Thank you for your replies.
Dark Penguin, I have a bit of a void in my practical application of my understanding of getting the best parameters for an image. I think I need tutorial "Applying ISO 101",
I am still a little stuck in the film 100, 200, 400, 800 thing, and need a bit of help to get around that. From what I understand, unless one is photographing under controlled lighting situations, nearly every image would benefit from having its' own custom ISO setting.
I would normally set my ISO to the 'old ASA' amount, and generally then my aperture, and if the Tv calculated and histogram looked ok...that was the image I settled with. If I wanted a specific shutter speed, I would do the same if the Av was acceptable enough. I'm sure I could do better than that, with the ability to customise and control settings more appropriately, in particular the ISO - so that is what I was asking.
Andrew, thank you so much for your detailed reply. It helped me to understand that I am on the right track with my understanding. I probably need to just keep pushing myself to experiment changing the ISO, (because I was not used to changing it very much), so I get a better feel for the effect it has with different combinations of aperture and shutter speeds, and get my mind set out of the way I am accustomed to thinking. I didn't know if there were 'tricks of the trade' for ascertaining and fine tuning the best ISO for an image.
When I have combinations of Aperture and shutter speed that aren't quite right, I'll just have to be more mindful to dial in changes with the ISO to get the parameters which suit best.
Thank you again.
Julie