Bounce flash underexposure in P-TTL

CarlJF · 07-20-2016, 05:51 PM

Lately, I'm experimenting with bouce flash off the wall with my 360FGZ (version I) in P-TTL and K-500. But, no matter how hard I try, I always get heavily underexposed shots by 2-3 stops... I've used both wireless and P-TTL cable with the same results (since the 360 head can't swivel, I use it off camera and pointing at 45 degrees toward the wall on the left or right). I also got the same underexposure no matter which lenses is mounted. Now, I'm able to get a correct exposure under the same condition if I set the flash in manual at 1/2 power, so the underexposure isn't because the flash isn't powerful enough. If I use direct flash, exposure is fine, the problem only appears with bouce flash. I have two 360 and both give me the exact same result, so it's probably not a flash malfunction. And yes, the camera exposes perfectly without flash, so it's not the dreaded aperture control failure.

The camera is in manual mode, 1/160, F4 or F5.6, ISO400. The flash was set as Slave, Wireless in either P-TTL or manual mode, with the K-500 on board flash in controller mode.

So, is anyone have an idea on what can be the cause of this underexposure in P-TTL ? Do I miss something on how P-TTL bounce flash works ?

**mattt** · 07-20-2016, 06:33 PM

You know you've got enough power.... If I was using an automated exposure on a white surface I would exposure bias +2 (on the flash head). Just like shooting in snow or beach you'd exposure bias the camera to tell its a brighter surface than middle grey.

mcgregni · 07-20-2016, 06:39 PM

Well, it works by metering for the light the camera sees reflected back 'through the lens' . This can vary of course depending on surface reflections and tones. This the same for direct or bounce flash, so we can assume that your bounce surfaces are somehow causing the camera to 'see' more light than there really is, and so not provide enough power.

I presume you're using autofocus lenses? Thats a requirement for P-TTL. Certainly your camera settings seem very sensible ..... what sort of total distance is involved with your flash lights pathway to the subject?

As an experiment, try zooming out to include a wider view of the scene .... your flash exposure should increase a bit as a result. Conversly, zooming in very close to a subject is likely to reduce the flash exposure value. The reason is that with wider scenes the meter sees more 'dead space', often not well lit dead space (like dull walls and shadowed areas), and so tries to light more with the flash. In any case, this test will confirm whether in general the P-TTL metering is working as expected.

Try ISO 800 or 1600 also ..... you may find that will encourage the system to give a higher flash exposure, and it will reduce your battery drain also. Assuming this gets you a bit closer to the brightness you want, then apply plus flash compensation as needed. It is not uncommon to need +1.0 or so compensation in bounce situations.

stevebrot · 07-20-2016, 07:22 PM

Originally posted by mcgregni

I presume you're using autofocus lenses? Thats a requirement for P-TTL.

??? I use A-series lenses all the time. Flash attenuation is not as good (pre-flash is full strength regardless of distance), but overall, it works.

Steve

mcgregni · 07-20-2016, 07:43 PM

Hi Steve, no doubt some people can get good results with A-series lenses, but in general I believe the 'official line', based on the Pentax Flash Manuals and the various discussions and testing that was carried out after my Flash Guide was released, was that P-TTL depended on the contacts, AF point and subject distance information introduced with the KAF mount.... so F-series types are the earliest lenses that are compatible.

Thats an 'official line' .... although maybe there can't even be an 'official line'

. Lets face it, a number of us were having to make it up as we went along, considering the extreme lack of technical data and official information published at the times of the various technology updates (eg KAF, F series, FA series, KAF2, P-TTL).

Anyway, for someone who's having concerns with P-TTL exposure, I think it would be safer to stick to autofocus, at least until things are working reliably.

