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02-04-2011, 07:40 AM   #1
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Exposure Metering With Flash

Quick question...
When metering a shot using flash in Pttl, is the meter info on my camera from the ambient only or is the meter showing a number that it calculated/predicted with the flash blasted into the scene?

02-04-2011, 08:52 AM   #2
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I think the exposure is actually measured During the flash output using your ambient metering conditions. What happens is the duration of the flash is adjusted in P-TTL. So if your camera is set for 1/180 sec at f5.6, your flash will be on only long enough to give you proper exposure. That's how I Think it works anyway.

02-04-2011, 11:00 AM   #3
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The only thing I can say for sure is that in M mode,the camera will measure based on available light. Set e.g. an aperture of 8 and a shutter speed of 1/60s in a dark(ish) environment and notice the exposure (in my case it was -3 eV).

Next pop the flash up; the reading was still -3eV in my case. This was tested on a K100D both with with internal flash as well as with AF540 (to verify on the flash that P-TTL was actually active in M mode).

When the photo is taken, P-TTL comes into play and a reading is done to calculate the flash duration for correct exposure.
02-06-2011, 06:38 PM   #4
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P-TTL works by pre-flash metered calculation of exposure. That first little flash during the exposure - that's the pre-flash. It's actually fired during the exposure but it's of such tiny intensity it doesn't add to the exposure. The camera instantaneously calculates the metering based on your fixed settings (any or a combination of aperture, shutter speed and ISO) and alters the variable settings and/or flash intensity accordingly.

If you're in Av mode, the shutter speed will be fixed at approximately 1/(focal length) unless you have HSS engaged on the flash. So if you've got Av fixed, Tv fixed (not HSS) and ISO fixed (not in auto mode), then the camera tells the flash to adjust the flash intensity according to the exposure required, calculated by the metering mode you've set on the camera.

M mode's another story. Based on the EV compensation set, the camera can only meter off ambient light.

Hope this hasn't been too confusing.

02-06-2011, 07:51 PM   #5
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So if using a P-ttl flash you're better off shooting in Av or Tv than M-mode?

QuoteOriginally posted by Ash Quote
Hope this hasn't been too confusing
A little... I'm still getting to grips with flash... Neither or my guns have P-ttl... One is manual and the other has an 'auto-mode' and is pretty old but seems to work out how much power to give out as long as I shoot between 80 and 160 of a second... (I worked this out by playing around for 2 days solid)
02-06-2011, 08:28 PM   #6
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Dave, shutter speed plays no role in getting the flash portion of exposure correct. What determines that is aperture.
02-06-2011, 09:20 PM   #7
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Mike's right. Just stick with the flash sync speed of 1/180sec and you'll ensure all the flash's power is included in the exposure.

In P-TTL, you can be on whatever mode you wish for the total control you need. M mode is only useful in my opinion for dragging the shutter longer than the flash sync speed of 1/180 - this is particularly important to get a well balanced background lit by the ambient light around the scene.
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