Measuring HSS Flash With A Meter

[rpcrowe]

Please tell me if you think that my brain is in gear correctly regarding this supposition…

Given:

The Godox (family) XPro trigger has a function labeled TCM which will convert the flash value in the TTL mode to the power output value in the M mode.

If I have my camera set in manual mode to a certain shutter speed and a certain f/stop and if I have my flash set to TTL; then firing the camera should cause the flash to provide the correct amount of power to correctly expose the image at that f/stop and shutter speed regardless if HSS is required for the image parameters or not.

Most flash exposure meters will not accurately measure HSS flash power. The exception to that rule is the latest and most expensive of the Sekonic meters.

Supposition:

Converting the TTL values with the TCM function should give me the flash power setting to provide the correct exposure I have dialed into my camera. Therefore if I had my camera set for 1/500 second at f/8 using HSS and my subject was ten feet from my flash and if using the TCM function my flash power reading was 1/64 power; I could interpolate that the flash at 1/64 power would provide an exposure of f/8 at 10 feet using HSS.

Shooting like this at various f/stops and at various distances and measuring the flash power at those distances, f/stops and shutter speeds with my present exposure meter, could I not develop a correction curve to interface between what the flash is actually producing when using HSS and what my meter is reading…

I know that I could make up a chart that would give me the correct f/stop for distances and flash power. I have seen a YouTube video in which a chart like this was used to good effect in determining the exposure with flash used in HSS. I don’t know how that chart was developed but think that my system should help me develop such a chart.

[george013]

Please tell me if you think that my brain is in gear correctly regarding this supposition…

Given:

The Godox (family) XPro trigger has a function labeled TCM which will convert the flash value in the TTL mode to the power output value in the M mode.

If I have my camera set in manual mode to a certain shutter speed and a certain f/stop and if I have my flash set to TTL; then firing the camera should cause the flash to provide the correct amount of power to correctly expose the image at that f/stop and shutter speed regardless if HSS is required for the image parameters or not.

Most flash exposure meters will not accurately measure HSS flash power. The exception to that rule is the latest and most expensive of the Sekonic meters.

Supposition:

Converting the TTL values with the TCM function should give me the flash power setting to provide the correct exposure I have dialed into my camera. Therefore if I had my camera set for 1/500 second at f/8 using HSS and my subject was ten feet from my flash and if using the TCM function my flash power reading was 1/64 power; I could interpolate that the flash at 1/64 power would provide an exposure of f/8 at 10 feet using HSS.

Shooting like this at various f/stops and at various distances and measuring the flash power at those distances, f/stops and shutter speeds with my present exposure meter, could I not develop a correction curve to interface between what the flash is actually producing when using HSS and what my meter is reading…

I know that I could make up a chart that would give me the correct f/stop for distances and flash power. I have seen a YouTube video in which a chart like this was used to good effect in determining the exposure with flash used in HSS. I don’t know how that chart was developed but think that my system should help me develop such a chart.

Just a start. Nobody knows the flash value in TTL mode. It's determined "on the fly". In case of Nikon DSLR using a pre-flash.

George

[Manfred M]

Richard - there are two issues, not just one that have to be understood in doing what you are attempting to do. At HSS what the flash is effectively doing is acting as a continuous light source for the period of exposure.

1. The actual output of the flash meter when at different shutter speeds. I suspect it is not going to be linear as the flash's capacitor will have a limit as to how much charge can be stored. I would expect this to be lower at maximum and high power levels and lower at smaller power levels.

2. The higher the shutter speed, the smaller the slit left open to expose the light from the flash to the sensor, the greater the amount of light required to make the exposure.

In a lab, with the appropriate test gear, it should be possible to measure both of these variables and come up with what you are looking to do. Your proposed approach only addresses one of the variables in HSS with your Godox unit.

[apersson850]

In theory, it sounds doable. But in reality, with today's digital cameras, isn't it easier to make a few exposures and check them out instead?

Regarding the doability of your project:
HSS works by emitting several flashes during the exposure. It's easy to envision, if you think about an exposure time which renders the shutter slit half of the sensor size. That is, if flash sync is at 1/250 s, we use 1/500 s.
In a perfect world, the flash could then flash twice. Once to lit the upper half of the image, and once again to lit the lower. Each of these two flashes would be identical to the single flash required at 1/250 s. It's just that at 1/500 s, half of the light in each flash is wasted on that part of the image which isn't exposed at that moment.

