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03-13-2019, 03:41 PM   #16
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QuoteOriginally posted by mcgregni Quote
....


Bruce, be fair please, this is what you told us in your first post, so I think we should be forgiven for understanding you are trying to use Av with Auto-ISO.
You may have said you had changed it later, but the posts are quite long, so its likely that people are responding to the opening one primarily.


No one is denying that its not possible to get some good flash shots using Av with P-TTL modes .... I use that sometimes myself too. But I agree with Steve that it is more suited to "snapshots" .... it is not suited to any situation where we need to take full control over the mix of ambient and flash brightness that is recorded. Whether results in dynamic situations using Av / P-TTL can be more than snapshots really depends on the skill and creativity of the photographer I feel.


Don't forget to check out the impact of the Camera Flash Modes, which can have a major influence on the amount of ambient light allowed to be recorded.


You asked about why the Auto ISO figure was not displayed prior to taking flash pics .... well, its because the figure is not calculated until the pre-flash. And there will be different decisions made based on a range of factors .... camera flash mode, focal length, amount of ambient light are a few of them.
Ok I'm sorry if any of what I said came off rude, sometimes though it's like ppl skim read long posts and don't fully follow what is being said or what was actually done and happening. Sometimes to explain the events you need to provide context and details is all. But I thank your patience and apologise.

I have noticed also with the Auto-ISO that even if you have the flash set to manual and not P-TTL (but still in Av mode obviously for the Auto-ISO), it still doesn't show the ISO you're expected to get, but now in Manual there is no pre-flash. So I think the Auto-ISO value is just given when no flash is attached, and soon as flash is detected a couple of things happen. You lose the Auto-ISO parameter settings (Slow, Fast or Normal) and just get 'Normal' whether you like it or not, and then the Auto-ISO predicted value is gone, regardless of flash mode.

03-13-2019, 04:07 PM   #17
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Having a flash ready signal going to the camera through the hotshoe changes everything. The metering system has to decide whether it tries to balance ambient and flash exposures, or whether the flash is going to provide everything.

This depends of course on what restrictions on ambient exposure are being imposed by the program line and ISO parameter settings. The Camera Flash mode and lens focal length will likely have the most impact in practice though.
03-13-2019, 04:47 PM - 1 Like   #18
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QuoteOriginally posted by BruceBanner Quote
When introducing P-TTL flash your exposure triangle goes into a square with a 4th variable contributing to the shot,
When I delved into flash seriously for the first time a few years ago I learned (from Nigel) that the best way to think about it is that when you use flash you are making two separate exposures at the same time. Ambient and Flash. Thinking about it this way helped me a lot. The point being that anything beyond the range of your flash will not get lit by it. Equally a change in shutter speed will not affect the illumination provided by the flash. So you dont really have an exposure "square" when using flash, but two triangles that intersect: Shutter/aperture/ISO (ambient) and Aperture/ISO/Flash output and distance (Flash)

QuoteOriginally posted by BruceBanner Quote
Could it be that if the flash in P-TTL mode is firing it's weakest version and the exposure will still be too much it then has to bump something up for the shot so raises the shutter speed?
No, because the shutter speed will not affect the exposure provided by the flash. The flash duration is too fast for any change in shutter speed to have an effect.

QuoteOriginally posted by BruceBanner Quote
However I think a lot of people forget that ISO can be fixed in Av Mode
I use fixed ISO the majority of the time in most modes (except TAv)

I think the point here is that when you use any mode other than Manual you are asking the camera to do a balancing act between the ambient light and the flash. With virtually zero ambient you will get a flash lit shot. With a bright scene the camera will use the flash as fill-flash. Between the two you will get different variations.
03-13-2019, 05:18 PM   #19
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QuoteOriginally posted by pschlute Quote
When I delved into flash seriously for the first time a few years ago I learned (from Nigel) that the best way to think about it is that when you use flash you are making two separate exposures at the same time. Ambient and Flash. Thinking about it this way helped me a lot. The point being that anything beyond the range of your flash will not get lit by it. Equally a change in shutter speed will not affect the illumination provided by the flash. So you dont really have an exposure "square" when using flash, but two triangles that intersect: Shutter/aperture/ISO (ambient) and Aperture/ISO/Flash output and distance (Flash)



No, because the shutter speed will not affect the exposure provided by the flash. The flash duration is too fast for any change in shutter speed to have an effect.



