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02-05-2011, 10:56 AM   #1
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Question about P-TTL flash

Ok, I'm going to describe my 'understanding' and please correct me and fill in the gaps if you will....

So TTL means 'Through the Lens' so the metering that controls the camera flash is more accurate. P-TTL is a more advanced version of this... (big gap here).

When I have my flash set to P-TTL and active as an off camera slave it is 'controlled' by my on camera flash which triggers it by 'flashing' before the shot...

Now in camera, the metering of a shot in a dim room is usually indicating that the shot is very underexposed. Because it actually 'meters' at the time of the shot... So if I press the green button to set exposure settings in manual mode, it is NOT taking the the flash into account at this time....

And if I do this, and then shoot I will likely end up with a much slower shutter speed then necessary... But the shutter speed does not matter with flash... because the flash is sooo fast that the exposure will really only catch the moment of the flash... (ANOTHER big big gap...)

Or will the camera compensate for the flash and adjust the shutter speed at the time of the shot after doing it's P-TTL thing??

Help please...

Ken

02-05-2011, 11:33 AM   #2
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Your understanding is pretty good but there are a few gaps

TTL has a dedicated sensor that reads light (both flash and natural light) reflected off the sensor at the time of exposure and ends the flash when exposure is reached. This is not always when the shutter closes and if there is a lot of light after the flash ends and the shutter speed is low there is a blurring due to natural light plus some over exposure

P-TTL measures ambient light and flash (from preflash) using the cameras normal metering sensors and calculates the needed duration to achieve correct exposure considering the exposure by natural light

If you press the green button this setsthe exposure so thatthe camera does not need flash so the resulting flash will be short. If you do this in a lighting condition that demands a slow shutter speed and will permit blurred images
02-05-2011, 12:25 PM   #3
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Not exactly what you were asking about, but related.

An often under reported difference between TTL & P-TTL is the pre-flash causing some peoples eyes to begin closing by the time the image is captured. This is sometimes referred to as lazy eyes and makes the subjects look terrible (I am one of those it happens to all the time).

I always wish for TTL to be brought back for this and a few other reasons.
02-05-2011, 02:28 PM   #4
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QuoteOriginally posted by betaPhoto Quote
Not exactly what you were asking about, but related.

An often under reported difference between TTL & P-TTL is the pre-flash causing some peoples eyes to begin closing by the time the image is captured. This is sometimes referred to as lazy eyes and makes the subjects look terrible (I am one of those it happens to all the time).

I always wish for TTL to be brought back for this and a few other reasons.
I want the next camera to offer both P-TTL and TTL so that I can get flash support for K and M42 lenses. My *istD is Getting a little long in the tooth

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