RAART · 07-20-2016, 09:45 PM

in my experience all Pentax camera tends to underexpose when flash is bounced. I tried P-TTL and it does not work for me. Since then I switched to manual and I am good (just if I have to bounce the flash). Direct flash is ok. Nikon or Canon are so much better there. I hope that Pentax will catch up with them at some point.

stevebrot · 07-20-2016, 10:59 PM

Originally posted by mcgregni

Hi Steve, no doubt some people can get good results with A-series lenses, but in general I believe the 'official line', based on the Pentax Flash Manuals and the various discussions

I was just going on the compatibility matrices in the manuals for my K10D and K-3 and personal experience going back to 2007. I believe the same compatibility matrix is common to all Pentax AF cameras. I have not gone through them all, but at least some of the P-TTL flash manuals (e.g. AF 360FZG) include specific instructions for P-TTL use with manual focus lenses. On the flip side, the flash manuals include a statement indicating that "correct output" in P-TTL is only available with AF lenses. Apparently the camera and flash documentation teams don't talk with each other. As for the "official line", it is my understanding that Ricoh/Pentax will only vouch for compatibility within the current product line, regardless of what the manuals might say.

My experience is that P-TTL performance with A-series lenses is generally acceptable, but less reliable than with AF lenses. With the OP's AF360FZG non-PTTL auto flash may be the preferred option with A-series lenses if P-TTL is not working well.

Steve

mcgregni · 07-21-2016, 12:19 AM

That does seem a fair assessment of the current state of confusion

Perhaps its only the latest flash model manuals that spell out 'AF only' ? I've often wondered if such certainty only arose after they started testing the new flash models with a range of lenses?!

Unfortunately we have nothing to suggest there has been any technological or physical developments in the P-TTL algorithms or mount contacts with the latest flashes or more recent cameras and lenses.

Happyman · 07-21-2016, 03:41 AM

And you haven't, by any chance, set the negative exposure compensation on the camera body itself? It can be done even in manual mode, and when shooting with P-TTL flash, it will influence the flash output. At least on the K-5.

CarlJF · 07-21-2016, 07:19 AM

Thanks for your help guys!

For the lenses, I was using the DA55-300 and DA70 ltd. My goal was to see how the 55-300 would fare for casual portraiture, but it rapidly turned out in a bounce flash test... As a sidenote, the 55-300 was okay when stopped down, but obviously not has goos as the DA70. But still good enough that I don't feel the need to carry the DA70 during my vacation just in case I need to a quick portrait...

I will make a few more tests with FEC and Av mode when I will be back home later this evening. I've done a few shots yesterday by setting +1 FEC on the camera and +1 on the flash but it still was far from a acceptable exposure, on the histogram it looked more like a 0.5 stop exposure increase than two full stops... Like if the camera would refuse the fire the flash at more than 1/8 power in P-TTL... I've made one shot in full auto mode and although the exposure was fine, the camera choose to raise the ISO up to 6400 instead of using the flash at full power... Like if the camera only wanted to use the flash as fill light and not as key light... And thus kept the power low to not overpower the ambient light by too much.

RAART · 07-21-2016, 07:29 AM

Your flash should be set as master not slave...

lister6520 · 07-21-2016, 09:05 AM

One thing to watch out for - there is some odd logic in the software which changes the exposure calculation when it detects the flash head is not in the straight ahead direction (there is some microswitch or something in the flash head to detect this). I'm not quite sure what that logic is meant to achieve but as far as I can tell most of the time it seems to be counterproductive. I guess they tried to use it to tweak the exposure some way to take into account the different type of illumination but got the math wrong.

I've had the problem often with a K-r and K-30 using a Metz flash.

A simple experiment to do : instead of tilting the head to bounce the light instead leave it pointing forward and use a vanity mirror or something similar at 45 to direct the light upwards in the direction you would have tilted the head. This achieves the same effect as tilting the head but fools the camera into thinking it is direct illumination so it does not modify the calculation. I think I had posted the results in this forum somewhere but haven;t managed to find it.