I use fixed ISO the majority of the time in most modes (except TAv)

I think the point here is that when you use any mode other than Manual you are asking the camera to do a balancing act between the ambient light and the flash. With virtually zero ambient you will get a flash lit shot. With a bright scene the camera will use the flash as fill-flash. Between the two you will get different variations.
No, that's not the point here. The point here is why does the system change the shutter speed after shutter is pressed and we have an answer for that now

Shutter speed of course does have an effect in the shot, the flash might freeze frame and capture light, but the longer we leave that shutter open for the more ambient light comes into the shot and therefore the overall exposure and intent of the image changes. The variables I'm seeing with only a 1/4 stop difference between what the camera suggests we're going to get and what we actually get in P-TTL of course is not having a dramatic difference in exposure or intent, but I wouldn't altogether ignore the shutter speed just because we're using flash, I think that's a tad misleading no?

03-13-2019, 05:30 PM   #20
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QuoteOriginally posted by BruceBanner Quote
but I wouldn't altogether ignore the shutter speed just because we're using flash, I think that's a tad misleading no?
i did not say you should ignore the shutter speed. i simply pointed out that the shutter speed will only affect the ambient light in your scene.
03-13-2019, 06:19 PM   #21
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QuoteOriginally posted by pschlute Quote
i did not say you should ignore the shutter speed. i simply pointed out that the shutter speed will only affect the ambient light in your scene.
It can contribute to ambient blur too. Ever seen that trick where you deliberately lower shutter speed so that the flash freezes people well but then afterwards the ambient lights and bokeh can be deliberately manipulated to looking in motion due to low shutter speeds? Sometimes you want it, sometimes you don't, so shutter speed is important still, even with flash.
03-14-2019, 10:43 AM - 1 Like   #22
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QuoteOriginally posted by BruceBanner Quote
Could it be that if the flash in P-TTL mode is firing it's weakest version and the exposure will still be too much it then has to bump something up for the shot so raises the shutter speed?
QuoteOriginally posted by pschlute Quote
No, because the shutter speed will not affect the exposure provided by the flash. The flash duration is too fast for any change in shutter speed to have an effect.

Peter is right with this; its important to keep emphasising that exposure time does not influence the flash exposure level in normal operations .... only in HSS mode will exposure time have an effect on flash exposure.


If the meter detects an overexposure situation even though the flash would output its lowest possible level, then it would need to adjust another variable, ie aperture or ISO, in order to allow a good flash exposure. Which of these varies would depend on the mode of course, and whether ISO is available to vary (auto-ISO parameter settings).


If the need is to reduce flash exposure, and the flash is already at its lowest possible output, then in Av mode the camera only has one thing to vary .... ISO, which would have to be lowered. The only alternative is for the photographer to step back more and increase the distance, then try again with a new pre-flash and test shot.


Regarding shutter speed auto control, then this is determined mainly by the camera flash mode setting and the lens focal length. The system aims to provide a steady handheld shot as the first requirement, and in dim conditions this will result in a poor ambient/flash balance, unless the photographer overrides this with Slow Speed Sync mode.
03-14-2019, 01:13 PM   #23
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QuoteOriginally posted by mcgregni Quote
Peter is right with this; its important to keep emphasising that exposure time does not influence the flash exposure level in normal operations .... only in HSS mode will exposure time have an effect on flash exposure.


If the meter detects an overexposure situation even though the flash would output its lowest possible level, then it would need to adjust another variable, ie aperture or ISO, in order to allow a good flash exposure. Which of these varies would depend on the mode of course, and whether ISO is available to vary (auto-ISO parameter settings).


If the need is to reduce flash exposure, and the flash is already at its lowest possible output, then in Av mode the camera only has one thing to vary .... ISO, which would have to be lowered. The only alternative is for the photographer to step back more and increase the distance, then try again with a new pre-flash and test shot.


