as what's needed for monochrome subjects at further distances. Scenario 1 needs less power but better color temperature. Scenario 2 needs maximum power and color temperature isn't very critical since.. grey is grey. 

2 hours ago, DreiFish said:

Agree that GN32 (true, in water) would be plenty enough. Seeing as the Inon S-220s doesn't get you there (but are probably close in power to the old Z240s), the only options I see is to step up to the YS-D3 (730g) or Retra Pro Max (878g w/ 4 batteries). I wonder how the Seacam 60D compares here. It's only 550g, so marginally heavier than the Inon S-220s. Seacam rates it as GN8 in water (which probably is accurate/conservative). That would put it more or less in the same ballpark as my test results for the S-220 in water. Which is to say.. good enough for 90% of use cases.  GN8 gets you F8/ISO 100 or F11/ISO 200 or F16/ISO 400. It's good enough if not shooting sunballs. 

 

Just to make sure my comment on the Seacam 60D is not burried in the „personal fight“ (I am very sorry for that!):

 

Make sure you test them before buying! They have no seperate dials for mode and power setting. They were the first and only strobes I actually googled the operation manual as I needed them to operate as off camera slave strobes. 
My impression (did not test them really) was that they don‘t compare to the 150D and are not just a smaller version of them. But that is just an impression using them as off camera slave strobes on less than 10 dives. 

But to each their own and maybe it is the best compromise for you!

1 hour ago, ChrisH said:

Just to make sure my comment on the Seacam 60D is not burried in the „personal fight“ (I am very sorry for that!):

 

Make sure you test them before buying! They have no seperate dials for mode and power setting. They were the first and only strobes I actually googled the operation manual as I needed them to operate as off camera slave strobes. 
My impression (did not test them really) was that they don‘t compare to the 150D and are not just a smaller version of them. But that is just an impression using them as off camera slave strobes on less than 10 dives. 

But to each their own and maybe it is the best compromise for you!

 

Thanks for the head's up. I really would like to try the 60Ds before committing that amount of money. Unfortunately, I don't know anyone using them or that would have a demo or rental set on hand.

 

Only review I've been able to find was from Adam Handlen, and that one is fairly subjective. Based on everything I've read, I'd assume they're comparable in power output to the Inon S-220s. Perhaps even weaker. That wouldn't be much of an improvement.

Before wet lenses, when I used to set a primary lens for the dive, my default plan was to dive WA early/late in the day and macro in the middle of the day. Reason being, by avoiding WA in the middle of the day I didn't need as much strobe power to dominate over natural light. 

 

Its not just daylight that gets blue filtered, all light including strobe light gets blue filtered. No matter how much strobe power there is, no matter how high the ISO, subjects further away tend to blue.

 

Hence by getting close and avoiding too much natural light, you don't need massive strobe power for WA. For me, more powerful strobes = faster/longer burst at reduced power.

 

One final thought, rather than bigger strobes, consider more strobes. You don't see it so often now, but it used to be some photographers would have 4 strobes mounted to cover the corners or to dismount for an off-camera fill. Even if you don't use them all at the same time, an advantage of more cheaper strobes is redundancy on a trip. Again not so important now strobes are more reliable.

 

 

1 hour ago, John Liddiard said:

Before wet lenses, when I used to set a primary lens for the dive, my default plan was to dive WA early/late in the day and macro in the middle of the day. Reason being, by avoiding WA in the middle of the day I didn't need as much strobe power to dominate over natural light. 

 

Its not just daylight that gets blue filtered, all light including strobe light gets blue filtered. No matter how much strobe power there is, no matter how high the ISO, subjects further away tend to blue.

 

Hence by getting close and avoiding too much natural light, you don't need massive strobe power for WA. For me, more powerful strobes = faster/longer burst at reduced power.

 

One final thought, rather than bigger strobes, consider more strobes. You don't see it so often now, but it used to be some photographers would have 4 strobes mounted to cover the corners or to dismount for an off-camera fill. Even if you don't use them all at the same time, an advantage of more cheaper strobes is redundancy on a trip. Again not so important now strobes are more reliable.

