P-TTL flash - Camera in Manual mode - ISO shouldn't change exposure right?

José Ramos · 05-23-2010, 11:13 AM

Hello there.

I'm have a Pentax K10d and a Sigma EF-530 DG ST

I'm new to this flash thing. I was experimenting with it, and I don't understand one thing: either in manual or in aperture priority mode, with the flash in TTL mode, changing the ISO will change the exposure of the image. Considering that the flash is in TTL mode, and that the manual camera settings haven't changed, wasn't the flash supposed to compensate for the ISO difference, so that the final image/exposure output would be the same?

**Canada_Rockies** · 05-23-2010, 11:48 AM

I believe that you need to be in Program mode or its equivalent to be able to use P-TTL properly so that the flash can control the aperture of the lens.

Russell-Evans · 05-23-2010, 05:02 PM

The flash should compensate and use less power to get the same metered exposure. If you are testing this on a tripod, be sure to block light entering in through the finder as this will throw off the metering.

Thank you
Russell

enoeske · 05-23-2010, 05:10 PM

Additionally, this "equal exposure" only pertains to that which the flash can hit. If you are expecting the background to remain at the same exposure, without changing anything but the ISO, it won't. The background will change, but the subject hit with the flash should be the same.

stgmgr · 05-29-2010, 09:14 PM

I was using my Pentax 540 flash this evening, set to P-TTL mode.
Camera was set to Aperture priority.
I selected f13 in order to capture some DoF in my subject that was no more than 8 feet away.
Resulting image was very dark!
Shouldn't TTL compensate?
Isn't that what Through The Lens means??

Ash · 05-29-2010, 10:00 PM

Depends on a few factors - flashes are not magical, there is a considerable knowledge-set that is assumed to get reliable results.
f/13 is a small aperture and at low ISO it would require a lot of light to adequately expose a subject even 3 metres away.
Posting your underexposed image with intact EXIF would help explain things better.

05-23-2010, 11:13 AM	#1
José Ramos Junior Member Join Date: Aug 2007 Location: Portugal Posts: 37	P-TTL flash - Camera in Manual mode - ISO shouldn't change exposure right? Hello there. I'm have a Pentax K10d and a Sigma EF-530 DG ST I'm new to this flash thing. I was experimenting with it, and I don't understand one thing: either in manual or in aperture priority mode, with the flash in TTL mode, changing the ISO will change the exposure of the image. Considering that the flash is in TTL mode, and that the manual camera settings haven't changed, wasn't the flash supposed to compensate for the ISO difference, so that the final image/exposure output would be the same?

05-29-2010, 09:14 PM	#5
stgmgr Inactive Account Join Date: Aug 2008 Location: MiltonBrampton Posts: 84	Similar Problem? I was using my Pentax 540 flash this evening, set to P-TTL mode. Camera was set to Aperture priority. I selected f13 in order to capture some DoF in my subject that was no more than 8 feet away. Resulting image was very dark! Shouldn't TTL compensate? Isn't that what Through The Lens means??

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05-23-2010, 11:48 AM	#2
Canada_Rockies Loyal Site Supporter Join Date: Nov 2007 Location: Sparwood, BC, Canada Photos: Gallery \| Albums Posts: 12,384	I believe that you need to be in Program mode or its equivalent to be able to use P-TTL properly so that the flash can control the aperture of the lens.

05-23-2010, 05:02 PM	#3
Russell-Evans Veteran Member Join Date: Nov 2007 Posts: 1,553	The flash should compensate and use less power to get the same metered exposure. If you are testing this on a tripod, be sure to block light entering in through the finder as this will throw off the metering. Thank you Russell

05-23-2010, 05:10 PM	#4
enoeske Veteran Member Join Date: Jul 2009 Location: Surprise, Az Photos: Gallery Posts: 2,135	Additionally, this "equal exposure" only pertains to that which the flash can hit. If you are expecting the background to remain at the same exposure, without changing anything but the ISO, it won't. The background will change, but the subject hit with the flash should be the same.

05-29-2010, 10:00 PM	#6
Ash Moderator Join Date: Mar 2007 Location: Toowoomba, Queensland Photos: Gallery \| Albums Posts: 23,914	Depends on a few factors - flashes are not magical, there is a considerable knowledge-set that is assumed to get reliable results. f/13 is a small aperture and at low ISO it would require a lot of light to adequately expose a subject even 3 metres away. Posting your underexposed image with intact EXIF would help explain things better.

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