Regarding shutter speed auto control, then this is determined mainly by the camera flash mode setting and the lens focal length. The system aims to provide a steady handheld shot as the first requirement, and in dim conditions this will result in a poor ambient/flash balance, unless the photographer overrides this with Slow Speed Sync mode.
Never said he was wrong

Yesterday I was in the garden taking shots of my kid bouncing on the trampoline with direct on camera flash. I wasn't in HSS (1/160) but I have that 'ghosting' trail effect seen slightly (especially in the extremities hands and feets) of where the flash froze the subject vs the ambient light shutter speed recording at 1/160. I simply meant to convey that the shutter speed is still relevant in flash photography from a blur/motion perspective as well as contributing to ambient light. That is all. It's just the way he presented that information it might have sounded like you could have your shutter speed at whatever you like because flash will freeze and override shutter speed etc... it's ONLY going to affect ambient light, and I was meaning to chime in and add that it's still relevant to the shot in the ways that shutter speed without flash is (ie blur, motion etc).

03-14-2019, 01:18 PM - 1 Like   #24
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QuoteOriginally posted by mcgregni Quote
Peter is right with this; its important to keep emphasising that exposure time does not influence the flash exposure level in normal operations .... only in HSS mode will exposure time have an effect on flash exposure.


If the meter detects an overexposure situation even though the flash would output its lowest possible level, then it would need to adjust another variable, ie aperture or ISO, in order to allow a good flash exposure. Which of these varies would depend on the mode of course, and whether ISO is available to vary (auto-ISO parameter settings).


If the need is to reduce flash exposure, and the flash is already at its lowest possible output, then in Av mode the camera only has one thing to vary .... ISO, which would have to be lowered. The only alternative is for the photographer to step back more and increase the distance, then try again with a new pre-flash and test shot.


Regarding shutter speed auto control, then this is determined mainly by the camera flash mode setting and the lens focal length. The system aims to provide a steady handheld shot as the first requirement, and in dim conditions this will result in a poor ambient/flash balance, unless the photographer overrides this with Slow Speed Sync mode.
In Av mode, the camera can also change the shutter speed to reduce the combined brightness of the flash+ambient. Sure, it's true that changing the shutter time does not change the amount of light delivered by the flash, but it can reduce the amount added by the ambient to the flashed part of the scene to prevent blow-out (at the cost of dimming the most distant parts of the scene).

The proof of is in Bruce's own data: Av mode was claiming it would use a certain shutter speed before the pre-flash and then changed the shutter speed after the preflash.

This does not surprise me given the use of the powerful Cactus RF60x at ISO 1600 and f/1.9.

The Cactus RF60x is a bright flash (GN of at least 33 meters even at the 24mm wide-angle setting and ISO 100). At minimum power (1/128), the GN would be 3 meters @ ISO 100 so it would be a GN 12 at ISO 1600. With an aperture of f/1.9, anything less than 6 meters from the flash would be very brightly lit.
03-14-2019, 02:00 PM   #25
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Just wondering how metering mode might effect this. The flash is only lighting the subject not the background. -1 ev on the frame vs -1 ev on a face in pttl I feel should make for big differences. Preflash would certainly read different.
03-14-2019, 02:13 PM   #26
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QuoteOriginally posted by photoptimist Quote
In Av mode, the camera can also change the shutter speed to reduce the combined brightness of the flash+ambient. Sure, it's true that changing the shutter time does not change the amount of light delivered by the flash, but it can reduce the amount added by the ambient to the flashed part of the scene to prevent blow-out (at the cost of dimming the most distant parts of the scene).

The proof of is in Bruce's own data: Av mode was claiming it would use a certain shutter speed before the pre-flash and then changed the shutter speed after the preflash.

This does not surprise me given the use of the powerful Cactus RF60x at ISO 1600 and f/1.9.