 

 

I have used four strobes regularly it improves angle of coverage however that is indeed not the biggest issue so I abandoned this track 

the whole issue of angle of coverage is extremely overrated and fixed by a piece of plastic in front of the strobe if you have enough power left you are good to go 

5 hours ago, DreiFish said:

 

Agree with this. Also, I think this thread (despite veering towards personal fights) did highlight that what's needed for close, colorful reef shots is not the same as what's needed for monochrome subjects at further distances. Scenario 1 needs less power but better color temperature. Scenario 2 needs maximum power and color temperature isn't very critical since.. grey is grey. 

According to the reviews on backscatter you posted the retra are 5800 and the sea and sea 6600

to bring a 6600 light source to 5500 you need 1/8 CTO that has a loss of 0.23 stops you would still be above 22 compared to the retra native 32

With regards to the color temperature this doesn’t have to be confused with CRI

color temperature matters for the water in the background it makes no difference to the subject itself

5 hours ago, TimG said:

Gentlemen, whatever temperature strobes are being used, can we lower please the temperature of this discussion?

Made my day 🤣 🤣 💙 💙 

 

Very good remark - so many opinions here fighting for dominance. Time to rest my case.

 

Just mentioning that backscatter released also cooling filters with their new hybrid strobe and have a nice demo on YT.

 

Our cooling filter while we post in WPX

 

1 hour ago, Adventurer said:

Just mentioning that backscatter released also cooling filters with their new hybrid strobe and have a nice demo on YT.

 

I think it is this video?

 

Actually a good overview and comparison! The blue filters are really something new for strobes, although I have to say I personally don't like the look of the wide angle pictures that they show in the video. But that might just be me and also maybe in the future somebody will make use of them to produce some shots that might change my opinion. We will see. But I did like the look of the macro shot with the blue filter! The background did look better with the blue filters.

22 hours ago, ChrisH said:

And again: in this picture I see no red. The fish have no red.

Sorry @ChrisH but I have to ask: where you ever diagnosed protanopia ?

 

If not, I probably need to see the doctor because, I clearly see RED in @Interceptor121 Bohar snapper shot. Am I the only person seeing the red there?

28 minutes ago, Adventurer said:

Sorry @ChrisH but I have to ask: where you ever diagnosed protanopia ?

 

If not, I probably need to see the doctor because, I clearly see RED in @Interceptor121 Bohar snapper shot. Am I the only person seeing the red there?

 

Guys,  before things get personal why don't you agree on definition of red perhaps? There is RED, middle red and not so much red? Perhaps depending on the distance? Perhaps one red is good for one and not good for the other?🤷‍♂️😁

Ahem, red or not so red - I‘ve seen many really nice pictures of fish schools in this thread. The best way to bring back color to a Rouget is to fry it in a pan, IMHO (that‘s the French name, no idea what fish that is in English). Delicious!

 

I am intrigued by the concept of a blue filter on the flash. I assume this works by allowing you to adjust the white balance with more red overall, hence recovering more of the remaining red from ambient light in the „non strobed“ parts of the frame. 
If you used a warm color strobe, you would cool the rest of the frame, which gives the nice negative space blue. If you used a cooler color strobe, you could push the red further overall, hence deeper penetration of color. 
Thus, do I understand this right: It‘s like the red filter, just inverted logic?

Then, do I also understand it right that this will work best in shallow waters (say, up to about 15 m) and that in theory you need to get the right blue filter for the depth you are shooting?
 Maybe we‘ll see a new line of magic filters, this time for the strobes?
 

1 hour ago, Klaus said:

Ahem, red or not so red - I‘ve seen many really nice pictures of fish schools in this thread. The best way to bring back color to a Rouget is to fry it in a pan, IMHO (that‘s the French name, no idea what fish that is in English). Delicious!

 

I am intrigued by the concept of a blue filter on the flash. I assume this works by allowing you to adjust the white balance with more red overall, hence recovering more of the remaining red from ambient light in the „non strobed“ parts of the frame. 
If you used a warm color strobe, you would cool the rest of the frame, which gives the nice negative space blue. If you used a cooler color strobe, you could push the red further overall, hence deeper penetration of color. 
Thus, do I understand this right: It‘s like the red filter, just inverted logic?

Then, do I also understand it right that this will work best in shallow waters (say, up to about 15 m) and that in theory you need to get the right blue filter for the depth you are shooting?
 Maybe we‘ll see a new line of magic filters, this time for the strobes?
 

This shot is taken with ambient light filters and filter on the camera too what it does is bring the color to the entire image not just the flash so it seems ambient light but it is not

 

 

I unfortunately don‘t know what you are referring to with „ambient light“ filter, but it certainly looks nice. And shallow, too - so there is at least some red left in the ambient light coming from the sun.

