Flash Power Question

LC2

LC2

Negan
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Tim
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Something I've noticed with both my flash guns (Godox TT350F & TT685F).
When in TTL, the flash is able to be brighter than when using manual settings.

Scenario.
  • Indoors or outdoors doesn't matter
  • ISO set manually (generally 100)
  • Aperture set manually
  • Shutter Speed set manually - normally the max sync speed of 1/250th (but also observed at 1/125th to).
  • In Camera flash settings left as defaults.
  • Observed on both the X-T2 & X-T3
  • Flash on camera, pointed Up (not forward) into an fstoppers flash disc to diffuse the flash (I'm running and gunning at conventions, rather than in a studio set-up, having a softbox on a stand is not normally an option (think 50k+ stuffed into the half of the ground floor at the Excel)).

  • Flash set to 1/1 power (max on the Godoxs) and depending on the distance from of the flash from the subject it may be underexposed.
  • If it is underexposed, keep the distance the same and swap to TTL, where you can go +3 from "correct" exposure, the image can often be exposed brighter (distance is still important, but...)

I will quite often use this trick when manual does give enough light, but... I would rather be able to get this extra light in manual as TTL can be tricked (reflective surfaces etc.)

I've noticed that the High Speed Sync icon is showing on the flash gun, but I'm staying below the sync speed (and I thought HSS reduced power).

Any idea what's happening / what I've got set up strangely / and what I could change to enable this "extra" exposure in manual?
 
Shouldn't be possible. Just guessing, but possibilities might include change of distance and shutter speed (possibly TTL related, or working in aperture-priority).

At close range, just a few inches change of distance can make a noticeable difference. The other thing is shutter speed, because at full power the flash duration will be around 1/300th sec and if you're at 1/250sec shutter speed the shutter will only be fully open for about half that time, ie the sensor will not be completely uncovered long enough* to capture the full flash pulse.

*Edit: something that seems to get overlooked these days when shooting at max x-sync outdoors, and applies to every focal-plane shuttered camera. Broadly speaking, the shutter curtains take about 3ms to travel from top to bottom of the frame (typical range 2.5-4.0ms) therefore at 1/250sec shutter speed the sensor is not fully uncovered for long enough to capture the full flash output and as much as half the light can get wasted, resulting in an overall loss of brightness most noticeable at the bottom of the frame. But drop the shutter speed a bit, or turn the flash down to reduce flash duration (with IGBT-controlled flash units) and the problem goes away.
 
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HoppyUK said:
Shouldn't be possible. Just guessing, but possibilities might include change of distance and shutter speed (possibly TTL related, or working in aperture-priority).

At close range, just a few inches change of distance can make a noticeable difference. The other thing is shutter speed, because at full power the flash duration will be around 1/300th sec and if you're at 1/250sec shutter speed the shutter will only be fully open for about half that time, ie the sensor will not be completely uncovered long enough to capture the full flash pulse.
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I agree, it shouldn't be possible.
As per original post, the camera is set up fully manual (typically I shoot iso 100, 1/250th, f2, 35mm prime).
Agree, distance would make a difference, but I notice this most often doing full length shots, which on the 35mm puts me 6' (or maybe a little more) away from the model. I understand the impact of the inverse square law, and feel that the possible change of a few (maybe 6) inches distance that could change while swapping from M to TTL shouldn't make too much of a difference.

Interesting thought on the shutter sync speed. Needs some testing at 1/125th maybe to reduce any impact of the shutter being fully open.
I guess some testing on a tripod with a fixed subject is needed.
 
1/1 power is the absolute max for every flash; TTL cannot go +3 above that.

My best guess is that it has to do with the flash showing HSS... Which flash do you have? I would have to get mine out and check (if it's the same), but I know some Godox units can be placed into (forced) HSS mode when in manual control. Being in HSS mode (pulsed flash) does significantly reduce the max power capability; quite possibly below what the flash would use in standard/TTL mode.
 
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sk66 said:
1/1 power is the absolute max for every flash; TTL cannot go +3 above that.

My best guess is that it has to do with the flash showing HSS... Which flash do you have? I would have to get mine out and check (if it's the same), but I know some Godox units can be placed into HSS mode when in manual control. Being in HSS mode (pulsed flash) does significantly reduce the max power capability; quite possibly below what the flash would use in standard/TTL mode.
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Godox TT350F & TT685F.
Reading the instructions, HSS is controlled from the camera's flash settings screen (if I'm reading it right), which I've left as the defaults (which I think does enable it if it's required. Options are Auto HSS, First & Second Curtain Flash.

So somehow I'm potentially getting less than full power on 1/1 in Manual perhaps?
Puzzled...
 
Canon's early A-TTL Speedlights (EZ series) also had a potentially higher output (assumed to be 1/2 stop) in TTL mode. When the camera was used in 'Program' mode the aperture was chosen from a series of full stops (f/1.4, f/2, f/2.8 etc) and the flash power adjusted from a lookup table. It was then possible to add in some FEC at +/- 1/2 stop. The lookup table only possessed 10 values and some 'exposures' shared the same aperture with either a slight under or over exposure being the result if no correction was applied.
 
