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06-30-2009, 08:59 AM   #1
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Flash exposure issue

My AF540FGZ with K20D acts funny.


I used to use it quite successfully in P-TTL mode on my former camera (K10D).

With K20D (v.1.3) it underexposes badly.


Here is what happens:

In dimly lit room, shooting at furniture, people, floor, anything really at the distance of 10ft in P-TTL, Av at F8, ISO Auto 200-800, most camera settings default shots are consistently underexposed by several points. Underexposure is worse with the lens closed down (F11) and so-so, but still present with the lens opened up to F4. Same story for various lenses.


The same settings on camera with the flash set to A or M produce beautiful images. Flash is more than capable to lit subject at this distance.


But here is a twist: the MODE switch on the flash most of the times toggles between P-TLL and SB mode only! (so A and M modes are not available). I said most of the times because occasionally it will go to A and M for example when the K20D is turned OFF. But then again sometimes Ill set it to A, half-press the shutter and it throws me back to P-TTL.

Ive cleaned the contacts on the flash and on the camera a few times with alcohol no change.

I believe that either my K20D is broken or my flash went rogue. I am hoping to hear suggestions as of which one is it.

Any suggestions before I send it to Pentax for service? Id hate to loose a camera for a summer!

07-01-2009, 12:11 PM   #2
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It will allow you to chose A or M only after the meter in the camera has timed out. The camera need not be off, but if it is metering only P-TTL and SB are selectable.
As far as the exposure issue - I can only tell you that my 360 does exactly the same thing. On-board flash, or 360 in "auto", exposure is great. 360 in P-TTL, underexposed.
On another flash topic - I am still wondering why auto zoom is available with the flash in "manual" but not with the flash in "auto". That seems bass-ackward to me.
07-01-2009, 02:35 PM   #3
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QuoteOriginally posted by slawek Quote
But here is a twist: the MODE switch on the flash most of the times toggles between P-TLL and SB mode only! (so A and M modes are not available). I said most of the times because occasionally it will go to A and M for example when the K20D is turned OFF. But then again sometimes Ill set it to A, half-press the shutter and it throws me back to P-TTL.
What mode is the camera body in? I don't remember the specifics, but please try again with the camera body in M or X mode.

About P-TTL in general, it is supposed to be easy. But it is so unpredictable I use it ONLY when I have to. My 15 year old (film) Minolta 7xi with 5400 flash produces more consistent exposures.
07-07-2009, 08:38 PM   #4
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problem solved

It turned out to be a faulty rechargeable batteries issue (2 separate sets of Energizer and Duracell - both freshly charged) . When I put in my trusty Eneloops the problem dissapeared .

A few things I have learned in the process (other than using ONLY good quality batteries):

1. It will allow you to chose A or M only after the meter in the camera has timed out.
2. Set overexposure to +2 in camera before shooting with P-TTL
3. Use AUTO mode for more predictable results
4. Use Manual mode for even more predictable and consisten results

Thanks to everybody for their input.

And now I am off to DealExtreme to order one more set of Eneloops (DealExtreme: $12.61 Sanyo Eneloop 2000mAh Ni-MH Rechargeable AA Batteries (4-Pack))

07-07-2009, 09:24 PM   #5
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FEC can only be set to +1.0 in camera.
Do you mean EV compensation? If so, this does not alter flash output or shutter speed in flash photography using auto modes (Av, Tv, P, etc) as far as I'm aware...

But you're right - P-TTL is inconsistent, and I too would favour manual mode on camera and in flash, which is quite sad for Pentax - this would be one feature I believe should be resolved sooner rather than later (K-7 users can tell us whether this has happened or not...)
07-08-2009, 07:54 AM   #6
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QuoteOriginally posted by Ash Quote
FEC can only be set to +1.0 in camera.
Do you mean EV compensation? If so, this does not alter flash output or shutter speed in flash photography using auto modes (Av, Tv, P, etc) as far as I'm aware...

But you're right - P-TTL is inconsistent, and I too would favour manual mode on camera and in flash, which is quite sad for Pentax - this would be one feature I believe should be resolved sooner rather than later (K-7 users can tell us whether this has happened or not...)
Check out my review of the Vivitar DF-383 flash. I have the same problems and had to use BOTH EV and FEC to get correct exposure in bounce and swivel mode. IT IS SAD P-TTL does not work right even with Pentax flashes.
07-08-2009, 03:00 PM   #7
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FEC is the only thing that matters when using flash. EV changes don't make a difference as the shutter speed is fixed according to the focal length in Av mode and at 1/180 in X mode.