1 hour ago, Klaus said:

Ahem, red or not so red - I‘ve seen many really nice pictures of fish schools in this thread. The best way to bring back color to a Rouget is to fry it in a pan, IMHO (that‘s the French name, no idea what fish that is in English). Delicious!

 

I am intrigued by the concept of a blue filter on the flash. I assume this works by allowing you to adjust the white balance with more red overall, hence recovering more of the remaining red from ambient light in the „non strobed“ parts of the frame. 
If you used a warm color strobe, you would cool the rest of the frame, which gives the nice negative space blue. If you used a cooler color strobe, you could push the red further overall, hence deeper penetration of color. 
Thus, do I understand this right: It‘s like the red filter, just inverted logic?

Then, do I also understand it right that this will work best in shallow waters (say, up to about 15 m) and that in theory you need to get the right blue filter for the depth you are shooting?
 Maybe we‘ll see a new line of magic filters, this time for the strobes?
 

 

Yes, correct. The concept is not exactly new -- it's been used with video lights for a long time. Keldan is the biggest proponent, but on the old Wetpixel forum, there's a long thread on "ambient light filters" using Rosco gels to match the torch color temperature to that of the ambient light at different depths (works best down to around 12m). 

 

I did some experiments 2 years ago and came up with a pretty decent formula using cheap gels to get to similar end result as you get from the $100 Keldan ambient light filters. No reason same couldn't be used with strobes instead of video lights. 

1 hour ago, Klaus said:

 

I am intrigued by the concept of a blue filter on the flash. I assume this works by allowing you to adjust the white balance with more red overall, hence recovering more of the remaining red from ambient light in the „non strobed“ parts of the frame. 
If you used a warm color strobe, you would cool the rest of the frame, which gives the nice negative space blue. If you used a cooler color strobe, you could push the red further overall, hence deeper penetration of color. 
Thus, do I understand this right: It‘s like the red filter, just inverted logic?

Then, do I also understand it right that this will work best in shallow waters (say, up to about 15 m) and that in theory you need to get the right blue filter for the depth you are shooting?
 Maybe we‘ll see a new line of magic filters, this time for the strobes?
 

I would say there no such thing as free lunch in UW photography 😁 The cool filters will not bring reds - the opposite they will block them. So in theory when you push reds in post the closest to you part of image won’t get ‘red hot’ and the color expression of the image will get more uniform. It also means though that you can forget about real reds and the image will be kind of - without RED. If it makes sense. 

8 minutes ago, RomiK said:

I would say there no such thing as free lunch in UW photography 😁 The cool filters will not bring reds - the opposite they will block them. So in theory when you push reds in post the closest to you part of image won’t get ‘red hot’ and the color expression of the image will get more uniform. It also means though that you can forget about real reds and the image will be kind of - without RED. If it makes sense. 

This is true when the filter is on the camera lens - but the backscatter one is ion the strobe. So it removes the red from the strobe light, hence you can push the red channel in the camera image without turning  the foreground pink. No?

30 minutes ago, Klaus said:

This is true when the filter is on the camera lens - but the backscatter one is ion the strobe. So it removes the red from the strobe light, hence you can push the red channel in the camera image without turning  the foreground pink. No?

The blue filter makes the strobe light similar to ambient light which is fine at 10 meters this also lets you add a filter on the lens to have an even better effect as the image i posted

19 hours ago, DreiFish said:

This would be true if what we cared about was illuminating the subject, not restoring color. But in most cases, we don't need to illuminate the subject -- the sun does a plenty good enough job of that. What we need to do is restore color, i.e. get more red and yellow light on the subject. Cooler wavelengths of light might help the light penetrate further through the water, but they don't help with restoring color.

 

 

 

I would argue you care about both, the sun does a fine job of illuminating the upper surfaces but the sides of fish, corals etc don't get much illumination, then of course there's the undersides, you only have to look at how dark it is under even a small overhang.  

On 5/30/2024 at 5:22 PM, Adventurer said:

I expected this to arouse everybody and not receive flowers for my statement as just a minority of shooters has understood the physics behind this.

 

Underwater photographers are less open to this idea and the correct physics approach.

 

It‘s more accepted and understood in the underwater video community were some execute this by putting blue cooling filters on their video lights.

