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05-04-2021, 09:25 AM   #1
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Pentax AF 360FGZII underexposing on P-TTL

My 360FGZII seems to be underexposing by anywhere from 1-2 stops when in P-ttl mode. Tried AV, P, and Auto, does the same thing. At first I thought it was the modifier, but even with bare flash. Using EC on the flash doesn't seem to make any difference, but using EC on the camera does if I up it to +2 or more. I'm out of ideas.....any suggestions

05-04-2021, 09:37 AM   #2
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Here are the usual questions:
  • What camera?
  • What lens?
  • Did it previously work?


Steve
05-04-2021, 09:43 AM   #3
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QuoteOriginally posted by stevebrot Quote
Here are the usual questions:
  • What camera?
  • What lens?
  • Did it previously work?


Steve
K1MII
Pentax 70-200 f/28
I don't believe it has, but attributed the underexposure to user error.
05-04-2021, 09:45 AM   #4
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It is difficult to say without images. But is sounds like the exposue settings you use is not enough to get ambient/background light correctly exposed.

If there is anything in the foreground then it will be overexposed if using to much power from the flash so the flash will be limited in power, and the exposue for the background is controlled by the camera exposure settings.

05-04-2021, 10:04 AM   #5
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QuoteOriginally posted by Smoke665 Quote
K1MII
Pentax 70-200 f/28
I don't believe it has, but attributed the underexposure to user error.
In green (auto) mode, user error should not be a consideration. Do you have similar problems with other lenses or on another camera (if you have one)?

P-TTL is generally pretty predictable as long as it is being used with AF lenses and subject distance and nature of the subject* does not exceed the power of the flash. Adding modifiers and/or bounce may complicate things.


Steve

* low reflectivity subjects are best addressed using manual technique.
05-04-2021, 10:32 AM   #6
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If you have your aperture closed down to much, you will reach a point where the flash just canít put out enough light. Iíve seen this happen with my 320ws studio strobe. Depending on the modifier (they usually cost you at least 1 stop), the aperture and ISO you use will greatly affect the amount of light output required.
05-04-2021, 11:11 AM   #7
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More questions:
-subject
-distance

Have you tried using a calculated "manual" setting ?
05-04-2021, 11:29 AM   #8
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QuoteOriginally posted by Smoke665 Quote
My 360FGZII seems to be underexposing by anywhere from 1-2 stops when in P-ttl mode. Tried AV, P, and Auto, does the same thing. At first I thought it was the modifier, but even with bare flash. Using EC on the flash doesn't seem to make any difference, but using EC on the camera does if I up it to +2 or more. I'm out of ideas.....any suggestions
Can you post some picture examples.

The usual cause of u/e is too great a subject to flash distance.

05-04-2021, 11:39 AM   #9
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QuoteOriginally posted by stevebrot Quote
P-TTL is generally pretty predictable as long as it is being used with AF lenses and subject distance and nature of the subject* does not exceed the power of the flash. Adding modifiers and/or bounce may complicate things.
Yeah I thought that initially, but here's a shot of the wall. Distance is 5' set to program mode, ISO 200, 83mm, f/3,2, 1,/125. and it's under by a good 2 stops.


now the same on all manual ISO 200, 83mm, f/2.8, 1/125, with the flash on manual at 1/32 power.


Last edited by Smoke665; 05-04-2021 at 11:54 AM.
05-04-2021, 01:39 PM   #10
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Tough wall, all those ridges make any reflected light travel in every direction- hence "fooling" the sensor.

But, just what "color depth is the wall" ? Close to the second shot or in between?
05-04-2021, 02:27 PM   #11
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QuoteOriginally posted by DonV Quote
Tough wall, all those ridges make any reflected light travel in every direction- hence "fooling" the sensor.

But, just what "color depth is the wall" ? Close to the second shot or in between?
actually closer to the lighter. If the second shot was about -1/2 it would be close
05-04-2021, 03:36 PM - 1 Like   #12
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When I use my flash in TTL (P-TTL), I almost always use my camera in manual mode. If the camera is in an Auto mode (SV or TV for example), the camera/flash results may be less consistent in some circumstances. If your EV (camera exposure) or flash exposure is not adjusted to minus or plus, than you are not affecting the outcome yourself. You may want to test on a subject that has an ample amount of contrast to its characteristics. Usually P=TTL is very effective if using the camera in manual mode.

Last edited by C_Jones; 05-04-2021 at 04:04 PM.
05-04-2021, 05:59 PM - 1 Like   #13
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I had the opposite problem, overexposure. Solved it by going manual.
6 Days Ago   #14
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QuoteOriginally posted by C_Jones Quote
When I use my flash in TTL (P-TTL), I almost always use my camera in manual mode. If the camera is in an Auto mode (SV or TV for example), the camera/flash results may be less consistent in some circumstances. If your EV (camera exposure) or flash exposure is not adjusted to minus or plus, than you are not affecting the outcome yourself. You may want to test on a subject that has an ample amount of contrast to its characteristics. Usually P=TTL is very effective if using the camera in manual mode.
Doing a little reading last night on how P-TTL actually works. Apparently there's more user input required then the marketing hype leads you to believe. Haven't had the opportunity to test it out, but it seems using both camera EC and flash EC is necessary. I also suspect that it is somewhat dependent on what your camera exposure metering mode is set to. Normally I leave it on Spot, but I was recently at an event where I switched to Matrix, and forgot to change it back.

The same writeup I was reading also mentioned using camera manual and setting the flash to P-TTL. I was unaware that was possible. I had erroneously assumed that you had to use both on Manual. I forsee more experiments ahead.
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It is not at all unusual to need to adjust the exposure by 1-2 stops when using flash, it all depends on the distance/subject/lens/flash itself.
I have one or two units that I leave a small exposure adjustment set at all times; other combinations are spot on.

Many users observe that the old TTL method and/or a flash with its own built in sensor are more likely to give first time correct exposure than p-ttl.

You are correct- it is not that "simple".
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