The Cactus RF60x is a bright flash (GN of at least 33 meters even at the 24mm wide-angle setting and ISO 100). At minimum power (1/128), the GN would be 3 meters @ ISO 100 so it would be a GN 12 at ISO 1600. With an aperture of f/1.9, anything less than 6 meters from the flash would be very brightly lit.
Thanks for doing the math and posting this. I crafted a similar comment yesterday, but did not post due to a feeling that it would be wasted effort. The case from the original post of Av mode, f/1.9 and fixed ISO 1600 has sufficient hints to provide an explanation of what is happening here. The same settings in M mode will likely have resulted in the flash signalling error in the viewfinder.


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03-14-2019, 02:17 PM   #27
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QuoteOriginally posted by swanlefitte Quote
Just wondering how metering mode might effect this. The flash is only lighting the subject not the background. -1 ev on the frame vs -1 ev on a face in pttl I feel should make for big differences. Preflash would certainly read different.
These are excellent points. Truth is that we don't know if different logic is used for multi-segment flash metering with P-TTL.


Steve
03-14-2019, 02:24 PM   #28
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QuoteOriginally posted by photoptimist Quote
In Av mode, the camera can also change the shutter speed to reduce the combined brightness of the flash+ambient

Yes, its good to acknowledge that flash output combines with ambient brightness, so the two are effectively slapped on top of one another. And there is a risk of blow out where flash is introduced on top of, for example, an already naturally bright face ..... however, I wonder if it may be over-promoting the sophistications of our P-TTL metering system a bit too far to suggest that the system can recognise when a flash cannot reduce its output sufficiently, and consequently take a decision to use shutter speed (if allowed) to reduce ambient to a level that would allow the flash to expose correctly ...? That does sound rather far fetched, however of course there's no proof about this as we lack any technical information about this sort of thing.


It sounds to me more like the sort of considered and assessed decision a photographer would have to make when finely tuning things using manual mode!
03-14-2019, 02:54 PM   #29
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QuoteOriginally posted by mcgregni Quote
Yes, its good to acknowledge that flash output combines with ambient brightness, so the two are effectively slapped on top of one another. And there is a risk of blow out where flash is introduced on top of, for example, an already naturally bright face ..... however, I wonder if it may be over-promoting the sophistications of our P-TTL metering system a bit too far to suggest that the system can recognise when a flash cannot reduce its output sufficiently, and consequently take a decision to use shutter speed (if allowed) to reduce ambient to a level that would allow the flash to expose correctly ...? That does sound rather far fetched, however of course there's no proof about this as we lack any technical information about this sort of thing.


It sounds to me more like the sort of considered and assessed decision a photographer would have to make when finely tuning things using manual mode!
It may be far-fetched and yet the camera did use a different shutter time in Av mode after the pre-flash.


Using any kind of camera automation is like delegating a task to an idiot cousin (whom we might call Kayone). Sure, he's friendly. Sure, he can handle simple tasks with reliability. But then some situations just stump him and the results are not pretty. So the task for the photographer is to learn the quirks of the idiot cousin (auto-exposure system) to know when to trust him and when to switch to full M mode. That's what Bruce seems to be doing here and he's finding some interesting quirks in our favorite cousin.
03-14-2019, 05:31 PM - 1 Like   #30
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QuoteOriginally posted by photoptimist Quote
Using any kind of camera automation is like delegating a task to an idiot cousin (whom we might call Kayone).
Hmmmm....perhaps Kayone has an evil brother named Kaytwee. I just spent an hour or so with my camera doing attempts at reproducing Bruce's results using fixed ISO and Av mode. I had no trouble reproducing the shutter speed shift and was even able to do it at ISOs as low as 800. A common feature is that the "actual" higher shutter speed flashes in the viewfinder display while the flash is recycling and is that the flash exposure itself is mildly overexposed when compared to the same framing at one stop lower ISO or other frames where the shift does not occur.

I have not been able to reproduce in P mode.

I am not certain whether this is a "hail Mary" attempt by the camera to salvage overexposure or prevent ambient exposure blur or a bug in the P-TTL implementation.

Note: Av mode is sort of unique in that the flash protocol overrides the metered shutter speed based on what lens is attached. At least that is what the user guide says.


Steve

Last edited by stevebrot; 03-14-2019 at 05:37 PM.
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