LC2 said:
Godox TT350F & TT685F.
Reading the instructions, HSS is controlled from the camera's flash settings screen (if I'm reading it right), which I've left as the defaults (which I think does enable it if it's required. Options are Auto HSS, First & Second Curtain Flash.

So somehow I'm potentially getting less than full power on 1/1 in Manual perhaps?
Puzzled...
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I would change the camera setting to disable HSS... I guess that would be first curtain; the HSS symbol should go away on the speedlight.
 
Test with your camera on a tripod, photograph a painted wall or similar indoors, which will show up any irregular exposure.
But I would be very surprised if Richard's theory turns out to be wrong, it's the only logical explanation..

Also, be aware that not all shutters move at the same speed, some are fast enough to actually work at the quoted maximum sync speed, others aren't so good.
 
Garry Edwards said:
Test with your camera on a tripod, photograph a painted wall or similar indoors, which will show up any irregular exposure.
But I would be very surprised if Richard's theory turns out to be wrong, it's the only logical explanation..

Also, be aware that not all shutters move at the same speed, some are fast enough to actually work at the quoted maximum sync speed, others aren't so good.
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There's not a focal plane shutter in existence that moves fast enough to capture the full max power light output of most flash units at 1/250sec (4ms) shutter speed or even 1/200sec (5ms), given that the shutter curtain takes around 3ms to uncover the sensor. Some shutters take a little longer but I've never seen anything less than 2.5ms.

So with a t.1 flash duration of commonly 1/300th-1/200th (3.3-5ms) there's only about 1/500sec (2ms) at best for the flash to dump 100% output and it obviously doesn't fit into the time window. It's very easy to check and to see how dropping the shutter speed both increases overall brightness and reduces slight shading at the bottom of the frame (as the second curtain closes over the fading tail of the flash pulse). Canon recommends 1/60sec to be sure - it's in every camera handbook.

It's a real problem for working outdoors when you often need both max x-sync speed and max flash output.
 
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Anyone interested in seeing how fast focal plane shutters move, here are a couple of links to slo-mo videos.

There are timings shown on some but an easier way to calculate the curtain travel time is to look at the 1/1000sec clip and pause it when the exposed area is half way down. Then measure the width of the gap, measure the distance from top to bottom of the frame, and divide the first number into the second. That will give you the total travel time in milliseconds.

For the record, from those videos I reckon the Nikon D3s, which is a fast full-frame pro camera with x-sync up to 1/250sec, takes 3.4ms. And the Canon 7D, another fast camera with 1/250sec max x-sync speed but APS-C format, takes 2.6ms.

View: https://www.youtube.com/watch?v=CmjeCchGRQo


View: https://www.youtube.com/watch?v=0dVmGVa3Rbw
 
HoppyUK said:
Anyone interested in seeing how fast focal plane shutters move, here are a couple of links to slo-mo videos.

There are timings shown on some but an easier way to calculate the curtain travel time is to look at the 1/1000sec clip and pause it when the exposed area is half way down. Then measure the width of the gap, measure the distance from top to bottom of the frame, and divide the first number into the second. That will give you the total travel time in milliseconds.

For the record, from those videos I reckon the Nikon D3s, which is a fast full-frame pro camera with x-sync up to 1/250sec, takes 3.4ms. And the Canon 7D, another fast camera with 1/250sec max x-sync speed but APS-C format, takes 2.6ms.

View: https://www.youtube.com/watch?v=CmjeCchGRQo


View: https://www.youtube.com/watch?v=0dVmGVa3Rbw
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Interesting, thanks for posting it. This confirms my own pretty basic practical tests, carried out many years ago with the Nikon bodies I had at the time.
APS sensor - Fuji S3 Pro and S5 Pro
Full frame sensor - Kodak DCS14N, D700 and D3.

The D3 left everything else for dead.
 
On a brighter note (haha) to my posts above, in practise things are not quite as bad as you might think if you've done the sums. Because the flash pulse rises very quickly to peak brightness right at the start and then fades away more gradually, the lion's share of the light output still makes its way to the sensor. In my experience, the loss of brightness is typically around one third or a quarter of a stop. And given the few situations where both max power and max x-sync speed are needed, ie outdoors in bright ambient light, then in practise the ambient light has a strong tendency to wash over the whole image and basically conceal any shortcomings of the flash. In other words, you may never notice it ;)
 
Really useful info, thankyou :)
Still confusing that it (certainly appears) that I get more light in TTL than manual, but I think I need to do more scientific tests using the info from you and Garry to see exactly what's happening, and to see if I can change settings to make the two configurations more consistent.
 
