Backscatter myths

[Guest]

1 minute ago, fruehaufsteher2 said:

If you, Interceptor, just could reduce using the word „you“ and reduce blaming or insulting others, your posts would be much more helpful and a win for this forum 

I have not insulted anybody those are facts

you dont swim head on into a group of fish with the motivation of getting your perfect strobe distance 

that is not needed and a total lack of understanding of fish behaviour 

 

i say things as they are and waste no time i also do not tolerate people harrassing fish in the name of taking a photo and then pontificate how to do things or not do

 

it is a sad state of things under water photographers understand very little about the fish they think they like nothing to be happy about at the same time education must continue with zero tolerance 

[ChrisH]

I know that is off topic, but I really can‘t let somebody just accuse me harassing wildlife for „getting a shot“. It is a really bold statement from somebody who was not there when I took the shot. 
 

As can clearly be seen in the picture, the snappers face my direction, they move towards me, not the other way around. If you approch them by moving towards them they will turn away. I was hovering there and waiting for at least 10-20 minutes, waiting for them to get used to me. I had two people just rush through the school, bringing me kind of back to my starting point. When they get used to a diver, they sometimes will just ignore your presence and swim around you. I did not manage to have that situation very often, but it is possible. The shot would in my opinion not be possible as it is by swimming towards or into the school! 
 

But yes, I guess there always has to be some „cheating“ involved if people don‘t understand how a shot is made. 

[fruehaufsteher2]

3 minutes ago, ChrisH said:

I know that is off topic, but I really can‘t let somebody just accuse me harassing wildlife for „getting a shot“. It is a really bold statement from somebody who was not there when I took the shot.

Several of the colleagues here have been blamed for being false, lying, doing something that they shouldn't do... as long as this is coming always from the same colleague the best way is to ignore. You have nothing to blame yourself for and nothing to apologise for. The picture exactly reflects your statement and there is really NOTHING in it that could be recognised as a disturbance of nature in any way.

[RomiK]

1 hour ago, ChrisH said:

I know that is off topic, but I really can‘t let somebody just accuse me harassing wildlife for „getting a shot“. It is a really bold statement from somebody who was not there when I took the shot. 
 

As can clearly be seen in the picture, the snappers face my direction, they move towards me, not the other way around. If you approch them by moving towards them they will turn away. I was hovering there and waiting for at least 10-20 minutes, waiting for them to get used to me. I had two people just rush through the school, bringing me kind of back to my starting point. When they get used to a diver, they sometimes will just ignore your presence and swim around you. I did not manage to have that situation very often, but it is possible. The shot would in my opinion not be possible as it is by swimming towards or into the school! 
 

But yes, I guess there always has to be some „cheating“ involved if people don‘t understand how a shot is made. 

 

HaHa so funny your picture is great as the fish is coming onto you in the formation while the OP claiming know it all and harassing people thinking otherwise shows pictures (in the other thread) where either fish is running away from him or he is successfully braking the formation 🤣🤣 so much for understanding the fish 🤣🤣

[Adventurer]

23 hours ago, Interceptor121 said:

have read here again that 'using the edge of the strobes' avoids back scatter

That is indeed not true.

👏 policeman Massimo at work 🦾  It's noteworthy.

 

23 hours ago, Interceptor121 said:

Pointing the strobes outwards accomplishes no purpose whatsoever other than wasting power

 

 Good point and true. ✅

However (as always) there are some exceptions to thumb rules.

 

Here is a really odd one:

You might have seen pictures with little or no backscatter when the photographer was shooting with the strobes completely outwards. Meaning he casted the light totally to his left and right and nothing on the wide angle subject. In very bad visibility this actually works. It will produce a light cloud left and right from your camera and can be very much compared when pointing your strobe indoors towards the wall or ceiling. The light will bounce and softly hit the subject.

