HSS will minimize Backscatter

[Adventurer]

„HSS will minimize Backscatter compared to a single strong pulse of the underwater flash.“

 

What do you think about this?

[TimG]

Interesting thought. Where did this come from?

[humu9679]

Assuming all variables being the same - strobe angle, conditions, distance to subject, adjusted exposure - how can that be?

[Tobyone]

What is "HSS"?

[bvanant]

High speed sync could theoretically reduce backscatter depending on many things. HSS works by using a bunch of very short pulses. If the particles that cause the backscatter move between the HSS pulses then you could imagine that BS (that's what this is by the way) might be reduced. In practice in a BW dive in quite snotty conditions, not much reduction in backscatter.

Bill

[Tobyone]

If each flash froze the one backscatter particle , wouldn't multiple flashes create more points of reflection from the individual particle?

[Klaus]

Yes, but they are dimmer in absolute terms. What counts, though, is the relative intensity when the backscatter is compared with the subject. HSS compensates the fact that at faster exposure speeds, the shutter curtains move as an open stripe from top to bottom and multiple strobes (as in stroboscopic) illuminate the sensor as the curtains travel down.

Thus:

The backscatter-causing particles and the subject will receive the same amount (relative to each other) of light in both cases - HSS and « normal » strobe. If the backscattering particles move in an HSS relevant way, i.e. extremely fast  (we’re talking about shutter speed above 1/250 nowadays), then there is too much current and you should get out of the water asap. It should also blur the particles in a similar way for the standard strobe, if not more (full dump is longer than the stroboscopic action). And apart from solid structures, your subject will likely experience the same ripping current. It’s unlikely to be a great shot…
 

I think there are more important reasons to use HSS than backscatter reduction, but in the end it’s user experience than counts - and I cannot provide that for underwater. But top-side I can attest that HSS is not the solution for when it snows 😞

 

 

[Chris Ross]

Sounds like the claim is that each pulse of HSS light is very short and if the particle moves between pulses it will receive less light.  The problem is that HSS pulses are at very high frequency in the kHz range and likely the particle won't move very far between pulses. 

 

A quick analysis, assume a 20MP m43 sensor shooting at 0.3x .  Pixel pitch is 3.3 microns and the backscatter particle needs to move one full pixel which is 3.3/0.3 = 11 microns to not appear on the same pixel at the next pulse.  Assume the HSS is operating at 20 kHz so each flash is 1/20000 = 0.00005 sec.  To move 11 micron in this time particle needs to move at 0.000011m/0.00005 sec = 0.22 m/s.  Not particularly fast, that is 0.42 knots. 

 

However the particle is probably bigger than 11  micron to be conspicuous, lets say it's 1mm across so it needs to move 1/2 a mm so it doesn't cover any of the same pixels in the next pulse, so that's 0.0005mm/0.00005 sec, so that's 10 m/sec which is 20 knots.  Any slower and it leaves a short trail as it is illuminated each time the flash pulses. 

 

This analysis is not exact of course and only gives you an idea of how quickly things need to move to be not be recorded by more than one flash pulse.  In a HSS image at 1/250 shutter speed and HSS at 20 kHz, the total time the shutter is open is 0.004 sec and at 20 kHz that's 80 flashes.  You do get a moving window of the focal plane shutter and  that means each bit of image receives only some of these pulses, but the total light must be the same if the subject is to be exposed correctly.

[Klaus]

I think you need to factor in the optics in the calculation - 11 um on the sensor is more in the water, unless you are shooting 1:1 macro. I would assume that the need for HSS occurs more frequently at the fish portait-to-reefscape scale, so then the particles need to be more like 10x these speeds?
If this really made a difference, the forum would be full of examples and our GAS would be over and done with already… 

[Chris Ross]

33 minutes ago, Klaus said:

I think you need to factor in the optics in the calculation - 11 um on the sensor is more in the water, unless you are shooting 1:1 macro. I would assume that the need for HSS occurs more frequently at the fish portait-to-reefscape scale, so then the particles need to be more like 10x these speeds?
If this really made a difference, the forum would be full of examples and our GAS would be over and done with already… 

I've already factored that in by calculating the the amount a particle would need to move at 0.3x magnification to move one pixel width on the sensor which is 3.3 microns/0.3 x = 11 microns.

[Klaus]

But for a reef scene it would be 0.03 or even less - 0.3 is 1:3 so still a close-up, right?

Whatever, those who can shall try but I seriously doubt it’s worth the effort.

[humu9679]

16 hours ago, Chris Ross said:

However the particle is probably bigger than 11  micron to be conspicuous, lets say it's 1mm across so it needs to move 1/2 a mm so it doesn't cover any of the same pixels in the next pulse, so that's 0.0005mm/0.00005 sec, so that's 10 m/sec which is 20 knots.  

Diving conditions may be terrifying to bring this theory to practical use. 

