Backscatter myths

[Guest]

10 minutes ago, Klaus said:

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

 

Lol dont irradiate the scattering particle and pull back the strobes?

that also doesn’t irradiate the target so no light on the target either

then pull back is BS if your lens doenst have a wide horizontal field of view thats not even required 

the reason you pull back the strobe is to avoid the lens to see the cloud generated by the high energy particle collision at the strobe surface it doenst fix anything else 

 

recipes dont exist and the tyndal idea is silly because water is not a colloid is a solution 

in a minute we will be diving not in Milky water but in milk

Edited June 4 by Interceptor121

[Klaus]

Oh c‘mon it’s obvious - same idea as uplighting when you shoot vertical without moving the strobes. You need to turn down the power of the bottom strobe to avoid blasting bottom-up. Nothing to to with the edge of the beam, it‘s a separate thing. 
Backscatter comes from the particles close to the lens and as nicely described above will be affected much more by pulling back the strobe than the object further away. This does not make the backscatter disappear, but helps to make it less offensive! Sure there is no magic recipe or setting - but every little bit contributes and together they can do a lot. And of course it will depend of the situation.

Will the pull-back help when shooting Macro? No, because the particles and the subject are at a similar distance from the lens. And shooting from a close distance is most beneficial anyways.

Will the pull-back help for CFWA? Perhaps not for the backscatter between the lens and the subject (as in macro) but the cone of light gets wider and that may be desirable. And there are lots of particles at a similar distance than the subject above and around it, maybe these will become a bit less visible.
Will the pull-back help with wide-angle at a medium distance (say 1-2 m)? I think yes, but I suppose it should complement rather than replace moving the strobes out on their arms. So at that distance, moving them out AND back may be best.

Then as things get even more distant, the situation may ask for moving the strobes out as much as possible with the arms you brought, leaving no leeway for also pulling them back. But this is a situation I have never tried myself.

[Guest]

Some instructional links

 

Diffuse reflection (source of back scatter)

https://en.wikipedia.org/wiki/Diffuse_reflection

 

Lambertian Reflectance (a perfect diffuser has this behaviour)

https://en.wikipedia.org/wiki/Lambertian_reflectance

 

Inverse Square Law (light power relationship with distance)

https://en.wikipedia.org/wiki/Inverse-square_law

 

Color of water or else does not matter to back scatter but matters to color temperature of the strobe. As light travels temperature gets cooler due to selective absorption

 

From link 1 a particle hit by light will reflect in all direction as well as give a specular reflection

 

Pointing the strobe in a way that is not aligned with the lens only minimises the specular reflection (light bounces back to the strobe that in the same line of the lens means back into the lens) does not change the rest.

So shooting a strobe always results in backscatter by diffuse reflection

 

With regards to pulling the strobe back relatively to the lens that does a few things in addition to creating more distance between the subject and the strobe which reduces backscatter but also reduces the light and therefore per se is NOT a benefit

1. Brings closer to the camera the area where the two strobes overlap and reduces the area lit between the lens and the strobe. This however can backfire if the strobes are too far back the camera rig will cast a shadow

2. Avoids the lens to see the strobe. As more particles hit the water near the strobe surface you literally see a glow there that your lens if super wide would see. There is no  need to pull back strobes if your lens is narrow and does not see the strobes 

 

Practical situation

fisheye lens with 142 field of view horizontal and high power strobes: highly likely your lens will see the strobes pull the strobes back a little

Rectilinear lens with 96 degrees horizontal field of view: no need to pull back strobes 

Macro lens with 20 degrees fov no need to pull back strobes

 

Why cross strobes increase backscatter? Because the strobes overlap with the camera field of view is increased and the light density is higher

 

Minimise back scatter? instead of shooting ISO 100 shoot ISO 800 and turn the strobe 3 stops down. However this may result in a lot of noise in the image and reduced dynamic range. What is worse?

 

 

[Klaus]

The particles that cause my grieving are so big that they probably reflect rather than scatter (for what my blobs are concerned at least). The milk -effect may be what I would call haze in an image. This may be influenced by the pull-back as well, but I still have a long way to go before worrying about this.

 

 

[Guest]

11 minutes ago, Klaus said:

Oh c‘mon it’s obvious - same idea as uplighting when you shoot vertical without moving the strobes. You need to turn down the power of the bottom strobe to avoid blasting bottom-up. Nothing to to with the edge of the beam, it‘s a separate thing. 
Backscatter comes from the particles close to the lens and as nicely described above will be affected much more by pulling back the strobe than the object further away. This does not make the backscatter disappear, but helps to make it less offensive! Sure there is no magic recipe or setting - but every little bit contributes and together they can do a lot. And of course it will depend of the situation.

Will the pull-back help when shooting Macro? No, because the particles and the subject are at a similar distance from the lens. And shooting from a close distance is most beneficial anyways.

Will the pull-back help for CFWA? Perhaps not for the backscatter between the lens and the subject (as in macro) but the cone of light gets wider and that may be desirable. And there are lots of particles at a similar distance than the subject above and around it, maybe these will become a bit less visible.
Will the pull-back help with wide-angle at a medium distance (say 1-2 m)? I think yes, but I suppose it should complement rather than replace moving the strobes out on their arms. So at that distance, moving them out AND back may be best.

