HSS will minimize Backscatter

[Chris Ross]

13 hours 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 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

I think you will find this will depend on the shutter speed.  at very high shutter speeds like 1/4000 sec the band crossing the sensor is quite narrow.  Most often in UW work the SS will be in a 1/320 - 1/500 range and all you are trying to achieve is prevent the black band in the image and the opening that moves takes up a large portion of the sensor and everything receives multiple flashes  I suspect even at the highest shutter speeds it might receive 5-10 pulses or nore depending on pulse frequency and speed of the shutter curtain.

[Architeuthis]

4 hours ago, Chris Ross said:

I think you will find this will depend on the shutter speed.  at very high shutter speeds like 1/4000 sec the band crossing the sensor is quite narrow.  Most often in UW work the SS will be in a 1/320 - 1/500 range and all you are trying to achieve is prevent the black band in the image and the opening that moves takes up a large portion of the sensor and everything receives multiple flashes  I suspect even at the highest shutter speeds it might receive 5-10 pulses or nore depending on pulse frequency and speed of the shutter curtain.

 

I find it difficult to find in the Internet a description how, exactly, HSS is working. Therefore my argumentation may have faults, but I can see none of them so far...

According to what I was able to find out about HSS just now, the pulse frequency in HSS is mode is very high, up to 100 kHz, what makes the individual pulses merge and causing the flash to provide a single pulse, with, more or less, constant (but a little fluctuating) intensity that lasts for the duration of the entire exposure (=much longer than the shutter speed adjusted).

 

=> This would mean that in HSS mode the individual pixels are exposed to flashlight for exactly the time that is adjusted via the shutter speed. i.e. a single pulse ...

[Chris Ross]

On 2/11/2025 at 6:58 PM, Architeuthis said:

 

I find it difficult to find in the Internet a description how, exactly, HSS is working. Therefore my argumentation may have faults, but I can see none of them so far...

According to what I was able to find out about HSS just now, the pulse frequency in HSS is mode is very high, up to 100 kHz, what makes the individual pulses merge and causing the flash to provide a single pulse, with, more or less, constant (but a little fluctuating) intensity that lasts for the duration of the entire exposure (=much longer than the shutter speed adjusted).

 

=> This would mean that in HSS mode the individual pixels are exposed to flashlight for exactly the time that is adjusted via the shutter speed. i.e. a single pulse ...

HSS works by imitating a continuous pulse.  The capacitor can generally only do  a limited pulse length and if just fed through the normal circuit for longer would run out of charge.  So it pulses and this has to be like a duty cycle controller to reduce the current drawn and so enable the pulse to run for the entire time the shutter is open.

 

With the high frequency each line of pixels receives multiple pulses as the shutter curtain travels upwards, as the shutter curtain window narrows with faster shutter speeds each line of pixels progressively sees less pulses this is why power goes down each pixel sees a lesser number of pulses as they are uncovered for less time.  For example if the frequency is 100 kHz, then there will be 100,000 x 1/320 = 312 pulses on every pixel.

 

[Architeuthis]

1 hour ago, Chris Ross said:

HSS works by imitating a continuous pulse.  The capacitor can generally only do  a limited pulse length and if just fed through the normal circuit for longer would run out of charge.  So it pulses and this has to be like a duty cycle controller to reduce the current drawn and so enable the pulse to run for the entire time the shutter is open.

 

With the high frequency each line of pixels receives multiple pulses as the shutter curtain travels upwards, as the shutter curtain window narrows with faster shutter speeds each line of pixels progressively sees less pulses this is why power goes down each pixel sees a lesser number of pulses as they are uncovered for less time.  For example if the frequency is 100 kHz, then there will be 100,000 x 1/320 = 312 pulses on every pixel.

 

 

The pulses, I guess, refer to electricity. The gas glowing in the flashtube will not stop emitting light inbetween pulses, maybe light intensity jitters a little at these extremely high frequency (remember the (rather slow) exponential decay of light intensity after a single pulse, shown sometimes here by our flash engineeres, who have the tools to record light intensity in time at high resolution))?

 

In the end, a particle producing backscatter is illuminated by a flash pulse that lasts for 1/320s in HSS mode in your example. This compares to illumination by a flash pulse, lasting a little shorter than maximum shutter sync speed (depending on flash type and light strength selected), in normal flash mode. So the end result is expected to be pretty much the same...

Edited February 12 by Architeuthis

[Chris Ross]

1 hour ago, Architeuthis said:

 

The pulses, I guess, refer to electricity. The gas glowing in the flashtube will not stop emitting light inbetween pulses, maybe light intensity jitters a little at these extremely high frequency (remember the (rather slow) exponential decay of light intensity after a single pulse, shown sometimes here by our flash engineers, who have the tools to record light intensity in time at high resolution))?

 

In the end, a particle producing backscatter is illuminated by a flash pulse that lasts for 1/320s in HSS mode in your example. This compares to illumination by a flash pulse, lasting a little shorter than maximum shutter sync speed (depending on flash type and light strength selected), in normal flash mode. So the end result is expected to be pretty much the same...

Exactly.  You might get a slight variation in light output with each pulse but effectively it stays on, just the pulsing reduces and spreads out the capacitor so it can stay on longer.  I would guess that HSS sync capable flashes mostly work by pulsing the capacitor for as long as they receive light from the trigger (rather than trying to exactly match the master flash out put exactly.

[Adventurer]

6 hours ago, Chris Ross said:

I would guess that HSS sync capable flashes mostly work by pulsing the capacitor for as long as they receive light from the trigger

Well,.. HSS implementation seems to differ quite a bit depending on the strobes used.

If you consult the Backscatter HF-1 manual, it says that the HSS mode always delivers same power output. This also means the picture will darken with increased shutter speeds heading towards 1/8000.

Contrary to Marelux Apollo III 2.0 when in M-HSS mode you can use the strength dial and influence something.

[Klaus]

From what I know of HSS in dry-land you will always want the maximum output - or more still. Turning it down may be technically feasible (Marelux?) but rarely useful.

Back to the original question - backscatter: Since we agree that HSSis essentially the same as using an LED torch, does anyone have comparative experience between strobe and LED-light for the white thingies?

 

Edited Monday at 09:06 PM by Klaus
Typo

[Adventurer]

7 minutes ago, Klaus said:

Since we agree that HSSis essentially the same as using an LED torch,

No, we certainly do not agree to this.

 

I have not seen an an led torch which was able to out-compete strobes (even when they are in HSS mode).

 

And I have seen and used 20.000 lumen video lights validated in an Ulbricht sphere.