Enhance AV Max setting for AutoISO

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The following text was taken from a comment I made in Issue#154. I'm breaking 
it out as a specific request so that the requirement is not lost in the mix.

I have problems with the way AV max currently works. For instance, with my 
55-250mm lens, I set AV max to 4.5. I then switch to my 50mm F1.8 and set the 
AV max to 1.8. When I switch back to the 55-250mm, if I don't remember to set 
the AV max to 4.5, I end up with very slow selected shutter speeds (safety 
shift enabled). In the AutoISO menu, AV max still shows 1.8.

Would recommend the following enhancements:
1) Provide an "Auto" setting for Av Max. When the camera is turned on, the max 
aperture of the attached lens should be checked, and AV max set to that value. 
From there, the current behavior is fine.
2) Let the user set Av max to any value down to, say F1.2. If the attached lens 
is not capable of F1.2, then use the AV max "Auto" logic. This way, I can set 
my AV Max to say F2.8. When the 50mm is attached, AV Max will be set to F2.8 
(because I always want to use the lens stopped down a bit). When my 55-250mm is 
attached, it can't be opened to F2.8, so use "Auto" logic.
3) An option that will be useful to use with AV max = Auto would be a "stop 
down" value. This option would allow the user to set a fixed stop down amount 
from the maximum aperture of the attached lens.

Original issue reported on code.google.com by colinban...@gmail.com on 16 Jan 2013 at 3:55

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No, your request wasn't lost, it was addressed in a different manner:

The root of the problem is that 400plus' AutoISO can only act after the camera 
has measured the scene and set an aperture; and obviously, the camera is not 
going to set an aperture larger than what the lens can support. Thus, setting a 
limit equal to (or larger than) the lens' maximum aperture useless, because 
that limit will never be surpassed and AutoISO will never kick in.

But we can detect when the current settings (ISO + speed + aperture) produce an 
underexposure (let's say we can "see" the exposure meter from the viewfinder); 
then I thought about a "Safety Shift for AutoISO" feature (see issue #209): 
raise the ISO when 400plus detects an underexposure, disregarding any aperture 
limits. In practice, this is exactly your "Auto" mode, in the case when the 
limit is out of the lens' limits.

Then, we can think that all this as a particular case of what you comment on 
your third point: just by using an offset equal to zero. So, at the end, this 
all comes to having a "Max Av" value relative to the lens' maximum aperture, 
instead of the current absolute value. And the good news is that this seems 
quite easy to implement.

I'll have a look at the user manual from other cameras anyway, and see how does 
AutoISO work for them; but I think that in next version "Max Av" will be a 
relative value. Thanks for your comments.

Original comment by eduardo....@gmail.com on 16 Jan 2013 at 6:47

• Changed state: Accepted
• Added labels: Milestone-M2

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Issue 209 has been merged into this issue.

Original comment by eduardo....@gmail.com on 17 Jan 2013 at 6:56

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Revision r1648 implements the "relative" mode for AutoISO + Tv:
* "Max Av" is now the maximum aperture (relative to the maximum aperture 
supported by the lens) that 400plus will allow before raising the ISO.
* I still have to find a reliable method to detect underexposures; until then 
"Max Av" must be grater than 0, or AutoISO would never kick in.
* If "the maximum aperture supported by the lens" changes (user changes the 
lens, or it is a zoom and user changes focal length), AutoISO will adapt to the 
new limits.

Original comment by eduardo....@gmail.com on 19 Jan 2013 at 12:42

• Changed state: Started

[GoogleCodeExporter]

We cannot detect under/over-exposures the way I thought; and when we do it, 
AutoISO starts to swing back and forth between two ISO values. So, having 
AutoISO for Tv mode, with an offset equal to zero, does not seem possible.

I'm closing the issue as finished for now.

Original comment by eduardo....@gmail.com on 19 Jan 2013 at 1:13

• Changed state: Fixed