[bvanant]

Actually I was thinking of Brownian motion, of relatively small but reflective particles. Certainly particles can move on the order of a few cm/s. If the particle moves enough rotationally then you might guess that it will have different reflectivity for different orientations. But remember I did say it was BS.😁

Bill

[Adventurer]

18 hours ago, bvanant said:

Brownian motion

I think everyone’s intuition that particles or subjects have to move to emit less backscatter reflection is not mandatory.

 

If you are familiar with astro photography image stacking you might develop a good intuition on what might be enough for the initial statement to be true.

 

In HSS the light pulses are usually less strong so they will cause a less strong reflection compared to a classic single strobe pulse (equaling the quantity of light that all HSS pulses together have).

 

As the shutter travels down (line by line)

during the (less strong) pulses, less backscatter reflection is captured, compared to a single strong strobe burst, giving out all at once and not in tiny slices.

 

[Chris Ross]

7 hours ago, Adventurer said:

I think everyone’s intuition that particles or subjects have to move to emit less backscatter reflection is not mandatory.

 

If you are familiar with astro photography image stacking you might develop a good intuition on what might be enough for the initial statement to be true.

 

In HSS the light pulses are usually less strong so they will cause a less strong reflection compared to a classic single strobe pulse (equaling the quantity of light that all HSS pulses together have).

 

As the shutter travels down (line by line)

during the (less strong) pulses, less backscatter reflection is captured, compared to a single strong strobe burst, giving out all at once and not in tiny slices.

 

This is true, but the same strobe light is illuminating both the subject and the backscatter, so if the backscatter doesn't move the pixels where it is recorded receive the same relative amount of light as the subject portion that is occupying the same slice of the image where the shutter is open.    If the subject is the same brightness in the final image the backscatter must also be the same brightness.  The only way it can be less bright is if it is recorded again on a different pixel or group of pixels on the sensor.

 

Another way to think of it is that the very high frequency pulsing on and off approximates continuous lighting so the result is similar to increasing the natural light coming in albeit from different directions and coverage.

 

To take the astro photo stacking analogy, if you get a satellite trail through one frame of a 20 frame stack it's only 1/20th as bright in that stack as it was only recorded on those pixels in that frame.  But you don't get that in backscatter unless the particle moves enough to not be recorded again on those pixels in the next frame.

[Adventurer]

In HSS we are not in the long exposure realm as with astro satellites moving across the frame.

 

We are in a very fast space of collecting light, as a reference look at:

 

sensor read speeds

HSS pulse frequencies

sensor size

pixel pitch on sensor 

derived line read speed

 

Also consider that there are two dimensions for a strobe to be bright*:

 

the timescale

the peak energy output

 

* the strobes total brightness is the integral under that curve.

 

With „normal“ relatively long exposures and classical max x-sync speeds of 1/200sec or 1/160sec strobes may have a loooooong burntime to archive a high guide number.

In HSS all that is different.

[Chris Ross]

On 2/2/2025 at 1:50 PM, Adventurer said:

In HSS we are not in the long exposure realm as with astro satellites moving across the frame.

 

We are in a very fast space of collecting light, as a reference look at:

 

sensor read speeds

HSS pulse frequencies

sensor size

pixel pitch on sensor 

derived line read speed

 

Also consider that there are two dimensions for a strobe to be bright*:

 

the timescale

the peak energy output

 

* the strobes total brightness is the integral under that curve.

 

With „normal“ relatively long exposures and classical max x-sync speeds of 1/200sec or 1/160sec strobes may have a loooooong burntime to archive a high guide number.

In HSS all that is different.

I'm sorry I have no idea what you are talking about.  In HSS each pulse illuminates the subject and the backscatter particle with the same intensity of light.  If the particle moves to a different set of pixels between pulses it won't be as bright as the pixels where it used to be don't get more light.  If it stays on the same pixels it gets the same amount of light as the subject and is just as bright as it would be in a single exposure that provides the same exposure on the subject.  

 

The pulses may be short and have less light but they have to add up the same amount on your subject for the exposure to be the same.  This is achieved by using more of the stored energy in the capacitor and/or wider aperture and/or higher ISO. 

[Adventurer]

Chris there is a threshold when different matter (objects) in this universe start to reflect.

https://www.researchgate.net/publication/367009373_Threshold_When_Does_a_Reflection_Become_Noticeable

 

When there is water column between subject and light source -> this energy absorbing effect might be amplified.

[Klaus]

Maybe it helps to look at this from a different angle. What the HSS pulses achieve is in essence to turn a strobe into a constant light source like an LED torch. (This is a simplification but bottom line that‘s it for still photography.) Any BS reduction due to particle movement should thus be observable using video lights as well.

For macro that is indeed a valid option - and the longer the shutter speed, the more pronounced the effect should be, provided the subject stays rock-still. 
So, has anyone observed less backscatter when using video lights for macro shots compared with strobes?
I honestly don‘t know, I‘ve never read anything along those lines.

