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12-13-2007, 01:24 AM   #1
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P-TTL in M mode

I need to clarify two things about P-TTL flash and camera M-mode. Here's how, I think, things work:

1. In camera M-mode (manual aperture and shutter), does P-TLL control the duration of the flash burst to achieve proper exposure?
Aperture and shutter are not affected as set for ambient, flash bust duration is controlled depending on distance to subject.
Example: M-mode, ambient exposure f4/45 (underexposed by -3EV), foreground subject is 10 ft. away, flash needs to burn 1/2250s to properly expose.
With the same ambient settings, subject is 15ft away, the P-TLL should allow the flash to burn for 1/1000s to properly expose.
So, I dial in the ambient exposure once, and from then on P-TLL automatically controls the flash burst to achieve proper exposure.

2. Considering the above, if I want to use fill flash to control the shadows in bright sun to 2 f-stops dynamic range, do I set aperture and shutter for the ambient light and then set "flash exposure compensation" to -2 ?
Example: in M-mode, dial f8/250 to properly expose a person's head-shot, but shadows are too deep. I only want shadows 2 f-stops darker, so I set "flash exposure compensation" to -2. If I did flash compensation 0, the portrait would be very flat with virtually no shadows.
(in reality, the bright parts will be slightly overexposed because the ambient and flash light add up, but that's a negligible error in this case),

Regards,
Stan

12-13-2007, 01:56 AM   #2
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QuoteOriginally posted by stanjo Quote
1. In camera M-mode (manual aperture and shutter), does P-TLL control the duration of the flash burst to achieve proper exposure?
Yes.

QuoteQuote:
2. Considering the above, if I want to use fill flash to control the shadows in bright sun to 2 f-stops dynamic range, do I set aperture and shutter for the ambient light and then set "flash exposure compensation" to -2 ?
Example: in M-mode, dial f8/250 to properly expose a person's head-shot, but shadows are too deep.
For 1/250 s you have to use HSS (with external flash only). Otherwise ok, just experiment a bit.
12-14-2007, 12:48 PM   #3
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Thanks for the response. It's good to know I understand it correctly.

Do you know how the metering modes (spot, center-weight, matrix) affect P-TTL?
My guess is, it's the same as with ambient exposure metering.
12-14-2007, 02:05 PM   #4
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I think it's the same, but I never tested it.
I always use matrix.

A good starting setting would be: M-mode, ISO 400, f4 or f5.6 and 1/30 s.

12-14-2007, 04:25 PM   #5
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Is the P-TTL metering calibrated to 18% gray ?

I wonder if I need to apply +EV compensation to the flash when the object is all dressed in white.
12-15-2007, 02:34 AM   #6
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QuoteOriginally posted by stanjo Quote
Is the P-TTL metering calibrated to 18% gray ?

I wonder if I need to apply +EV compensation to the flash when the object is all dressed in white.
Yes, that's correct. This is the reason of most complaints about "underexposure"; which actually was the correct exposure, exactly how the metering was calibrated. If you want white to appear white, you need to dial in EV compensation; flash or not.
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