 

Unintended versions of these "light clouds" can be observed with optical slave triggered strobes using no cables in your dive group. Recently this has become rare, but when S-2000 wireless connection kit was new, you had a high chance to get a few "wasted" shots were the effect was observable.

[Guest]

3 hours ago, fruehaufsteher2 said:

Several of the colleagues here have been blamed for being false, lying, doing something that they shouldn't do... as long as this is coming always from the same colleague the best way is to ignore. You have nothing to blame yourself for and nothing to apologise for. The picture exactly reflects your statement and there is really NOTHING in it that could be recognised as a disturbance of nature in any way.

Which statement the snappers are yellow mine were not yellow so the idea the color is missing is flawed

and yes thats just a bunch of fish coming out of a rock scarce 50 in the frame how does that compare to w true school?

Here a combo of jordan snappers and yellow snappers in quantity of 20x of the close up picture there is no reef behind and besides the reef has no red coral

i need to remind myself that i was at more than 1.5 meters so this image doesn’t exist

your statements by the way are pure inventions. I may say someone doenst understanr something or is misinformed that has nothing to

do with lying 

[Davide DB]

Gentlemen, 

Do we need a cooling filter here as well?

 

 

[Andrej Oblak]

This topic could've been so much more... I was hoping for some actual and helpful tips on reducing backscatter, instead we have this.

 

Does this forum maybe have the "block member" functionality, where you wouldn't see certain member's posts? I see that more and more topics here are polluted with this nonsensical bickering who's right and who's wrong. Instead of exchanging knowledge and experience, usual trend lately is just one person trying to prove other person(s) that they are wrong.

 

To (hopefully) get back on topic, let me ask a question: does flash gun's tube type influence the amount of backscatter? I.e. circular vs straight. I own a set of Inon Z-330 and a brand new set of Retra Pro Max strobes. The first thing that I noticed is that Retras produce noticeably less backscatter than Inons. Positioning of the strobes is the same (I point Retras the same way as I did Inons - noting changed here), but I get noticeably less backscatter with circular tubes than with straight ones. Would a (lack of) diffuser play a big role here?

[TimG]

Thank you for this, Andrej. 
 

We believe it a key ingredient of the forum that members treat each with civility and courtesy. People can disagree but this should be done within those principles. Agree to disagree and move on.
 

Like most people, we enjoy and do not discourage amusing banter. We really expect members - who are adults - to understand and appreciate that and not overstep the line. 
 

We do have range of sanctions and have and will use them when necessary. As you will appreciate we do not publicise their use. 
 

We really hope members are not being put off by some of the exchanges. But if anyone does feel uncomfortable, please let the moderators know. 

[RomiK]

44 minutes ago, Andrej Oblak said:

 

 

To (hopefully) get back on topic, let me ask a question: does flash gun's tube type influence the amount of backscatter? I.e. circular vs straight. I own a set of Inon Z-330 and a brand new set of Retra Pro Max strobes. The first thing that I noticed is that Retras produce noticeably less backscatter than Inons. Positioning of the strobes is the same (I point Retras the same way as I did Inons - noting changed here), but I get noticeably less backscatter with circular tubes than with straight ones. Would a (lack of) diffuser play a big role here?

 

This may be just a theory and thought from previous YSD2/3 current Retra Pro X owner - the small rectangular bulbs of Inons Z and alike will produce more concentrated beam of light and although the published angle parameters might be the same it is still a point source.

 

And reflections are just simple function of angles.

 

Whereas circular tube will disperse light under wider variety of angles and therefore less light may fall on backscatter in angles that matter.

 

Just a thought trying to find some logic in things we all hate 🙂 

Edited May 31 by RomiK

[TimG]

2 hours ago, Andrej Oblak said:

I own a set of Inon Z-330 and a brand new set of Retra Pro Max strobes. The first thing that I noticed is that Retras produce noticeably less backscatter than Inons.

 

I find this too, Andrej - also having moved from Inon Z240 to Retra, initially Pro-X. I find even less of a backscatter issue using the Retra reflector attachment.