I had experimented quite a bit with this and got good exposure in all situations where tilting up the head got me underexposure. I'm not sure if they've fixed this problem with new firmwares or on the K-3, or if they even consider it a problem. I had tried this some years ago and haven't used flash much since then.

CarlJF · 07-21-2016, 09:56 AM

Originally posted by RAART

Your flash should be set as master not slave...

I will try this.

Originally posted by lister6520

One thing to watch out for - there is some odd logic in the software which changes the exposure calculation when it detects the flash head is not in the straight ahead direction (there is some microswitch or something in the flash head to detect this). I'm not quite sure what that logic is meant to achieve but as far as I can tell most of the time it seems to be counterproductive. I guess they tried to use it to tweak the exposure some way to take into account the different type of illumination but got the math wrong.

I've had the problem often with a K-r and K-30 using a Metz flash.

A simple experiment to do : instead of tilting the head to bounce the light instead leave it pointing forward and use a vanity mirror or something similar at 45 to direct the light upwards in the direction you would have tilted the head. This achieves the same effect as tilting the head but fools the camera into thinking it is direct illumination so it does not modify the calculation. I think I had posted the results in this forum somewhere but haven;t managed to find it.

I had experimented quite a bit with this and got good exposure in all situations where tilting up the head got me underexposure. I'm not sure if they've fixed this problem with new firmwares or on the K-3, or if they even consider it a problem. I had tried this some years ago and haven't used flash much since then.

You may have a point here. This reminds me that yesterday, since I was only doing some quick tests, I've just put the flash in a small plastic stand and tilt the head. What I usually do is rather to put it on lightstand or a ball head, leaving it in its standard 90 degree position. I even was wondering why I suddenly got all these underexposed shots when I never noticed it before. This could well be the reason: I've tilted the flash head when I usually rather use the ball head...

mcgregni · 07-21-2016, 10:14 AM

Originally posted by RAART

Your flash should be set as master not slave...

I was a bit confused about the configurations being used .... It sounds like the flash being used is actually an off-camera slave triggered by the camera built-in flash in control mode ... ? It is perhaps more usual to bounce a 'Master' flash from the hotshoe ....

I just wonder if there is too much wider light spill reaching the meter and making it think there's more brightness than there really is on the subject. Bouncing a slave could certainly do this more so than the light spill from an on-camera flash.

CarlJF · 07-21-2016, 11:33 AM

Originally posted by mcgregni

I was a bit confused about the configurations being used .... It sounds like the flash being used is actually an off-camera slave triggered by the camera built-in flash in control mode ... ? It is perhaps more usual to bounce a 'Master' flash from the hotshoe ....

You understood correctly. The flash was used off camera in slave mode and bouced off a wall. I totally agree that is more usual to bounce from on camera flash, but since the 360 v1 doesn't have a swivel head, it's not possible to bounce it off the wall, which was what I wanted to try... However, I've used it many time bounced off the ceiling with nice results. Although I'm often wondering if I would be better off buying a flash with a swivel head, maybe a cheap Bolt, or a Metz 52... Since I can't made up my mind, I stick with the 360, which does a great job most of the time. The Bolt flash looks good on paper, but feedback from users is almost nonexistent, and although it's cheap I don't feel like taking a bet on it.

07-20-2016, 05:51 PM	#1
CarlJF Veteran Member Join Date: Sep 2014 Location: Quebec City Posts: 1,185	Bounce flash underexposure in P-TTL Lately, I'm experimenting with bouce flash off the wall with my 360FGZ (version I) in P-TTL and K-500. But, no matter how hard I try, I always get heavily underexposed shots by 2-3 stops... I've used both wireless and P-TTL cable with the same results (since the 360 head can't swivel, I use it off camera and pointing at 45 degrees toward the wall on the left or right). I also got the same underexposure no matter which lenses is mounted. Now, I'm able to get a correct exposure under the same condition if I set the flash in manual at 1/2 power, so the underexposure isn't because the flash isn't powerful enough. If I use direct flash, exposure is fine, the problem only appears with bouce flash. I have two 360 and both give me the exact same result, so it's probably not a flash malfunction. And yes, the camera exposes perfectly without flash, so it's not the dreaded aperture control failure. The camera is in manual mode, 1/160, F4 or F5.6, ISO400. The flash was set as Slave, Wireless in either P-TTL or manual mode, with the K-500 on board flash in controller mode. So, is anyone have an idea on what can be the cause of this underexposure in P-TTL ? Do I miss something on how P-TTL bounce flash works ?