[Klaus]

1 hour ago, Adventurer said:

 

When there is water column between subject and light source -> this energy absorbing effect might be amplified.

That certainly works when you are using a snoot, but HSS?

Seems unlikely. There‘s just no difference between 1 and 10x 0.1 if summed up. And if 0.1 is below the detection threshold each time due to absorption, then that is the same for the subject. Commonly, we refer to this as underexposed.
The only chance would be to have the particles move - then the subject may get 10x 0.1 but the particles will only leave a trail of, say, 2x 0.1 on each pixel. Waterpixel, to be precise 🙂

But is it true that backscatter manifests itself as streaks of lower intensity when one uses video lights to illuminate a still? 

 

[Chris Ross]

4 hours ago, Adventurer said:

Chris there is a threshold when different matter (objects) in this universe start to reflect.

https://www.researchgate.net/publication/367009373_Threshold_When_Does_a_Reflection_Become_Noticeable

 

When there is water column between subject and light source -> this energy absorbing effect might be amplified.

This is talking about human vision and it doesn't apply to pixels they store every photon they receive.  This threshold refers to enough contrast that human vision can say it's a different shade or brightness.  The object still reflects the light it just can't be detected against the background brightness - a signal to noise issue.

 

If you can pick up the object as backscatter with a single pulse shot and it stays on the same pixel then you will also see it with HSS.  I estimated that a 1/250 sec exposure sees something like 80 pulses and the object needs to move very rapidly to get to an entirely new set of pixels.  If a particle sees only one pulse it is about 6 stops under exposed roughly speaking.

 

Regarding video lights they are usually dimmed by a similar mechanism to HSS, rapid pulsing, so 50% strength comes from the light being powered on about 50% of the time, the cycling is rapid enough we can't see but can be a problem with video causing flicker. 

[Adventurer]

14 hours ago, Klaus said:

Any BS reduction due to particle movement should thus be observable using video lights as well.

 

I think we can rule out the particle movement thing (left to right etc.) as things happen so ultra-fast, when getting strobed.
Maybe electron rotation will do it's cause but not bigger molecules moving through the frame.

[Adventurer]

14 hours ago, Klaus said:

So, has anyone observed less backscatter when using video lights for macro shots compared with strobes?
I honestly don‘t know, I‘ve never read anything along those lines.

Yes Klaus, actually that is an experience I have had for many years.

There can be backscatter in video clips but actually is not as strong as the strobe.

 

And while you mention that:

In a really dirty conditioned water, when I take two shots and I manually ramp up my strobes.

The backscatter discs may very often appear larger when you fire at them with more strobe power.

[Architeuthis]

On 1/10/2025 at 1:21 PM, Adventurer said:

„HSS will minimize Backscatter compared to a single strong pulse of the underwater flash.“

 

What do you think about this?

 

 

Who ever has come to this conclusions and what are the logical arguments behind it? Is there a single practical evidence on a real photo?

 

HSS does not mean that a distinct region on the sensor is repeatedly exposed by the pulse of flashes, produced in HSS mode. Every single pulse of the strobe in HSS mode exposes a single, small region on the sensor and only once. The different regions of the sensor are exposed sequentially in order to achieve a shutter speed that is shorter than the "regular" flash syncronization speed, as the shutter is not able to syncronize the entire sensor region at once at a speed higher than sync speed (exceptionally fast sync speed is achieved via global shutter, as Sony A9III)...

This means a single particle producing backscatter is exposed only once, is it in HSS or in "normal" mode. A photo produced in HSS mode should be identical to a photo produced by a single flash, when the flash is set to low power (providing short flash duration equivalent to a single HSS flash pulse)...

 

=> It is not visible to me how sequential flash exposure of different and consecutive regions of the sensor should give a different backscatter pattern, when compared to an exposure produced with a single flash...

 

 

Wolfgang

Edited February 10 by Architeuthis

[Architeuthis]

25 minutes ago, Architeuthis said:

 

 

Who ever has come to this conclusions and what are the logical arguments behind it? Is there a single practical evidence on a real photo?

 

HSS does not mean that a distinct region on the sensor is repeatedly exposed by the pulse of flashes, produced in HSS mode. Every single pulse of the strobe in HSS mode exposes a single, small region on the sensor and only once. The different regions of the sensor are exposed sequentially in order to achieve a shutter speed that is shorter than the "regular" flash syncronization speed, as the shutter is not able to syncronize the entire sensor region at once at a speed higher than sync speed (exceptionally fast sync speed is achieved via global shutter, as Sony A9III)...

This means a single particle producing backscatter is exposed only once, is it in HSS or in "normal" mode. A photo produced in HSS mode should be identical to a photo produced by a single flash, when the flash is set to low power (providing short flash duration)...

 

=> It is not visible to me how sequential flash exposure of different and consecutive regions of the sensor should give a different backscatter pattern, when compared to an exposure produced with a single flash...

 

 

Wolfgang