Strobes & Diffusers Myths and Legends

Last night I went to the pool with a rectilinear lens to test a dome but also to check on strobes.

The following three shots are a Sea and Sea YS-D2J without diffuser with 100 diffuser and with 120 diffuser but higher ISO to compensate additional light loss all shots at 1/250 f/8

From what I could see the strobe without the diffuser looks like the bulb basically! And I cannot see any difference between 100 and 120 degrees

I then run the images into a false color monitor sorry for phone pictures. The last shot I raised ISO to 200 to compensate for the diffuser loss. If you are not familiar with false color light gray is 50% green is 45 dark grey is 30-40 blue 20 to 10 purple zero

So without the diffuser it looks pretty uneven but still cover the vertical axis around 75 degrees. With diffuser without raising power it becomes smoother, raising the power to compensate for the absorption gets to 90 degrees 50% centre 25 border between blue and dark grey. So in essence the diffuser makes the ugly shape of the bulb a nice round emitter that at 1 meter distance looks like a point radiant following the inverse square law

It is also interesting that I could not notice any difference between 100 and 120 whilst I could see that on land and that even without diffuser the angle of coverage does not drop like a lens but stays pretty much unchanged

When I do the same with video light I can see a distinct drop of angle of coverage with strobes it seems the energy transmits to the water particles and generates a scattering especially when the diffuser is on and the angle of coverage seems to increase even compared to air which is rather weird. In conclusion I have never felt the need of strobes with round tubes, dome shapes etc because it looks like a simple tube powerful enough combined with a thick enough piece of opal plastic works wonders

Is there something I am missing?

Contrary to popular belief a diffuser does not increase the angle of light by shifting light to the edges of the beam. Light can only be shifted through optics, diffusers are much "dumber" devices.

Diffusers are opaque and they inadvertently decrease the power and by decreasing power where it's abundant a more even distribution across the whole angle can be seen. This evenness is then translated into more "angle" because with a normal exposure in the center we get a less steep fall off on the edges BUT the overall light output is decreased as a result. 

What a diffusers actually does, and this is not advertised nearly as much, is that it softens the light by making the shadows less hard. This softening is usually what we are pleased to see.

Straight flash tubes have advantages and disadvantages. Straight flash tubes are limited as they can not take as much energy because of their compact shape. Some manufacturers take two straight tubes but even like that a power of 200 Ws or more is not possible to handle only with a reasonable amount of straight tubes. Their advantage is the before mentioned compactness and also price point to some extent - they are cheaper. 

Circular flash tubes also have advantages and disadvantages. Circular tubes are usually bigger and require a more powerful capacitor to get a similar amount of light on an area that straight tubes would cover with ease. This is because by default a circular tube will cover a larger area and thus spend its energy on a wider angle. The advantage being that they can handle more power and as default their light spread is more evenly distributed.

1 hour ago, Jerry Diver said:

Contrary to popular belief a diffuser does not increase the angle of light by shifting light to the edges of the beam. Light can only be shifted through optics, diffusers are much "dumber" devices.

Diffusers are opaque and they inadvertently decrease the power and by decreasing power where it's abundant a more even distribution across the whole angle can be seen. This evenness is then translated into more "angle" because with a normal exposure in the center we get a less steep fall off on the edges BUT the overall light output is decreased as a result. 

What a diffusers actually does, and this is not advertised nearly as much, is that it softens the light by making the shadows less hard. This softening is usually what we are pleased to see.

Straight flash tubes have advantages and disadvantages. Straight flash tubes are limited as they can not take as much energy because of their compact shape. Some manufacturers take two straight tubes but even like that a power of 200 Ws or more is not possible to handle only with a reasonable amount of straight tubes. Their advantage is the before mentioned compactness and also price point to some extent - they are cheaper. 

Circular flash tubes also have advantages and disadvantages. Circular tubes are usually bigger and require a more powerful capacitor to get a similar amount of light on an area that straight tubes would cover with ease. This is because by default a circular tube will cover a larger area and thus spend its energy on a wider angle. The advantage being that they can handle more power and as default their light spread is more evenly distributed.

A collimated ray of light can only be widened through a lens but this is nothing to do with how diffusers work because light is not just waves but also particles.

A diffuser is a radiating surface that accumulates light and then distributes it on a different shape so it does increase angle of coverage it is the primary mean for shops to create nice lighting when you go and try your clothes etc the light loss on a straight line depends on the fact the radiant body need to store the energy before releasing it and then this is diffused instead of being concentrated and some of this is lost by collisions within the diffuser body. Without diffusers you would have areas that are illuminated and others that are dark.

Diffusers tend to distribute at wider angle at expense of more energy retained so a diffuser on a strobe that is marked 100 or 120 is actually marked as 50 or 60 normally on land. 

This does not take into any effect the scattering of light in water that is not air and behaves differently which is the point I think is emerging from the shots and the false colour. Light is transmitted in air but scattered in water due to the fact you have particles, there are of course particles in air but the density is very little

So an opal diffuser with 50% transmission will require double the power and will increase the angle of coverage diffusing the light. 