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07-20-2016, 06:33 PM	#2
mattt Site Supporter Join Date: Feb 2011 Location: Niagara Photos: Gallery \| Albums Posts: 3,907	You know you've got enough power.... If I was using an automated exposure on a white surface I would exposure bias +2 (on the flash head). Just like shooting in snow or beach you'd exposure bias the camera to tell its a brighter surface than middle grey.

07-20-2016, 06:39 PM	#3
mcgregni Veteran Member Join Date: Sep 2012 Location: Surrey, England Posts: 2,603	Well, it works by metering for the light the camera sees reflected back 'through the lens' . This can vary of course depending on surface reflections and tones. This the same for direct or bounce flash, so we can assume that your bounce surfaces are somehow causing the camera to 'see' more light than there really is, and so not provide enough power. I presume you're using autofocus lenses? Thats a requirement for P-TTL. Certainly your camera settings seem very sensible ..... what sort of total distance is involved with your flash lights pathway to the subject? As an experiment, try zooming out to include a wider view of the scene .... your flash exposure should increase a bit as a result. Conversly, zooming in very close to a subject is likely to reduce the flash exposure value. The reason is that with wider scenes the meter sees more 'dead space', often not well lit dead space (like dull walls and shadowed areas), and so tries to light more with the flash. In any case, this test will confirm whether in general the P-TTL metering is working as expected. Try ISO 800 or 1600 also ..... you may find that will encourage the system to give a higher flash exposure, and it will reduce your battery drain also. Assuming this gets you a bit closer to the brightness you want, then apply plus flash compensation as needed. It is not uncommon to need +1.0 or so compensation in bounce situations.

07-20-2016, 07:22 PM	#4
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by mcgregni I presume you're using autofocus lenses? Thats a requirement for P-TTL. ??? I use A-series lenses all the time. Flash attenuation is not as good (pre-flash is full strength regardless of distance), but overall, it works. Steve

07-20-2016, 07:43 PM	#5
mcgregni Veteran Member Join Date: Sep 2012 Location: Surrey, England Posts: 2,603	Hi Steve, no doubt some people can get good results with A-series lenses, but in general I believe the 'official line', based on the Pentax Flash Manuals and the various discussions and testing that was carried out after my Flash Guide was released, was that P-TTL depended on the contacts, AF point and subject distance information introduced with the KAF mount.... so F-series types are the earliest lenses that are compatible. Thats an 'official line' .... although maybe there can't even be an 'official line' . Lets face it, a number of us were having to make it up as we went along, considering the extreme lack of technical data and official information published at the times of the various technology updates (eg KAF, F series, FA series, KAF2, P-TTL). Anyway, for someone who's having concerns with P-TTL exposure, I think it would be safer to stick to autofocus, at least until things are working reliably.

07-20-2016, 09:45 PM	#6
RAART Veteran Member Join Date: Apr 2013 Location: Oakville, ON Photos: Gallery Posts: 1,095	in my experience all Pentax camera tends to underexpose when flash is bounced. I tried P-TTL and it does not work for me. Since then I switched to manual and I am good (just if I have to bounce the flash). Direct flash is ok. Nikon or Canon are so much better there. I hope that Pentax will catch up with them at some point.