Then as things get even more distant, the situation may ask for moving the strobes out as much as possible with the arms you brought, leaving no leeway for also pulling them back. But this is a situation I have never tried myself.

The problem with verticals is nothing to do with this issue. It has to do with the fact that in a vertical on a typical subject like a coral head the bottom strobe may reflect on sand while the top may just go through water

This is becoming pure fantasy territory

 

Backscatter cannot be eliminated it a  feature of having particles in the water the only way to see less of it is to remove the water however this is also an issue because the scene may not be cointained by the lens field of view

If someone believes that pulling back the strobes 10cm does magic just to a few shots in real conditions to find out this is definitely not the case. You are just reducing the light and in addition you are putting some of the light closer to the lens which if the subject it not close actually increases particles diffusion

[Klaus]

I don’t think anyone claimed « magic » here.

but if you prefer, certainly this is useless - but I got the point. 

[Guest]

I realised I made a mistake bringing the strobes back from the line of the camera increase the area lit between the lens and the subject and therefore increases back scatter

 

See illustration in the first case only two particles are lit. however as the strobes go behind additional particles closer to the lens are hit by the strobe and therefore will diffuse more light

Bringing back the strobes is only useful to avoid seeing them with your lens

 

The advice of angling the strobes out brings the light further away compensating for that effect

So in short if you pull back the strobes you may want to very slightly angle out the strobes in case your subject is far however this also creates dark areas in the frame which in turn kills the whole purpose of even light in the frame

 

There is no such a thing as a free lunch

 

[Floris Bennema]

If you angle the strobes out so that only two particles are lit in the second case, the 'dark areas' will have the same size as in the first case. The benefit of the pull back is nicely explained in Seewolf's "Getankenexperiment"

[Guest]

7 hours ago, Floris Bennema said:

If you angle the strobes out so that only two particles are lit in the second case, the 'dark areas' will have the same size as in the first case. The benefit of the pull back is nicely explained in Seewolf's "Getankenexperiment"

As you angle out the strobes you create dark areas in the middle of the frame top and bottom

https://waterpixels.net/forums/topic/1315-backscatter-myths/?do=findComment&comment=6958

if you are extending out the strobes is because you are far and want to light a big scene not because you want to just shoot a single fish far 

I have already covered this earlier and the concept of lyndall diffusion is off this is not shooting in jelly

 

Edited June 5 by Interceptor121

[Floris Bennema]

If yo pull your strobes back the light spreads out earlier relatively to the camera. Reducing the dark areas effect of slightly angling out.  

[Architeuthis]

1 hour ago, Floris Bennema said:

If yo pull your strobes back the light spreads out earlier relatively to the camera. Reducing the dark areas effect of slightly angling out.  

 

13 hours ago, Floris Bennema said:

If you angle the strobes out so that only two particles are lit in the second case, the 'dark areas' will have the same size as in the first case. The benefit of the pull back is nicely explained in Seewolf's "Getankenexperiment"

 

If this is just two particles more (or less), I will be very happy...

 

But seriously now, I think the theories presented here are very important and explain a lot... 👍

In addition to these theories and also contributing to the undoubtetly (?) positive effect of pulling strobes back on backscatter, I believe that pulling the strobes "back", e.g. so that the front of the strobes is in line with the handles (maybe even behind the handles), uses the camera housing (and also the backside of the domeport) as some kind of "shade" that creates a shadow (with repect to flashlight) just in front of the domeport, minimizing backscatter from a very sensitive space...

 

Wolfgang

 

[DreiFish]

On 5/31/2024 at 10:35 PM, Interceptor121 said:

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

 

You can also increase the reach using beam-forming devices that limit the spread of the flash. Like the Retra Reflector, which cuts the beam spread to about 60 degrees.

 

 

This is probably a more effective method for truly far subjects than trying to get longer arms.

[Guest]

1 hour ago, DreiFish said:

You can also increase the reach using beam-forming devices that limit the spread of the flash. Like the Retra Reflector, which cuts the beam spread to about 60 degrees.

 

 

This is probably a more effective method for truly far subjects than trying to get longer arms.

Those devices are not for wide angle more for a portrait of a skittish fish

 

For me the situation is very clear having tested all sort of position in open water and in a pool I can confidently say I pull the strobes back only so that the lens does not see it, this is needed only with fisheye lenses it is not needed with WWL-1 and similar and rectilinear that have best case 100 degrees horizontal field of view

Angling out the strobes servers no purpose whatsover it only creates two dark areas top and bottom of the fisheye frame which is rather easy to do as the lens has around 90 degrees vertical field of view 

If you angle the strobes only 15 degrees out the distance to subject if this was one meter away becomes 1-73 meters so a whole 0.6 stops less so it also gets weaker in the centre

It is not easy to visualise until you get your strobes and go in a pool so you can see how the light actually casts, open water you have no idea what is actually happening and you dont have a point of reference

 

Doing tests in a pool is a great way to understand how lenses, strobes and your camera really work