From the shots you can also see that even without a diffuser the drop of angle of coverage of the strobe is not as sharp as you would expect with a ray of light coming long distance from the sun

if you look at the shots on the wall the strobe with the diffuser on is very similar to an ideal single point light and the fact that if looses intensity past 90 degrees is due to inverse square law not to the angle. In fact no strobes can cover a wall at 45 degrees incident angle without suffering a loss of a stop. As you go further away this becomes 3 stops that is 25% of the original intensity practically zero as you did not throw 100% in the centre

For those interested in physics a diffuser approximates a lambertian radiant so it does increase the angle of a collimated light

https://en.wikipedia.org/wiki/Lambert's_cosine_law

I stand corrected, there are small gains on the edges but this can happen only when a light source is very focused like on the YS-D2. Basically what is going on there is that a diffuser becomes the new light source. But a diffuser is not ideal for distributing the light output and will loose energy while doing so, especially at the center. I did a test with my old YS-D1 and I got about 0,2 F-stops more on the edge while the center beam was reduced by 0,8 F-stops.

If you take a circular tube with a good reflector (not the Ikelite, try Seacam or Retra) you will not gain or loose much in light output by adding a diffuser. Tested with the new Retra and their white diffuser I got practically the same exposure everywhere with only about 0,1 F-stops reduction. Their beam is already spread around and the only gains are with softer light output as mentioned before.

OK so what you are saying is that if you have a circular shape and well designed reflector you may have a smoother shape of light with the shape of the elements not showing? But I doubt that you will have zero hotspots unless the surface of the strobe is already diffusing instead of being clear. Looking at the design of retra you can see that it actually has an inbuilt diffuser inside the front around the element similar to keldan lights. When you submerge that in water however it will loose the field of coverage with that design so most likely it will make a difference to use a diffuser then. I am unsure and I do not have a retra strobe but this would need testing and not on land as air responds differently to scattering.

With regards to your statement of light intensity loss at centre that is irrelevant the strobe once it has a diffuser will distribute with the diffuser radiation curve that typically is following inverse square law as shown by the false color diagram. It is very smooth and has no hotspots otherwise those would show.

Coming back to my sea and sea or inon or any linear tube the images show that once you have a diffuser in water it works like a single light emitting radians following the inverse square law so I don't care I have a linear tube because I still have GN24 left and that is a lot of power. I have shots with two strobes and I can cover the frame of a fisheye lens except extreme corners but that is due to inverse square law not to the strobe itself on a flat surface.

The point of my post is that with a diffuser and a single bulb you get excellent light distribution with low cost

32 minutes ago, Interceptor121 said:

OK so what you are saying is that if you have a circular shape and well designed reflector you may have a smoother shape of light with the shape of the elements not showing? But I doubt that you will have zero hotspots unless the surface of the strobe is already diffusing instead of being clear. 

Yes, that is correct. No doubts needed, there are no hotspots with the retra or the seacam and both of them have a clear dome and no diffuser. Have a look at the test shots here: https://www.retra-uwt.com/pages/flashgun-light-comparison

The resulting images of the light beam are practically the same as the above shots from the pool. 

I see they've added another graph on the bottom that shows the full power output. Inon and sea&sea have an advantage in the center but everywhere else they drop about 1 F-stop. Even if the diffuser gets back 0,5 F-stops, which it does not, there is much less power.

Especially when we look at the first graph where the distribution is made with normal exposure in the center as we would have it when taking a picture. So in reality the drop off is around 2 F-stops!

50 minutes ago, Interceptor121 said:

The point of my post is that with a diffuser and a single bulb you get excellent light distribution with low cost

Straight tubes have their advantages but when it comes to pure light output performance and quality we need a circular flash tube, sorry.

43 minutes ago, Jerry Diver said:

Yes, that is correct. No doubts needed, there are no hotspots with the retra or the seacam and both of them have a clear dome and no diffuser. Have a look at the test shots here: https://www.retra-uwt.com/pages/flashgun-light-comparison

The resulting images of the light beam are practically the same as the above shots from the pool. 

I see they've added another graph on the bottom that shows the full power output. Inon and sea&sea have an advantage in the center but everywhere else they drop about 1 F-stop. Even if the diffuser gets back 0,5 F-stops, which it does not, there is much less power.

Especially when we look at the first graph where the distribution is made with normal exposure in the center as we would have it when taking a picture. So in reality the drop off is around 2 F-stops!

Straight tubes have their advantages but when it comes to pure light output performance and quality we need a circular flash tube, sorry.