07-20-2016, 10:59 PM	#7
stevebrot Otis Memorial Pentaxian Join Date: Mar 2007 Location: Vancouver (USA) Photos: Gallery \| Albums Posts: 42,007	Originally posted by mcgregni Hi Steve, no doubt some people can get good results with A-series lenses, but in general I believe the 'official line', based on the Pentax Flash Manuals and the various discussions I was just going on the compatibility matrices in the manuals for my K10D and K-3 and personal experience going back to 2007. I believe the same compatibility matrix is common to all Pentax AF cameras. I have not gone through them all, but at least some of the P-TTL flash manuals (e.g. AF 360FZG) include specific instructions for P-TTL use with manual focus lenses. On the flip side, the flash manuals include a statement indicating that "correct output" in P-TTL is only available with AF lenses. Apparently the camera and flash documentation teams don't talk with each other. As for the "official line", it is my understanding that Ricoh/Pentax will only vouch for compatibility within the current product line, regardless of what the manuals might say. My experience is that P-TTL performance with A-series lenses is generally acceptable, but less reliable than with AF lenses. With the OP's AF360FZG non-PTTL auto flash may be the preferred option with A-series lenses if P-TTL is not working well. Steve

07-21-2016, 12:19 AM	#8
mcgregni Veteran Member Join Date: Sep 2012 Location: Surrey, England Posts: 2,603	That does seem a fair assessment of the current state of confusion Perhaps its only the latest flash model manuals that spell out 'AF only' ? I've often wondered if such certainty only arose after they started testing the new flash models with a range of lenses?! Unfortunately we have nothing to suggest there has been any technological or physical developments in the P-TTL algorithms or mount contacts with the latest flashes or more recent cameras and lenses.

07-21-2016, 03:41 AM	#9
Happyman Forum Member Join Date: Oct 2011 Posts: 68	And you haven't, by any chance, set the negative exposure compensation on the camera body itself? It can be done even in manual mode, and when shooting with P-TTL flash, it will influence the flash output. At least on the K-5.

07-21-2016, 07:19 AM	#10
CarlJF Veteran Member Join Date: Sep 2014 Location: Quebec City Posts: 1,185 Original Poster	Thanks for your help guys! For the lenses, I was using the DA55-300 and DA70 ltd. My goal was to see how the 55-300 would fare for casual portraiture, but it rapidly turned out in a bounce flash test... As a sidenote, the 55-300 was okay when stopped down, but obviously not has goos as the DA70. But still good enough that I don't feel the need to carry the DA70 during my vacation just in case I need to a quick portrait... I will make a few more tests with FEC and Av mode when I will be back home later this evening. I've done a few shots yesterday by setting +1 FEC on the camera and +1 on the flash but it still was far from a acceptable exposure, on the histogram it looked more like a 0.5 stop exposure increase than two full stops... Like if the camera would refuse the fire the flash at more than 1/8 power in P-TTL... I've made one shot in full auto mode and although the exposure was fine, the camera choose to raise the ISO up to 6400 instead of using the flash at full power... Like if the camera only wanted to use the flash as fill light and not as key light... And thus kept the power low to not overpower the ambient light by too much.

07-21-2016, 07:29 AM	#11
RAART Veteran Member Join Date: Apr 2013 Location: Oakville, ON Photos: Gallery Posts: 1,095	Your flash should be set as master not slave...