[Klaus]

I am certainly  dry good at producing backscatter - I‘ll spare you of the image proofs. Whether one calls it edge lighting or simply moving the strobes out is semantics, not physics. I really prefer the wording in Alex‘ book: reduce the illumination of the VOLUME between the lens and the subject. The light of the (two) strobes should reach the subject, but not the water in front of it.

I think that is only possible when you get only the outer parts of the cone (to avoid „edge“) on the subject. And if you use two strobes, then you make two edges converge on the subject.

The theory is easy, putting it to practice is difficult for a beginner like me. But when it does work, it makes a HUGE difference. At least in the quarries I dive when not traveling.

Would it be possible to get back to „pleasing lighting“ and leave it with that?

 

[Guest]

54 minutes ago, Klaus said:

I am certainly  dry good at producing backscatter - I‘ll spare you of the image proofs. Whether one calls it edge lighting or simply moving the strobes out is semantics, not physics. I really prefer the wording in Alex‘ book: reduce the illumination of the VOLUME between the lens and the subject. The light of the (two) strobes should reach the subject, but not the water in front of it.

I think that is only possible when you get only the outer parts of the cone (to avoid „edge“) on the subject. And if you use two strobes, then you make two edges converge on the subject.

The theory is easy, putting it to practice is difficult for a beginner like me. But when it does work, it makes a HUGE difference. At least in the quarries I dive when not traveling.

Would it be possible to get back to „pleasing lighting“ and leave it with that?

 

The theory only works until your strobe arms length.

Imagine you have 8+12 segments your strobe goes out 85cm add 15 cm of the housing 1 meter the area that can be covered like that goes from 1 meter to 1.3 meters more or less

As you go more backwards the strobes appear close and you are hitting the subject mostly frontally as in my swimming pool example

In reality many people are not under one meter they are further away than they think mnost times

 

[Guest]

3 hours ago, Andrej Oblak said:

This topic could've been so much more... I was hoping for some actual and helpful tips on reducing backscatter, instead we have this.

 

Does this forum maybe have the "block member" functionality, where you wouldn't see certain member's posts? I see that more and more topics here are polluted with this nonsensical bickering who's right and who's wrong. Instead of exchanging knowledge and experience, usual trend lately is just one person trying to prove other person(s) that they are wrong.

 

To (hopefully) get back on topic, let me ask a question: does flash gun's tube type influence the amount of backscatter? I.e. circular vs straight. I own a set of Inon Z-330 and a brand new set of Retra Pro Max strobes. The first thing that I noticed is that Retras produce noticeably less backscatter than Inons. Positioning of the strobes is the same (I point Retras the same way as I did Inons - noting changed here), but I get noticeably less backscatter with circular tubes than with straight ones. Would a (lack of) diffuser play a big role here?

 

The inon have a parabolic front which increases diffusion and back scatter

The retra have the element right in the front and do not have diffusers therefore will produce less backscatter

 

In general dome shaped diffusers are only really useful at short range afterwards they become a liability as they scatter lots of light around and they do not increase angle of coverage either unless they are like a raduating ball (latest sea and sea dome diffusers)

 

 

[Klaus]

Sure - getting close seems to be the first thing that you read in any book on UW photography. So yes, if that is not possible (I still scare the fish much more than I‘d like to) then the image will be a compromise no matter what. I do cherish these as souvenirs, but I know that they are not ideal.

[Guest]

3 minutes ago, Klaus said:

Sure - getting close seems to be the first thing that you read in any book on UW photography. So yes, if that is not possible (I still scare the fish much more than I‘d like to) then the image will be a compromise no matter what. I do cherish these as souvenirs, but I know that they are not ideal.

