[amoringello]

My question as well. The focus mode should not impact P-TTL function since it is not part of the equation. The camera does not need to know focus distance in order to measure the intensity of the pre-flash burst and it is never part of the equation. That is, after all, why we use P-TTL. What is more, the AF system does not calculate actual distance, it merely establishes best focus regardless of distance.

Your underexposure at very close distance is not unexpected depending on what you mean by "close". Most on-camera flash units do not have wide enough coverage to illuminate a subject at closer than about a 18 inches. That is why specialized ring flashes and off-camera flash are the norm for close-up and macro work.
Steve

Yeah, I have seen in the EXIF that is merely indicates something like "close" or something like that. I don't know if it does anything more precise internally but at least from the EXIF it seems distance is a pretty rough estimate. Oddly, in some file's EXIF I see "Approximate Focus Distance" with a value of feet. Again, it is a pretty rough measurement as the value is "4" when the camera is definitely closer than four feet. But I cannot figure out how to get the camera to recreate that value in the EXIF so I cannot determine the accuracy.
Regardless, the measurement is probably equally inaccurate. :-(

FYI, segment of EXIF:
------------------------------------
:
:
Format : image/dng
Creator Tool : PENTAX K-3 Ver. 1.03
Metadata Date : 2014:03:23 18:56:43-04:00
Lens : smc PENTAX-DA* 16-50mm F2.8 ED AL [IF] SDM
Approximate Focus Distance : 4
Date Created : 2014:03:23 18:52:23
:
:
Subject Distance Range : Macro
:
:
------------------------------------


And I understand where you're coming from that the underexposure is not unexpected at close range but the specific cases being argued are handled perfectly with the K5. So it is not a limitation of the flash but a limitation of the camera.
There are certainly problems with determining proper exposure at "normal" flash power when the pre-flash is so close and so relatively bright that the camera sees only pure white due to the solar blast conditions of being extremely close. I'm simply not that close yet.

I guess the question would be, can you think of any argument supporting why the K3 cannot handle this situation that also explains why it works perfectly fine with the K5.

---------- Post added 03-29-14 at 09:59 AM ----------

What exactly do you mean by "manual focus lens"?
Note that unless your lens is at least an "A-lens", (supporting the P/KA mount) P-TTL won't be used at all.

he, he, yeah, the LensBaby is certainly not an 'A' lens so as far as the camera is concerned there isn't even a lens connected at all, and everything becomes fully manual.

---

Last edited by amoringello; 03-29-2014 at 07:06 AM.

[lister6520]

I'm guessing it is an attempt gone wrong to fix the K-5 overexposure issue you mention. I haven't reproduced it yet, but perhaps because I wasn't paying attention and missed the significance of near distance and autofocus. All the tests I did were just focused once with BBAF for the first picture or not focused at all. I'm guessing that in AF mode if you don't press the BBAF button before a shot it is internally treated as manually focused in most respects.

If I do manage to reproduce it I'll have a look at what is actually being communicated between the flash and the camera but it will have to wait a bit as I'm currently re configuring the equipment to figure out the GPS communication through the hotshoe.

[amoringello]

Does BBAF (I assume Back Button AutoFocus?) make a difference? I rarely use it. Tried for a week but just haven't found any change in my ability to get better photos.
It would be cool to know what's going on between the camera and flash.

Anyway, I went out for my weekly "charity product shoot" and had no problems at all.
I found a location to set up so I could keep back an extra foot or two, and used EV+/- when necessary to pump up the flash.
Things went very smoothly with very little re-shooting, now that I can anticipate the behavior a little better.

I still think the underexposure is wrong but if it is consistent, I can make things work.

[johnc]

I still think the underexposure is wrong but if it is consistent, I can make things work.

Once again - knowing is half the battle. The k-5 bounce flash overexposure was no great drama - easy to deal with tbh!

[awaldram]

OK, I guess I should repeat. I'm using an AF540FGZII on a K-3. The problem only happens with bounced flash when the camera is less than about 3 feet from the subject. It is very repeatable and never happens when I am further from the subject or if I use a manual focus lens.

I must be missing somthing here
from the flashes manuals

However, if the minimum distance obtained is 0.7m or less, the minimum distance will be 0.7m.

i.e under all circumstances you exceed min flash distance @.7m

So if you break the rules the flash doesn't work !!!!

[NotaxPen]

I must be missing somthing here
from the flashes manuals

However, if the minimum distance obtained is 0.7m or less, the minimum distance will be 0.7m.

i.e under all circumstances you exceed min flash distance @.7m

So if you break the rules the flash doesn't work !!!!