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07-21-2016, 09:05 AM	#12
lister6520 Veteran Member Join Date: May 2012 Posts: 733	One thing to watch out for - there is some odd logic in the software which changes the exposure calculation when it detects the flash head is not in the straight ahead direction (there is some microswitch or something in the flash head to detect this). I'm not quite sure what that logic is meant to achieve but as far as I can tell most of the time it seems to be counterproductive. I guess they tried to use it to tweak the exposure some way to take into account the different type of illumination but got the math wrong. I've had the problem often with a K-r and K-30 using a Metz flash. A simple experiment to do : instead of tilting the head to bounce the light instead leave it pointing forward and use a vanity mirror or something similar at 45 to direct the light upwards in the direction you would have tilted the head. This achieves the same effect as tilting the head but fools the camera into thinking it is direct illumination so it does not modify the calculation. I think I had posted the results in this forum somewhere but haven;t managed to find it. I had experimented quite a bit with this and got good exposure in all situations where tilting up the head got me underexposure. I'm not sure if they've fixed this problem with new firmwares or on the K-3, or if they even consider it a problem. I had tried this some years ago and haven't used flash much since then.

07-21-2016, 09:56 AM	#13
CarlJF Veteran Member Join Date: Sep 2014 Location: Quebec City Posts: 1,185 Original Poster	Originally posted by RAART Your flash should be set as master not slave... I will try this. Originally posted by lister6520 One thing to watch out for - there is some odd logic in the software which changes the exposure calculation when it detects the flash head is not in the straight ahead direction (there is some microswitch or something in the flash head to detect this). I'm not quite sure what that logic is meant to achieve but as far as I can tell most of the time it seems to be counterproductive. I guess they tried to use it to tweak the exposure some way to take into account the different type of illumination but got the math wrong. I've had the problem often with a K-r and K-30 using a Metz flash. A simple experiment to do : instead of tilting the head to bounce the light instead leave it pointing forward and use a vanity mirror or something similar at 45 to direct the light upwards in the direction you would have tilted the head. This achieves the same effect as tilting the head but fools the camera into thinking it is direct illumination so it does not modify the calculation. I think I had posted the results in this forum somewhere but haven;t managed to find it. I had experimented quite a bit with this and got good exposure in all situations where tilting up the head got me underexposure. I'm not sure if they've fixed this problem with new firmwares or on the K-3, or if they even consider it a problem. I had tried this some years ago and haven't used flash much since then. You may have a point here. This reminds me that yesterday, since I was only doing some quick tests, I've just put the flash in a small plastic stand and tilt the head. What I usually do is rather to put it on lightstand or a ball head, leaving it in its standard 90 degree position. I even was wondering why I suddenly got all these underexposed shots when I never noticed it before. This could well be the reason: I've tilted the flash head when I usually rather use the ball head...

07-21-2016, 10:14 AM	#14
mcgregni Veteran Member Join Date: Sep 2012 Location: Surrey, England Posts: 2,603	Originally posted by RAART Your flash should be set as master not slave... I was a bit confused about the configurations being used .... It sounds like the flash being used is actually an off-camera slave triggered by the camera built-in flash in control mode ... ? It is perhaps more usual to bounce a 'Master' flash from the hotshoe .... I just wonder if there is too much wider light spill reaching the meter and making it think there's more brightness than there really is on the subject. Bouncing a slave could certainly do this more so than the light spill from an on-camera flash.

07-21-2016, 11:33 AM	#15
CarlJF Veteran Member Join Date: Sep 2014 Location: Quebec City Posts: 1,185 Original Poster	Originally posted by mcgregni I was a bit confused about the configurations being used .... It sounds like the flash being used is actually an off-camera slave triggered by the camera built-in flash in control mode ... ? It is perhaps more usual to bounce a 'Master' flash from the hotshoe .... You understood correctly. The flash was used off camera in slave mode and bouced off a wall. I totally agree that is more usual to bounce from on camera flash, but since the 360 v1 doesn't have a swivel head, it's not possible to bounce it off the wall, which was what I wanted to try... However, I've used it many time bounced off the ceiling with nice results. Although I'm often wondering if I would be better off buying a flash with a swivel head, maybe a cheap Bolt, or a Metz 52... Since I can't made up my mind, I stick with the 360, which does a great job most of the time. The Bolt flash looks good on paper, but feedback from users is almost nonexistent, and although it's cheap I don't feel like taking a bet on it.