You get as close are reasonably possible. If the conditions are good the strobe off the axis of the lens does not generate backscatter regardless of the distance

If there are particles in the water you will keep seeing them in essence you still take the shot

Many many shots are taken further away than one meter people simply do not realise

The funniest ones are people diving with short arms than for sure you will be shooting frontally much sooner but on the other hand 12x2 are cumbersome

[Klaus]

8 hours ago, Interceptor121 said:

You get as close are reasonably possible. If the conditions are good the strobe off the axis of the lens does not generate backscatter regardless of the distance

If there are particles in the water you will keep seeing them in essence you still take the shot

Many many shots are taken further away than one meter people simply do not Realist

Again, quoting Alex Mustard here: You will always see particles giving backscatter when you fire a strobe. But if they are further away, the spots will be small and irrelevant or easy to clean. In contrast, if you light the particles CLOSE to the port, then they make big blobs that ruin the shot. I think he called this „terminal backscatter“. I‘ve also done this topside when taking family pics during snowfall, not pretty. But I am not sure they would have let me bring out the UW rig just to move the strobe out further…

And yes, arm length is also for me a compromise between convenience and efficacy. But in my limited experience every little bit helps and when you have lots of particles in the water then large landscapes are not an option anyways. Perhaps it may be wise to angle out the strobes a bit when you know that the arms should be longer than the ones you brought? 

[TimG]

I do agree on 2x 12” strobe arms being cumbersome especially when getting the system into the water. In addition I found the arm clamps struggle to hold the arms in place even at depth. I’ve always used 2x 8” plus 2x 5” and found this a good combination both for WA and macro. 
 

I bought 2x 12” to get more spread. Tried it a couple of times and gave up. I should put them in the Classifieds!

 

I do think though that Klaus should try using his housing to take skiing pics of the family. We would  need to see pics of the expressions on family faces…..  just for scientific purposes of course. 

[Guest]

6 hours ago, Klaus said:

Again, quoting Alex Mustard here: You will always see particles giving backscatter when you fire a strobe. But if they are further away, the spots will be small and irrelevant or easy to clean. In contrast, if you light the particles CLOSE to the port, then they make big blobs that ruin the shot. I think he called this „terminal backscatter“. I‘ve also done this topside when taking family pics during snowfall, not pretty. But I am not sure they would have let me bring out the UW rig just to move the strobe out further…

And yes, arm length is also for me a compromise between convenience and efficacy. But in my limited experience every little bit helps and when you have lots of particles in the water then large landscapes are not an option anyways. Perhaps it may be wise to angle out the strobes a bit when you know that the arms should be longer than the ones you brought? 

No because if you angle out you create a hole in the center

I have been on alex egypt workshop 3 times we are not at 1.5 meters from the snappers

there is good ambient light and if you hit the fish with the strobes the white balance works better at restoring colors

fish schools are normally shot with wwl-1 field of view so not a fisheye you are a few meters away with the right conditions it is not an issue

i have shot far more difficult schools in malpelo lower visibility and substantially large schools you get to the limit of the equipment there still the shots are decent despite particles

 

people get obsessed by backscatter but in reality is only an issue at close range where particles look bigger and brighter if reflective 

[Seewolf]

OK, let’s analyze backscatter from a scientific point of view. First of all, scattering has absolutely nothing to do with reflection. While reflection is directional (that’s how a mirror works), scattering is (almost) not. A particle, that is illuminated by a light source becomes a light source by itself and is irradiating more or less uniformly in all directions. Therefore, it is irrelevant from which angle a scattering particle is illuminated, all that matters is the intensity of the irradiating light at the place where the scattering particle is located.

You can easily prove this by yourself. Use a narrow beam dive light to irradiate a glass of water. There is no scattering, as long as there are no particles that scatter the light in the water (not completely true, I only consider Tyndall scattering). Now add a couple of drops of milk to the water, and you will see the light beam (Tyndall effect). You can move around the dive light and the intensity of the scattering will stay the same, irrespective of the angle of irradiation.