The retra diagrams are misleading for three key reasons:

1. Tests are performed on land which is not where you will use the strobes

2. You can't make comparison on beam angle shooting a flat surface and not net out inverse square law. The G area will always loose a stop because it is further away from the center. So the graphs are a mixed bag

3. The test graphs are without diffusers that may not make a difference on land for Retra due to design but will make a difference to other strobes and the gap will close further in water

So what really happens when you have a linear tube with a diffuser is that at 90 degrees or green zone you loose two stops in air, of which 1 is due to inverse square law and 1 is due to the diffuser strobe combo this is confirmed by independent tests from backscatter and other sources (with diffuser)

What my shots show is that the centre grey area is at 50% IRE and the edge that in that lens is 95 degrees is 20% blue area So the light loss in water is roughly 50-60% or 1-1.3 Ev 

On land that value is 2 Ev so somehow the scattering effect of water combined with the diffuser is further helping the angle of coverage of the strobe and softening further the beam. This also explains why I see no benefit with a 120 land diffuser

I have not found any academic work to back this up as nobody is really interested in this but that is the reason I was sharing.

1 hour ago, Interceptor121 said:

The retra diagrams are misleading for three key reasons:

1. Tests are performed on land which is not where you will use the strobes

2. You can't make comparison on beam angle shooting a flat surface and not net out inverse square law. The G area will always loose a stop because it is further away from the center. So the graphs are a mixed bag

3. The test graphs are without diffusers that may not make a difference on land for Retra due to design but will make a difference to other strobes and the gap will close further in water

I trust a test done at equal conditions no matter what the distances and medium are, just as long as they are the same for every strobe measured. In water there is a lot more trouble setting up a precise testing environment and thus the results will inadvertently vary more.

The water/air brake point is practically zeroed out because all strobes use domes. Only a completely flat port would noticeably reduce the angle of light coming out. No strobe has a flat port. 

We can observe images done with and without diffusers to get an idea about coverage. 

Finally, if someone who is making more than 300 photo dives per year and has abundant knowledge on underwater photography equipment tells us that the results coming from retra's tests are consistent with his experience shooting the same strobes underwater, there is no reason to dismiss the results or to say that they are misleading.

By the way, at some point I used almost all strobes in the test and can confirm they represent what I saw under water as well. The advantage of a wider light spread being that I can be much more relaxed about positioning because the width of the light beam will automatically correct any mistakes I made. 

Cheers 

That's not true only Inon have domes the other are similar to a flat surface. So the interface will matter

I have done test on land and in water and the behaviour is different an open water scenes on a non flat surface is not a test bench

And yes those diagrams are at best irrelevant and worst misleading this without even going into the details of the comparison. There is absolutely no point to look beyond 90 degrees because by inverse square low you are 1.6 Ev less already so in real life the whole curve past the first green point is totally zero value to anyone

What matters are the centre and the values at 30 60 90 degrees as per backscatter method as those constitute 95% of your frame who cares about the edges where you already have 1/3 power just because of inverse square low?

In addition to that strobes for wide angle need to be considered as a system as you have two radiating bodies not one so in the centre area the strobes overlap and a single strobe diagram becomes irrelevant anyway

I am every day amazed by the lack of rigour in any test and I am not at all surprised nobody goes and does test in a pool where the reality shows!

Two shots with a 8-18mm lens field of view 108 degrees diagonal, not a fisheye as I wanted straight lines

There were millions of bubbles in the pool sticking to my dome anyway the first is at around 3 meters distance at half power the second is around 1 meter

You can see that even at the edge is well exposed and drops to 30 only in the extreme corners

If I had to apply the reasoning of the retra website on the shot with the single strobe I could have not even reached the vertical limit of the frame and the power had to 25% so as I had 50% IRE in center this would have been 12.5 blue instead is solid in the black which means it is actually only one stop away so something is happening with the particles in the water AND the retra graph was measured for the strobe I have without diffuser which is not the way anybody uses those strobes

1 hour ago, Interceptor121 said:

That's not true only Inon have domes the other are similar to a flat surface. So the interface will matter

Retras have a dome front element.

And aren't Inon Z240s flat?

25 minutes ago, Barmaglot said:

Retras have a dome front element.

Am sorry to disappoint everyone but in order for a dome to conserve field of view the centre of curvature needs to be where the light is.

In the case of Inon Z330 this is true the surface if a full dome and the centre is where the tubes are however they have two elements so this will give somehow a weird result and most likely the shape of the tubes will show without a diffuser so diffuser is mandatory in that case

All the other strobes with hemispheres with the emitting element close to the surface will act as a (bent) flat interface and the beam will narrow in water. It is like a poorly designed dome port when the lens is close to the glass it looks flat

I do like the design with the diffusing element in the strobe of retra that will help more than the shape of the front of the strobe 

I test all my equipment in a pool repeating the same drills and I know very much what to expect. I wanted to share my experience as this has come up first time last year by chance, I had always taken for good what I read on the internet or heard from others but the I realised there is not a single test of strobes underwater in a pool. Tests on a reef do not constitute a repeatable environment 

By the way I do not care about retra etc etc. I just wanted to share my real life measured experience but then I got shot down with observations that are incorrect and in my nature I don't just accept them sorry