P-TTL is Pentax's big weak point.
07-08-2009, 06:39 PM   #8
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QuoteOriginally posted by Ash Quote
FEC is the only thing that matters when using flash. EV changes don't make a difference as the shutter speed is fixed according to the focal length in Av mode and at 1/180 in X mode.

P-TTL is Pentax's big weak point.
I thought that too, Ash. But If I am not mistaken I DID see a difference in exposure as I adjusted the EV with the FEC plused out. I am not home yet; at a relatives right now. I well double check to make sure.

I am still new to dedicated flash and I could be wrong.


Last edited by res3567; 07-08-2009 at 09:07 PM.
07-08-2009, 07:38 PM   #9
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Well. I checked again and with the flash set at 0 (no overexposure) P-TTL and camera EV (the little +/- buttton next to AF button set at 0, +1, +2 and +3 I can DEFINITELY see the difference:
0 is badly underexposed, +1 is somewhat underexposed, +2 and +3 are very similar and seem to be exposed correctly. I shot in Av mode set at F11.
Perhaps P-TTL works - it's just not calibrated corectly???
07-08-2009, 08:27 PM   #10
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QuoteOriginally posted by slawek Quote
Perhaps P-TTL works - it's just not calibrated corectly???
Try different scenes to see if you can duplicate the result.

My bet is that you can not.
07-08-2009, 09:08 PM   #11
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QuoteOriginally posted by res3567 Quote
I thought that too, Ash. But If I am not mistaken I DID see a difference in exposure as I adjusted the EV with the FEC plused out. I am not home yet; at a relatives right now. I well double check to make sure.

I am still new to dedicated flash and I could be wrong.
Yes Ash it is confirmed! EV does adjust the flash ouput. I just did a series of tests adjusting BOTH FEC and EV and I DID see a difference! My theory is that the camera is telling the flash to increase its output as the shutter speed does not change.
07-09-2009, 03:09 PM   #12
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QuoteOriginally posted by res3567 Quote
Yes Ash it is confirmed! EV does adjust the flash ouput. I just did a series of tests adjusting BOTH FEC and EV and I DID see a difference! My theory is that the camera is telling the flash to increase its output as the shutter speed does not change.
Yes, I see the same result. I found this thread looking for that very point. The camera's EV shouldn't logically have an effect, yet it does. That seemed wrong, so I came looking, but you guys are seeing it, too. Guess it's a "feature."

I just shot an "average living room scene." Sigma EF-530 DG ST in P-TTL mode, FEC 0, on a K10D. Camera in "P" mode, normal program line, ISO 100, EV 0. Camera selected 1/30s, f/3.5. Badly under-exposed.

Second shot, identical settings on camera and strobe, except the camera's EV was set to +2.0. Camera selected 1/30s @ f/3.5 again. Much better. Maybe 1/3 of a stop under-exposed, if that. Bizarre.

Last edited by Duck Dodgers; 07-09-2009 at 03:17 PM.
07-09-2009, 03:48 PM   #13
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That I cannot explain.
Well, I'l keep that in mind if FEC+1.0 just doesn't cut it!
07-09-2009, 06:27 PM   #14
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QuoteOriginally posted by Ash Quote
That I cannot explain.
Well, I'l keep that in mind if FEC+1.0 just doesn't cut it!
Maybe EV becomes FEC? I dont know either but it sure looks like that! Has anyone experienced this on a new Pentax flash?

Also using FEC along with EV produce great results. I adjust FEC first and its like EV gives the flash more of a boost. EV has to be telling the flash we need more power above FEC!
07-10-2009, 05:03 AM   #15
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This has been a pretty frequent topic. Point the flash up to bounce from the ceiling and you'll find the EV/FE compensation is no longer needed. Whatever it's doing, it's doing on purpose. The leading theory is that the P-TTL algorithm is too sensitive to bounced-back light caught by the matrix metering and is trying very hard to avoid the blown-out-flash look.

I'll have to experiment with my K-7. In theory, the much-better matrix metering should make this less of a problem.

Also in theory, it could be changed by a firmware update, but the fact that Pentax has kept it this way for so long seems to indicate that it's on purpose.

Note that if you have a Metz 58 AF-1, it will stay in A mode without jumping back to P-TTL.
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