How can we reduce scattering? (i) Don’t irradiate the scattering particle and (ii) pull back your strobe. (i) is trivial, but why does (ii) work? Let’s assume you have an ideal strobe, then the inverse-square law holds for the light intensity as a function of distance. If you double the distance between strobe and object to be illuminated, the light intensity decreases to ¼. This is true for both scattering particles and the object you want to photograph.

Now let’s do a “Gedankenexperiment”. You have your strobe, a scattering particle, and your object in line. The distance between strobe and scattering particle is 10 cm and the distance between strobe and object is 100 cm. In this case, the light intensity (number of photons per unit area) at the scattering particle is 100 times larger than at the object (because of the inverse square law and 10 times larger distance). If you now pull back the strobe by 80 cm, the distance between the strobe and scattering particle is 90 cm, the distance between scattering particle and object still 90 cm, and the distance between strobe and object 180 cm. Now the light intensity at the scattering particle is only 4 times that at the object and not 100 times! This is also true if you increase the power of the strobe to compensate for the larger distance to the object. Compared to the object, the scattering particle looks now much less bright.

The final “Gedankenexperiment” is to pull back the strobe to an infinite distance. We do have such a strobe, it is called the sun. In this case the light intensity is the same at the scattering particle and at the object. We still see the scattering, but it is much less intense than by using a strobe considering the inverse square law.

Of course, this is very much simplified. I am only talking about Tyndall scattering of particles larger than the wavelength of light, not about Rayleigh scattering at molecules.

[Guest]

30 minutes ago, Seewolf said:

OK, let’s analyze backscatter from a scientific point of view. First of all, scattering has absolutely nothing to do with reflection. While reflection is directional (that’s how a mirror works), scattering is (almost) not. A particle, that is illuminated by a light source becomes a light source by itself and is irradiating more or less uniformly in all directions. Therefore, it is irrelevant from which angle a scattering particle is illuminated, all that matters is the intensity of the irradiating light at the place where the scattering particle is located.

You can easily prove this by yourself. Use a narrow beam dive light to irradiate a glass of water. There is no scattering, as long as there are no particles that scatter the light in the water (not completely true, I only consider Tyndall scattering). Now add a couple of drops of milk to the water, and you will see the light beam (Tyndall effect). You can move around the dive light and the intensity of the scattering will stay the same, irrespective of the angle of irradiation.

How can we reduce scattering? (i) Don’t irradiate the scattering particle and (ii) pull back your strobe. (i) is trivial, but why does (ii) work? Let’s assume you have an ideal strobe, then the inverse-square law holds for the light intensity as a function of distance. If you double the distance between strobe and object to be illuminated, the light intensity decreases to ¼. This is true for both scattering particles and the object you want to photograph.

Now let’s do a “Gedankenexperiment”. You have your strobe, a scattering particle, and your object in line. The distance between strobe and scattering particle is 10 cm and the distance between strobe and object is 100 cm. In this case, the light intensity (number of photons per unit area) at the scattering particle is 100 times larger than at the object (because of the inverse square law and 10 times larger distance). If you now pull back the strobe by 80 cm, the distance between the strobe and scattering particle is 90 cm, the distance between scattering particle and object still 90 cm, and the distance between strobe and object 180 cm. Now the light intensity at the scattering particle is only 4 times that at the object and not 100 times! This is also true if you increase the power of the strobe to compensate for the larger distance to the object. Compared to the object, the scattering particle looks now much less bright.

The final “Gedankenexperiment” is to pull back the strobe to an infinite distance. We do have such a strobe, it is called the sun. In this case the light intensity is the same at the scattering particle and at the object. We still see the scattering, but it is much less intense than by using a strobe considering the inverse square law.

Of course, this is very much simplified. I am only talking about Tyndall scattering of particles larger than the wavelength of light, not about Rayleigh scattering at molecules.

