How to properly set aperture, iso and shutterspeed?

Dear all,

I'm trying to use libgphoto via the python bindings to step through
aperture (f-number), iso and shutterspeed.

E.g. for increasing ISO:
100 -> 125 -> 160 -> 200 etc. in one-third-stops.

So far I can poll my camera for a list of choices by getting the property
from the cameras config. So far so good. Even setting the values manually
(e.g. set_value('200')) works well.

My idea was to get a list of choices (get_choices()), look up the index of
the current value, increase the index by one, look up the new value and set
it. This works reasonably well for iso values.

However, there are some oddities when it comes to f-number and shutterspeed:

For example when the current shutterspeed is above 1/3, `get_value` will
return fractions of 10, e.g. 4/10, 10/10, etc. However, the list returned
by get_choices contains the value '1' instead of '10/10' hence the lookup
fails.

Is there an easy way to set shutterspeed and f-number to the next value or
do I have to somehow figure out what 10/10 means and translate it to 1? I
think I could use a static table for that but that seems to be laborious
and error-prone.

Thank you very much in advance and best regards,

  Jochen

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