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10-07-2015, 10:08 AM   #1
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Problem with Pentax AF540FGZ II

Hi,
I have a problem with my flash's P-TTL (PENTAX AF540FGZ II) when using the following flash settings:


-flash is attached to the camera (K3) bounced at 60-75 (bright celling)
-flash zoom set to 13mm with its wide angle panel
-any program mode except in manual of course
-flash mode: P-TTL 0,0EV - Leading Curtain Sync Mode
-short lens focus distance (about 1-2 meters) THIS IS IMPORTANT!


If I take a picture with these settings directly to a white wall or if a composition includes white parts I get very underexposed picture. If I change the lens focus distance to about >4 metersor change the flash zoom to >34mm or leave it in auto the flash starts working normally. It has been tested with DA35 and DA50. At DA50 the flash zoom must be set to >48mm or auto.


If I take the flash off the camera and put it in wireless mode near the body with the same settings I have no problems.
It also works normally if a composition doesn't include white parts (not too much).


I don't know if the problem is in the flash or camera. The firmware version is 1.21


Do you have any similar experiences with Pentax flashes?


Thank you.


Last edited by zan; 10-07-2015 at 10:14 AM.
10-07-2015, 03:48 PM   #2
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I get the feeling that there is a mismatch between the spread of light you are creating (with the bounce direction and flash head zoom setting) and the actual image frame (created by your focal length) .... In other words, are you actually bouncing enough light into your image circle? Maybe the wide flash zoom setting is causing too much light to disappear in the 'wrong' direction. Is there a clear line of travel across the bounce surfaces from the flash to the subject.? A longer flash zoom may help to direct more of the light towards the subject.

Remember that the bounce surfaces already spread the light (and dissipate it's power), so the extra wide zoom setting may be causing too much light loss. .... ?

Just like with ambient light metering, white surfaces will cause underexposure of automatic flash, so flash compensation would be needed.

The focus distance itself doesn't affect P-TTL exposures (this was tested recently in a staff article here following questions raised by my flash guide revision), but focal length is considered, with autofocus lenses.
10-07-2015, 06:52 PM   #3
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Why are you shooting slow speed sync?
10-08-2015, 12:59 AM   #4
zan
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Thank you for the answers.


Exposure settings were 1/60, F2.4, ISO (auto 100-3200) in TAV mode but no changes in TV, AV or P mode. I am talking about indoor shooting under tongsten lights.
I think the flash power is not a problem because if I put the flash very close to the camera (just few cm) in wireless mode all shots are ok. The camera sets the ISO to about 1600 otherwise it is about 3200 if the flash is turned off. This is normal.


BUT if I mount the flash to the camera with the same bounch direction and wide flash zoom settings it sets the ISO to just 100 and makes a picture very underexposed.


I don't know if the problem is in the flash or camera.

10-08-2015, 02:31 AM   #5
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QuoteOriginally posted by zan Quote
Thank you for the answers.


Exposure settings were 1/60, F2.4, ISO (auto 100-3200) in TAV mode but no changes in TV, AV or P mode. I am talking about indoor shooting under tongsten lights.
I think the flash power is not a problem because if I put the flash very close to the camera (just few cm) in wireless mode all shots are ok. The camera sets the ISO to about 1600 otherwise it is about 3200 if the flash is turned off. This is normal.


BUT if I mount the flash to the camera with the same bounch direction and wide flash zoom settings it sets the ISO to just 100 and makes a picture very underexposed.


I don't know if the problem is in the flash or camera.
Switch the camera to 'X' this will stop the camera attempting to 'mix' ambient to flash
Lock ISO to around 400

try now I think you'll find wireless and on camera behave the same

I suspect your issue is Ambient/WB and flash bounce

If you really want mixed ratio ambient to flash the figure our what exposure you 'need' fro ambient (flash off) and lock you ISO at least to a reasonable setting then add the flash, using exposure and flash compensation to get the ratio your after.

Given you current settings the flash cannot reach an acceptable exposure solution given the cameras shutter/aperture/ISO needed to achieve ambient exposure this may be because of environmental reflectivity or even just flash angle relative to lens.

If you still have issue post and image of your setup using camer phone/ P&S so we can see what you bouncing off , what the average light levels are like and distance your bouncing.

given you seem to be using white wall , white object to cause your 'issue' I'm confident its a Ambient exposure Vs Flash issue not a hardware problem.

As a general tip never use auto ISO when mixing flash with ambient (any 'auto' mode but 'X' sync) .... 'X' sync is provided for a reason for most flash use indoors this is where you should be 'X' is your friend.

Last edited by awaldram; 10-08-2015 at 02:38 AM.
10-08-2015, 02:35 AM   #6
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EDIT ...awaldram, your post came in just before mine! I think we're on the same page here .....


I would try to remove all automatic variables, just to be sure the actual physics aspects of what you're trying to achieve are possible.

I would switch to manual mode, set ISO 400, F5.6, shutter at 180thsec. In manual flash mode set 1/16th power. Assess the exposure and adjust the power until its just right. If you want more orange ambient light colouring then increase the shutter time (like 1/100, 1/30 ...). Let us know if you can get a successful and consistent exposure this way.

Then you can start to adjust each individual setting one by one to experiment .... Change the aperture and correct the flash exposure with power .... Then change the ISO and correct again. This way you'll get a feel for the effect of each adjustment.

I know you want to have everything automatic, and that should be possible, but this is the only way to assess that your set-up and flash to subject distances are realistic and within the equipments capabilities. Personally I would avoid any auto-ISO use for flash photography ....ISO and aperture and two things I'd always want control over with flash ....
10-08-2015, 03:19 AM - 1 Like   #7
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In the past I had posted what became a rather long thread about issues with the K3 and how PTTL had apparently changed from the K5.
The K3 seems to have more problem with exposure when up close (less than about four feet).

Metering is difficult when there is a lot of white. Metering systems as a whole are built to create a medium grey, so underexposure is often expected in this case.
You may need to increase exposure if there is a lot of white in the scene, as you would when photographing snow.

Trying to say the problem is strictly in the body or the flash is difficult.
The body is generally what does the calculations for adjusting flash power and metering the result, but if you move the flash or change the distance you are affecting the environment.
So the flash is not exactly the problem but it greatly affects how the body reacts.


PTTL has always had issues with not being able to meter well when there are specular highlights or bright white. (Pentax needs to revamp PTTL)
The mere glint off a link in a thin chain necklace can destroy a portrait, for example.
The K3 has improved in most cases but still has issues with metering and certainly different from the K5 and prior bodies.


I do some minor product photography for a resale shop. Not a perfect setting (not on a tripod, I'm in tight cramped areas, and never able to get exact same lighting for any two objects).
But what I have found seems to indicate that that the slightest change in distance or angle can vastly affect the metering.

Having the flash off-camera in wireless mode is probably enough to change the metering both how the camera decides what to do as well as the physical properties of how the light is reflected back to the camera.
10-08-2015, 03:59 AM   #8
zan
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Ok. Thanks


I am going to test the suggested settings in next days and let you know.

10-09-2015, 03:04 AM   #9
zan
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I am done with the testing. You are right. The manual modes are better for indoor shooting. I have more control over exposures and ratios between abmient
and flash light.


Thank you very much.
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