In fact rayleigh scattering applies to blue wavelenght in water

the term backscatter is also not scientific it really means reflection

if you point the light exactly at the same angle and on top of the lens you will see the particles however if you put the light off axis you see less

of them hence having arms is the single most important factor regardless of where you point the light 

at the end there is no way to show no particles if there are plenty 

you can get close reducing the amount of water and hence particles however this means the scene you have in mind doesn’t fit

the only technique that really works when there is a lot of sediment is extreme inward lighting see my article on uwpmag 

[Seewolf]

No, Rayleigh scattering has nothing to do with the blue color of water since it is not efficient. Scattering at objects that are much smaller than the wavelength of light, such as molecules, does depend on the wavelength (Rayleigh scattering). For blue light, it is a little bit less inefficient than for red light. This is what causes the blue color of the sky, but not the blue color of water. If you look at the sun at sunrise or sunset it is red because the blue light passing through a huge column of the atmosphere is scattered away. During the day the sun is yellow, more or less the natural color defined by the surface temperature of the sun. The column density of molecules for light passing perpendicularly through the atmosphere is too low to produce a noticeable effect.

If the blue color of water was caused by Rayleigh scattering, a distant dive lamp would look red, since the red light could pass the water and the blue light would be scattered. Just like the sun at sunset.

The blue color of water is caused by the absorption of light, not by scattering, just like a dye. It is the fourth harmonic of the OH stretching vibration of water that causes the absorption. Interestingly, the heavy isotopomer of water, D2O, is colorless, since the overtone of the OD stretching vibration is shifted out of the visible region. If it was scattering, it should be the same as normal water H2O.

[Guest]

4 hours ago, Seewolf said:

No, Rayleigh scattering has nothing to do with the blue color of water since it is not efficient. Scattering at objects that are much smaller than the wavelength of light, such as molecules, does depend on the wavelength (Rayleigh scattering). For blue light, it is a little bit less inefficient than for red light. This is what causes the blue color of the sky, but not the blue color of water. If you look at the sun at sunrise or sunset it is red because the blue light passing through a huge column of the atmosphere is scattered away. During the day the sun is yellow, more or less the natural color defined by the surface temperature of the sun. The column density of molecules for light passing perpendicularly through the atmosphere is too low to produce a noticeable effect.

If the blue color of water was caused by Rayleigh scattering, a distant dive lamp would look red, since the red light could pass the water and the blue light would be scattered. Just like the sun at sunset.

The blue color of water is caused by the absorption of light, not by scattering, just like a dye. It is the fourth harmonic of the OH stretching vibration of water that causes the absorption. Interestingly, the heavy isotopomer of water, D2O, is colorless, since the overtone of the OD stretching vibration is shifted out of the visible region. If it was scattering, it should be the same as normal water H2O.

Water absorbs red and scatters blue and am not talking about the color of water but what happens to a strobe with a diffuser on

 

with regards to backscatter this is a reflection due to diffuse scattering of the strobe light In particolar the blue wavelenght which make the particle look white

 

Now as the scattering is diffused meaning it goes in all direction it follows that shooting a strobe will always light particles in the water and depending on the angle of the lens you will see more or less of them. And in addition light scattering will also show particles in ambient light as my photos on the very first post of this thread

Can backscatter be avoided: no

Can backscatter be minimised: in some cases

Can you point your strobe at the subject: of course you can as long as the strobe is not directly in line with the lens. If there are a lot of particles the energy of the light will create scattering and you will see more particles than if you were not pointing directly that does not mean you should systematically used the edges of the strobe light

Edited June 2 by Interceptor121

[Floris Bennema]

Seemingly it's hard to say "Yes, you are right".

[Klaus]

On 6/2/2024 at 1:43 PM, Seewolf said:

 

How can we reduce scattering? (i) Don’t irradiate the scattering particle and (ii) pull back your strobe. (i) is trivial, but why does (ii) work? 

Brilliant and thanks for spelling this out in detail for once!

It makes perfect sense  now - and I wonder why I didn‘t get this by myself but just kept wondering about this strange advice of pulling the strobes back to avoid backscatter.
It will be much easier now for me to remember this below surface!

Klaus