 

I can just encourage everyone to burry the old recommendation that you need warm lights or strobes underwater.

 

Once you embraced the idea that you have an absorption related depth of field for underwater color, you will improve your imaging and minimize travel weight.

 

Actually you are the able to colorize subjects more distant then 1.5 meters from the camera. Alex Mustards „Bohar Snapper“ from the BBC Wildlife Photographer of the Year competition is a good example and also proof that you do not have to sacrifice cool blue water background when using this technique.

There's the physics of course, but this ignores the aesthetics, using blue filters on your light source does indeed provide an even colour profile throughout and it does suit some subjects.  But it mutes the reds through yellows and looks kind of flat - it suits some subjects but not others IMO.  For skin tones, vibrant soft corals the images look kind of dull that I have seen.  One of the big things underwater is to restore reds - all the warm colours but putting a blue filter on your light source  kind of defeats the purpose. 

 

I would agree that you don't necessarily need a warm strobe, but you might want one. 

On 5/30/2024 at 5:19 PM, DreiFish said:

Hey Chris,

 

You might be mixing up two different things. If you think of the cone of light, wider strobes have a wider cone of light, but the principles are the same -- moving the strobes back (without moving them out) doesn't actually reduce backscatter -- if anything, it increases the area between the dome and the subject that is illuminated. Only moving the strobes out to the sides will decrease the backscatter.

 

What moving the strobes back does (especially with a wide lens like a fisheye) is ensuring you don't get light hitting the dome itself (which produces reflections and artifacts way worse than backscatter). Anyway, it's a fine distinction, but from a physics perspective there's no way moving the strobes back can actually decrease the amount of backscatter in terms of lighting up particles in the water column between the dome and the subject. 

 

What it does do is mean that less light hits those particles because of the distance between them and the strobes (inverse square law). But.. that also means that less light hits your subject for the same reason, so you end up having to increase light power to compensate, which gets you back in the same place.

Basically, move the strobes back just far enough to avoid direct light hitting the sides of your dome and then move them outwards proportionally to the subject distance (or inwards for close subjects) to control backscatter. 

What I have seen if they are forward is the lens sees the particles very close to the strobe which then become very bright and intrusive.  perhaps I didn't express it right.  The main point of the post really is to point out that stating that you can't avoid backscatter by using edges because you can see unilluminated particles in the image is being pedantic.    The whole point is not to illuminate them so you don't notice them.

5 hours ago, Klaus said:

This is true when the filter is on the camera lens - but the backscatter one is ion the strobe. So it removes the red from the strobe light, hence you can push the red channel in the camera image without turning  the foreground pink. No?

The answer would be no and for explanation we can reach physics.   What makes color as perception by human eye? A surface reflection of the light which reaches it. No reflection - black. Total reflection - white. In simple terms. So if the filter blocks red portion of the light then this red won’t reach the surface which in turn will not reflect it. 
 

No filter or processing in the camera will change this as there is nothing to recover. The surface does not emit light. It only reflects what comes on it. No red in means no red out. By using the blue filter you get more uniform - but bland - foreground and background. Which is fine for some applications and not for others. Backscatter.com example visualizes this nicely. 

2 hours ago, RomiK said:

The answer would be no and for explanation we can reach physics.   What makes color as perception by human eye? A surface reflection of the light which reaches it. No reflection - black. Total reflection - white. In simple terms. So if the filter blocks red portion of the light then this red won’t reach the surface which in turn will not reflect it. 
 

No filter or processing in the camera will change this as there is nothing to recover. The surface does not emit light. It only reflects what comes on it. No red in means no red out. By using the blue filter you get more uniform - but bland - foreground and background. Which is fine for some applications and not for others. Backscatter.com example visualizes this nicely. 

Exactly, hence I think this can only work up to a certain depth where you still have some red left in the ambient light. Full spectrum strobe light throws a lot of red into the mix, hence the reflected colors pop. But only as far as the strobe reaches.
As far as I understand, when you reduce the strobe’s red content (I am fairly certain that these filters diminish but do not completely remove the red) you get a more equilibrated color balance between the strobe-lit part and the background. The picture will then be blueish at first, but as long as there is some red left in the ambient light, you can push that in post processing. I think that will result in deeper, but not infinite color penetration, as well as more noise in the red channel. 
I suppose one would not leave this blue filter on for the entire dive?