If the flash is bounced and the distance from the flash to the wall/ceiling back to the subject is about 7-8 feet, how is that less than the minimum distance of .7m?
If I exceed the minimum distance, in other words get too close, it should be overexposed, NOT underexposed.

As I said, with my manual focus A 28/2.0 I can shoot as close as it can focus (1 foot) and get properly exposed bounce flash pictures every time.

What possibly could explain the difference in behavior of the PTTL between autofocus and manual focus lenses?

[amoringello]

I must be missing somthing here
from the flashes manuals

However, if the minimum distance obtained is 0.7m or less, the minimum distance will be 0.7m.

i.e under all circumstances you exceed min flash distance @.7m

So if you break the rules the flash doesn't work !!!!

Don't know where you get that it doesn't work.
It has been a long thread but I think you've missed several aspects of this discussion;
- The first being that it works fine on the K5.
- Second, you've missed the part where the problem exists up to nearly four feet (FYI, 0.7 meters is only about 2.25 feet, via rough calc in my head)
- Also remember that with bounced flash, the flash distance is unknown.
In my case, a limited portion of the light is being bounced off the bounce card. A significant portion of the light is then going straight up where in most cases that is into the air and not bounced off of anything.
- And recently another K3 owner has found that even the K3 gets proper exposure when manual focus lenses are used. I have not been able to verify, but it is interesting to note.

[lister6520]

Pentax DSLRs require the flash to report whether it is facing straight forward or not and uses that information to influence how it works. This should supposedly give additional information to the metering and other algortithms to make a better choice. However as far as I can tell whatever way it is modifying its behaviour it always seems to be for the worse.

I have found this to be a problem with underexposure with the K-30 in the past and found a workaround, even if a somewhat clumsy one. Instead of tilting up the flash head I leave it pointed straight forward and then put a mirror in front of it tilted at 45 degrees upwards. It achieves the exact same effect optically but doesn't close the contact that signals the flash is in bounce mode. In other words the camera doesn't 'know' that the flash is bounding and thinks it is direct. Curiously it always works at least as good as and on occasions much better than it does when actually tilting the flash head.

They probably should just scrap whatever code they have implemented that tries to take into account the flash being in bounce mode and instead just let P-TTL do its thing without such clever 'improvements'

[stevebrot]

So if you break the rules the flash doesn't work !!!!

ROFL




Steve

[awaldram]

If the flash is bounced and the distance from the flash to the wall/ceiling back to the subject is about 7-8 feet, how is that less than the minimum distance of .7m?
If I exceed the minimum distance, in other words get too close, it should be overexposed, NOT underexposed.

As I said, with my manual focus A 28/2.0 I can shoot as close as it can focus (1 foot) and get properly exposed bounce flash pictures every time.

What possibly could explain the difference in behavior of the PTTL between autofocus and manual focus lenses?

The sensor is in the camera it makes no odds for Min distance whether you bounce or not.

If the distance is to short for the the camera to notify the flash to shut off then you will get over or underexposure dependant on the lighting situation as the pre-flash shuts down whihc will be depndant on processing invovlved (matrix/hss/bounced etc etc)

The fact you can exceed the spec in certain situations and it 'works' does not mean when you cannot there is an issue.!

[NotaxPen]

The sensor is in the camera it makes no odds for Min distance whether you bounce or not.

If the distance is to short for the the camera to notify the flash to shut off then you will get over or underexposure dependant on the lighting situation as the pre-flash shuts down whihc will be depndant on processing invovlved (matrix/hss/bounced etc etc)

The fact you can exceed the spec in certain situations and it 'works' does not mean when you cannot there is an issue.!

That makes sense, but I don't think you read what I wrote carefully.
I am NOT exceeding the spec!

The distance the light from the flash is traveling is about 7-8 feet. That is NOT less than .7m. According to my google calculation, .7m is 2.3 feet.

If I put the AF540FGZII on top of my K-3 and put it in PTTL mode with the flash head facing direct and not bouncing, it says the flash range is 2.3 - 18.2 feet (I'm at f 5.6) If I take a photo 2.3 feet from the subject with the flash head facing forward, the exposure is nearly perfect every time. The camera can obviously calculate correct exposure at 2.3 feet.

