Your focal length with a 1.5 tc fitted will be 1.5 x whatever length you set the zoom at. So at the 500mm end the focal length will be 750mm.
With your camera you will also have a crop factor which will alter the Field of view but this should not be confused with an increased focal length.
I do not have this lens but have read reports from users who say that image qualilty is badly effected by adding a tc.
As Derek as already said ISO does not come into it.
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