HoppyUK said:
On a brighter note (haha) to my posts above, in practise things are not quite as bad as you might think if you've done the sums. Because the flash pulse rises very quickly to peak brightness right at the start and then fades away more gradually, the lion's share of the light output still makes its way to the sensor. In my experience, the loss of brightness is typically around one third or a quarter of a stop. And given the few situations where both max power and max x-sync speed are needed, ie outdoors in bright ambient light, then in practise the ambient light has a strong tendency to wash over the whole image and basically conceal any shortcomings of the flash. In other words, you may never notice it ;)
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I think this is basically the difference between t.5 and t.1 times. At SS's near X-sync the entire sensor is exposed for the flash pulse/peak, but the flash exposure is basically reduced to t.5 duration; which is why that is the time usually quoted. And at longer SS's more of the tail is included in the exposure and the t.1 starts to play a significant part in the exposure (motion blur, etc). I.e. X-sync and t.5 times are close together and the SS basically defines your ability to freeze motion; but at longer SS's the flash duration/t.1 time defines your ability to freeze motion.

But that begs the question; is the stated GN associated with the stated t.5 time/power, or is it associated with the often unknown t.1 (my guess is t.1 or the full pulse as that looks/sounds better).
 
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LC2 said:
Really useful info, thankyou :)
Still confusing that it (certainly appears) that I get more light in TTL than manual, but I think I need to do more scientific tests using the info from you and Garry to see exactly what's happening, and to see if I can change settings to make the two configurations more consistent.
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There's really only two possibilities... Either the flash is not actually putting out full power when set to 1/1 in manual; which is highly unlikely IMO.
Or the flash is going into HSS pulsed flash earlier than you might expect, and you would normally probably never know it. The latter is controlled by firmware/programming, and there is no knowing exactly how the camera is programmed for the Auto HS mode (w/o extensive testing).

But I don't really see a reason why switching to TTL would cause the programming to not use HSS with the same exact settings/situation. The only thing I can guess is that the TTL pre-flashes tell the camera the exposure is good; whereas in manual it assumes HSS would be better for some reason... that sounds like a glitch in the firmware to me; which is not terribly unlikely IMO (either the flash's or the camera's; or in their interaction).
 
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sk66 said:
I think this is basically the difference between t.5 and t.1 times. At SS's near X-sync the entire sensor is exposed for the flash pulse/peak, but the flash exposure is basically reduced to t.5 duration; which is why that is the time usually quoted. And at longer SS's more of the tail is included in the exposure and the t.1 starts to play a significant part in the exposure (motion blur, etc). I.e. X-sync and t.5 times are close together and the SS basically defines your ability to freeze motion; but at longer SS's the flash duration/t.1 time defines your ability to freeze motion.

But that begs the question; is the stated GN associated with the stated t.5 time/power, or is it associated with the often unknown t.1 (my guess is t.1 or the full pulse as that looks/sounds better).
Click to expand...

I think t.5 times are often quoted simply because they make the flash duration look better, ie faster. The Guide Number will be based on a meter reading, ie the total output, or effectively t.0 if you like. Manufacturers tend to give us as little information as they can get away with.

FWIW in the detailed flash duration tests I posted a couple of years ago, particularly comparing quoted durations against the action-stopping potential of actual shutter speeds, I came to the conclusion that t.2 was the best match, ie t.5 overstates a lot, and t.1 understates a little. That would apply to all flash units at full power, also to all voltage-regulated studio heads at all powers, but not to IGBT-controlled units at anything less than full power. When IGBT heads are turned down a bit, there's very little difference between t.1 and t.5 times, and actual shutter speeds too, as the output is quenched abruptly.
 
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HoppyUK said:
I think t.5 times are often quoted simply because they make the flash duration look better, ie faster. The Guide Number will be based on a meter reading, ie the total output, or effectively t.0 if you like. Manufacturers tend to give us as little information as they can get away with.

FWIW in the detailed flash duration tests I posted a couple of years ago, particularly comparing quoted durations against the action-stopping potential of actual shutter speeds, I came to the conclusion that t.2 was the best match, ie t.5 overstates a lot, and t.1 understates a little. That would apply to all flash units at full power, also to all voltage-regulated studio heads at all powers, but not to IGBT-controlled units at anything less than full power. When IGBT heads are turned down a bit, there's very little difference between t.1 and t.5 times and actual shutter speeds as the output is quenched abruptly.
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Yes, but IGBT is the new norm and now costs so little to incorporate that, in time, it is bound to become the standard technology, so T2 will become redundant.
How long ago was it that the Paul Buff Einstein was launched? I got hold of a couple as soon as they became available, by a very convoluted route (the Company refused to export them) and I spoke to chief engineers in various Chinese firms, all of whom told me that the technology was impossible and that I was dreaming but here we are, just a few years later, and they have become the new standard:).

It seems (and perhaps is) a stupid thing to say, but I believe that flash development can go no further, although no doubt companies will continue to introduce new gimmicks. The next development has already happened, a combined flash and LED light (for example) https://www.lencarta.com/all-products/continuous-lighting/Godox-FV200 and the use of LED lighting is bound to increase and the use of flash is bound to decrease for a number of reasons.
 
HoppyUK said:
When IGBT heads are turned down a bit, there's very little difference between t.1 and t.5 times, and actual shutter speeds too, as the output is quenched abruptly.
Click to expand...
I've noticed that between ~ 1/2 and 1/8 the t.5 and t.1 times are typically very close; but at 1:1 t.5 is about 3x faster... no idea what t.2 might be; other than even closer to t.5 below 1:1.
 
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