A preliminary reply:
1. Some more detail about "somewhat different" would be helpful, perhaps two such pics as you describe.
But, I would say that in Av mode there is some further automation that you may not be aware of and so cannot
set in manual mode. This may possibly be due to Auto WB settings. It also depends on whether the scene changes
significantly in the interval between shots.
2. I am uncertain of what you mean here: "Another question is, if i use auto
iso, the camera tends to choose a much higher
iso setting..."
About using the lowest possible
ISO setting, that is a general rule to try to avoid introducing too much signal noise into the image.
Ie, higher
ISO usually > higher noise. If you need high
ISO, just use it. There is no fast rule about using auto
ISO and I would say that
nobody here adheres to any such.
Rather than giving you a long reply, have a look at the idea of
The Exposure Triangle in this Library article. Look at the other topics there as well.
3. For shooting "against the
light", there are two main ideas I can recall.
One is to "back-
light" you subject, such as sunlight shining through green grass. This would be close to a "fairly normal"
exposure,
and you'd likely notice some contrast difference compared to a "front-lit" shot of the same scene.
The other is to produce a silhouette, where the subject is in deep shadow and the background and rest of the picture is "more normal".
For this, you'd use an
exposure for the non-silhouetted subject area and let the subject go dark.
So, that's preliminary, and the best way is to try some shots yourself (mainly in manual mode) and post them here for discussion.
Bon chance!