So a flash meter, which is clever enough to measure only the first of the two flashes, would get the output right. But one that integrates the two together, would find the light twice as powerful.

However, in reality the flash can't fire just two flashes, since then the slightest bad luck with tolerances would render a black stripe across the center of the frame. Or a twice as bright band, if tolerances are on the other side.
So the flash has to fire a multitude of times, each a fraction of what's needed for the proper exposure of that part of the image. A separate flash meter can't measure that output unless it also knows for how long a certain part of the subject is exposed to the sensor. That is, how long the shutter slit passes the part of the image the meter is measuring.

Hence the flash has to fire at twice the rate if the exposure time is cut to half, or, if the rate of flashing is the same, with twice as high power each time. But to the image sensor, it all looks the same.

[george013]

In theory, it sounds doable. But in reality, with today's digital cameras, isn't it easier to make a few exposures and check them out instead?

Regarding the doability of your project:
HSS works by emitting several flashes during the exposure. It's easy to envision, if you think about an exposure time which renders the shutter slit half of the sensor size. That is, if flash sync is at 1/250 s, we use 1/500 s.
In a perfect world, the flash could then flash twice. Once to lit the upper half of the image, and once again to lit the lower. Each of these two flashes would be identical to the single flash required at 1/250 s. It's just that at 1/500 s, half of the light in each flash is wasted on that part of the image which isn't exposed at that moment.

So a flash meter, which is clever enough to measure only the first of the two flashes, would get the output right. But one that integrates the two together, would find the light twice as powerful.

However, in reality the flash can't fire just two flashes, since then the slightest bad luck with tolerances would render a black stripe across the center of the frame. Or a twice as bright band, if tolerances are on the other side.
So the flash has to fire a multitude of times, each a fraction of what's needed for the proper exposure of that part of the image. A separate flash meter can't measure that output unless it also knows for how long a certain part of the subject is exposed to the sensor. That is, how long the shutter slit passes the part of the image the meter is measuring.

Hence the flash has to fire at twice the rate if the exposure time is cut to half, or, if the rate of flashing is the same, with twice as high power each time. But to the image sensor, it all looks the same.

Global one could say that the guide number is corrected with a factor shutterspeed/sync speed. If the sync speed is 1/250 and the shutterspeed is 1/800 the correction factor will be (1/800)/(1/250) or 250/800. A guide number of 40 will become 12.5. I never checked it so maybe wrong somewhere.

But another point is that Richard wants to recalculate the flash values from TTL. That's impossible since no one knows what these will be. The calculations above works only for manual.

George

[Manfred M]

In theory, it sounds doable. But in reality, with today's digital cameras, isn't it easier to make a few exposures and check them out instead?

Regarding the doability of your project:
HSS works by emitting several flashes during the exposure. It's easy to envision, if you think about an exposure time which renders the shutter slit half of the sensor size. That is, if flash sync is at 1/250 s, we use 1/500 s.
In a perfect world, the flash could then flash twice. Once to lit the upper half of the image, and once again to lit the lower. Each of these two flashes would be identical to the single flash required at 1/250 s. It's just that at 1/500 s, half of the light in each flash is wasted on that part of the image which isn't exposed at that moment.

So a flash meter, which is clever enough to measure only the first of the two flashes, would get the output right. But one that integrates the two together, would find the light twice as powerful.

However, in reality the flash can't fire just two flashes, since then the slightest bad luck with tolerances would render a black stripe across the center of the frame. Or a twice as bright band, if tolerances are on the other side.
So the flash has to fire a multitude of times, each a fraction of what's needed for the proper exposure of that part of the image. A separate flash meter can't measure that output unless it also knows for how long a certain part of the subject is exposed to the sensor. That is, how long the shutter slit passes the part of the image the meter is measuring.

Hence the flash has to fire at twice the rate if the exposure time is cut to half, or, if the rate of flashing is the same, with twice as high power each time. But to the image sensor, it all looks the same.

All good points, but if you look at the Sekonic L-858D-U, it does exactly what you suggest is not possible. It has been designed to provide light readings for HSS flash. Even my entry level (and now discontinued) Sekonic L-358 was able to measure the impact of both flash and ambient light on a subject and give me the percentage that each light source was contributing to the image, which can be very useful in a mixed-light setup.

https://www.sekonic.com/united-state.../overview.aspx