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04-29-2012, 02:05 PM   #1
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P-TTL starter

I've read both Camera and Flash users manual, and still am stumped as to how P-TTL should work. Hardware info. K10D and a Metz AF-50 flash. Also using the standard kit lens. Using "X" and the flash on "M", I can get pretty decent images, (even some keepers). I've tried P-TTL using Program, TAV modes and my results are disappointing. Just looking for a few starter tips.

04-29-2012, 02:14 PM   #2
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Post some pictures with EXIF data intact to help us understand what the issue is.
Be aware that metering is the biggest determinate of exposure in those auto modes.
P-TTL works quite consistently if you're in the right metering mode for the type of scene you're shooting, and you understand concepts like auto-exposure lock, spot and centre-weighted metering.
04-29-2012, 03:53 PM   #3
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Hi Ash, I posted 2 images in the members gallery under jcamero, heading of P-TTL Test.
04-30-2012, 06:15 AM   #4
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I find that both my Metz 58 and Metz 54 don't like Auto-ISO. If I manually set the ISO, they work consistently in all programs. I also, agree with Ash, that if you use AE-Lock (which will lock the ISO), it should be the same as using M mode.

04-30-2012, 11:34 AM - 2 Likes   #5
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something to think about with pentax flash and auto modes.

THe designers have decided that the flash is principally meant to be a fill flash, and as a result the camera will attempt to meet the exposure criteria any way possible before using flash. As a result, It will adjust ISO first, increasing it to the auto ISO limit, then reduce either shutter or aperture or both following the rules of hand holding (1/focal length) or the MTF curve of the lens, and only then will it start increasing flash power.

I find it much more practical to use the camera in manual mode, where I set the shutter and aperture as I want, and let the flash do the rest. If I want a heavy flash contribution I will set the aperture and shutter so that the natural lighting is about 4 stops below and the flash provides almost all the scene lighting. If I want fill flash, then the natural lighting may be as close to within 1/2 stop of the requirement and the flash does the rest.

I have no issues with exposure using this method
04-30-2012, 07:29 PM   #6
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Hi jcamero,

Though short, this thread holds a lot of concise information to use P-TTL flash effectively. I think if you try using this info, you'll get it, and be happy with the performance.

Most threads like this devolve into "I just use fully manual -- P-TTL sucks!" testimonials, and I've never understood why. . .P-TTL has some flaws, but mostly for really advanced use, and it's really pretty useful if you take the time to understand it. Pentax should probably take some time to explain their thinking -- Lowell managed to do it very well in just two paragraphs.

If you continue to have problems, the best thing to do is post some pics (with exif intact) along with a description of what you were trying to achieve. I'm sure that someone will be able to offer some suggestions.

Scott
05-02-2012, 08:04 AM   #7
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QuoteOriginally posted by Lowell Goudge Quote
I find it much more practical to use the camera in manual mode, where I set the shutter and aperture as I want, and let the flash do the rest.
I find exactly the same (I have a Metz 36 AF-5). Which leads to the inevitable question: what is the value of P-TTL over manual flash?

I have a very good manual flash (Lumopro) so I'm inclined to lose the P-TTL one.
05-02-2012, 08:37 AM - 1 Like   #8
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QuoteOriginally posted by top-quark Quote
I find exactly the same (I have a Metz 36 AF-5). Which leads to the inevitable question: what is the value of P-TTL over manual flash?

I have a very good manual flash (Lumopro) so I'm inclined to lose the P-TTL one.
The advantage of P-TTL has nothing to do with manual exposure on the camera. We are talking different things here, whether you have automatic control of flash, or automatic control of the camera natural light exposure or both or neither.

Here is the issue in a nutshell.

As discussed earlier, using automatic exposure modes with flash leaves the camera attempting by any means permitted through the mode selected, to meet the exposure without flash. Then flash always fires at the lowest power. That is why I manually set the natural component of the light for flash exposure.

As for flash control, there are 4 modes potentially,
-Manual, where you take the guide number of the flash, your subject distance, the lens aperture and the ISO setting, and then calculate manually the flash power in increments of either full or 1/2 stops
- Auto, where you tell the flash the aperture, and ISO and it attempts to expose with its own sensor that probably has a different FOV than your lens, whether the light it receives back is enough for the scene
is sufficient to expose the image.
- TTL which is similar to Auto, except that the flash control sensor is seeing light reflected off either the film or the front filter of the image sensor in a DSLR
- P-TTL, which works completely differently, and uses a preflash, and open aperture metering to calculate the duration of the flash needed to illuminate the scene when blended with the natural lighting.

The problem with AUTO, is , as noted that the sensor is not metering off your scene that you see through the lens, and therefore might have errors because of reflective surfaces that are not in your scene, or because the subject is in the foreground and the sensor is trying to meter off the background very far away. Additionally, even if the flash shuts off when it gets the correct amount of light back, the natural exposure may still keep exposing the frame, leading to ghosting and over exposure.

TTL is better than auto, but still is a reactive system, and cannot compensate properly for the remaining light after the flash is shut off and the shutter remains open.

P-TTL is much better because it knows the ambient light, and measures the increase due to the contribution of the preflash, and all your camera settings to calculate the correct duration before triggering the shutter, it is predictive rather than reactive. But to do this, it needs to know the maximum aperture , shooting aperture and ambient light.

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