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Does the TTL Flash system know about shutter speed?

Kropotkin

Bicester, UK
71 posts

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Kropotkin Registered since 02nd Dec 2012
Wed 05-Jun-13 04:30 PM | edited Wed 05-Jun-13 05:08 PM by Kropotkin

Hi

This is (for me at any rate) one of those thorny problems which I can't seem to quite nail.

Let's take a simple case.

The scene combines both outside through an open window and a darker interior. The brightly lit outside is off centre in the frame. The camera is Manual mode. F8 125s. TTL. (SB-910 mounted on camera). This gives me a nice exposure on the sky outside and the interior is well lit.

Now I shoot again with 60s. Everything else the same. Now the outside is over-exposed due to the one stop more of exposure. The interior is lit exactly as before.

Without flash the one stop difference made the interior much lighter. So it looks to me that TTL 'knew' about this and fired less flash. If not the second shot would have a brighter interior. But it cannot know about this from the pre-flash. So, ergo, the TTL system works out what the current camera exposure is and based on that it decides how much to add?

This is the only explanation which fits the facts but does not square with what the manual seems to suggest. (It doesn't mention shutter speed being passed to the flash). Nor with what people say about the shutter speed not mattering with flash.

Sorry; this is one of those tricky, picky, questions but one which I feel I should understand. Can anyone help?


--Justin Wyllie

http://www.justinwylliephotography.com/googleplus
http://www.nelsonphotographic.co.uk

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