If I take a photo 2.3 feet to about 3.5 feet from the subject with the flash head turned so that it bounces off the wall/ceiling the photo is about 4-5 stops underexposed. When I have the flash head turned so that it is bouncing off the celing/wall, the distance is definitely more than the minimum of 2.3 feet and definitely less than the maximum of 18.2 feet.

When I back up a little so the camera is more than about 3.5 feet from the subject, the bounce flash exposures are correct every time.

when I take a photo with a manual focus lens and bounce the flash off the wall/ceiling, this error does not ever occur regardless of the camera to subject distance

There must be something other than the preflash influencing whether it exposes correctly or not.

[lister6520]

@Notaxpen
Try leaving the flash facing straight forward and using a mirror as described in my post just above. My theory is that you will get a good exposure. It seems that if the camera 'knows' that the flash is in bounce mode it messes up but if it is oblivious to that fact it works just fine.

That being said I've spent some time trying to reproduce the problem but I have not yet succeeded. I've tried camera to subject distances of 2,3 and 4 feet and flash to ceiling distance of 3 and 5 feet. I've tried AF, MF and also using the half press for AF (I usually use BBAF). I'm getting a good exposure every time. One may be of the opinion that the pictures are a little underexposed but nothing like what OP has been getting. The only way to get different exposure is when I shoot directly at less than two foot in which case I get some blown highlights - but I think that is expected as the flash just cannot power down enough.

Maybe worth mentioning that I'm using a Metz 58AF2 rather than a Pentax flash. I don't know if that might be making the difference. Or maybe the fault is intermittent and I just haven't happened upon it. Has anyone reproduced this problem with a non Pentax flash?


This is one of the pictures - not sure which combination of parameters but they all looked pretty much the same anyway (apart from the very near direct ones)


The picture is unaltered except scaling down the resolution. It is imported in lightroom from the OOC jpeg and exported just at reduced size with no processing whatsoever (or at least none that I am aware of)

[awaldram]

That makes sense, but I don't think you read what I wrote carefully.
I am NOT exceeding the spec!

When I back up a little so the camera is more than about 3.5 feet from the subject, the bounce flash exposures are correct every time.

when I take a photo with a manual focus lens and bounce the flash off the wall/ceiling, this error does not ever occur regardless of the camera to subject distance

There must be something other than the preflash influencing whether it exposes correctly or not.

1 Camera subject distance + processing = min distance (bounce irrelevant)
easiest processing direct min = 2.3ft anything else greater required distance. (bounce = more processing )
ergo
"When I back up a little so the camera is more than about 3.5 feet from the subject, the bounce flash exposures are correct every time"

2 Manual lens means no matrix metering , no p-ttl no pre trigger no processing so I don't understand your reference to it.

3 But you have proved that P-ttl and distance are the variables that effect exposure at min distance !!

---------- Post added 07-04-14 at 01:15 PM ----------

@Notaxpen

Maybe worth mentioning that I'm using a Metz 58AF2 rather than a Pentax flash. I don't know if that might be making the difference. Or maybe the fault is intermittent and I just haven't happened upon it. Has anyone reproduced this problem with a non Pentax flash?

)

af58af2 = min 1/256 = 1/33,000 pulse
fgz540 = min 1/64 = 1/14,000 pulse

min GN @ 50mm ioso100, fgz540 = GN5, af58af2 = 2.5

[Eyewanders]

Not trying to start anything here, but browsing through this thread simply makes me glad I never use TTL. The time spent here, IMO would be better served just nailing manual flash settings. If you're shooting primarily fixed subjects and product type shots, this is a no-brainer for me. Another case of the camera tech trying to be smarter than the photographer (and failing). The only time I really consider using TLL is in rapidly changing lighting conditions, different subjects, different distances and no time to dial it in manually.
(here ends the voice of dissent)

[Class A]

The time spent here, IMO would be better served just nailing manual flash settings. If you're shooting primarily fixed subjects and product type shots, this is a no-brainer for me.

+1

The only time I really consider using TLL is in rapidly changing lighting conditions, different subjects, different distances and no time to dial it in manually.

Same here.

Having said that, I once shot an event (action that I could not control) with a fully manual flash and it went fine. The client was super happy with the results. As long as you don't overexpose, a lot can saved by shooting RAW and being in the right ballpark.

The only real advantages I see in TTL are support for second-curtain sync and HSS. The RF60 allows you to emulate these features, but you'll have to punch in delay values manually which is not as convenient as just pressing the shutter button. If Pentax dealt with flash triggering like Nikon does, then TTL would not even be needed for second-curtain sync & HSS, but they don't...