Author Topic: Read focal length outside LV and adjust expo accordingly  (Read 5413 times)

Marsu42

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Read focal length outside LV and adjust expo accordingly
« on: November 20, 2013, 02:36:28 PM »
It seems the Canon fw can read the focal length and adjust the min. shutter speed in full auto mode, so there has to be a way to read this value somehow even if it hasn't been found yet ... this would be invaluable for example for the auto_iso module and so on which set a min. shutter speed in Av.

dmilligan

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Re: Read focal length outside LV and adjust expo accordingly
« Reply #1 on: November 20, 2013, 03:52:49 PM »
You would only need this for zoom lenes, for prime lenses you can just determine it from reading the lens name (which is available outside LV, I know it is b/c you can display it in the flexinfo). You probably want to read the lens name anyway to determine whether or not the lens is IS, since you can push the shutter a little longer if it is. Until it gets done you could just default to setting the min shutter based on the max focal length of the lens for zooms.

I do agree though, it would be nice to have.

Greg

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Re: Read focal length outside LV and adjust expo accordingly
« Reply #2 on: November 20, 2013, 04:40:16 PM »
I found something on the 500D (18-55 IS):

18mm :


55mm :


50mm 1.4 :


0x14CD2 - This is the maximum value of the aperture.
0x14CD3 - This is the minimum value of the aperture.

Marsu42

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Re: Read focal length outside LV and adjust expo accordingly
« Reply #3 on: November 20, 2013, 08:21:13 PM »
You would only need this for zoom lenes, for prime lenses you can just determine it from reading the lens name

Elementary, my dear Dr. Watson :-> ...

Until it gets done you could just default to setting the min shutter based on the man focal length of the lens for zooms.

... of course this is what I do, but for a 70-300 zoom this results in a very unnecessary high iso setting if zoomed in - for shorter or standard zooms it might not matter that much.

0x14CD2 - This is the maximum value of the aperture.
0x14CD3 - This is the minimum value of the aperture.

This prop is already known and static, you can read it from lens.c/h as raw_aperture_min & raw_aperture_max.

The problem with the current zoom length outside lv is that it isn't a prop (or it would have been found by now), but most likely is read from the af chip that hasn't been reverse engineered yet - someone would have to re the Canon fw and see how this reads the value from the af system, and this is way beyond my skill... so this feature request is more like a todo tag for the likes of a1ex or g3gg0.

Greg

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Re: Read focal length outside LV and adjust expo accordingly
« Reply #4 on: November 20, 2013, 08:43:55 PM »
I tried prop_dump(), I did not find any differences by changing the focal length.  :-\

dmilligan

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Re: Read focal length outside LV and adjust expo accordingly
« Reply #5 on: November 20, 2013, 09:39:37 PM »
for non-constant aperture zooms, you could figure it out based on the max aperture allowed

Marsu42

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Re: Read focal length outside LV and adjust expo accordingly
« Reply #6 on: November 20, 2013, 10:25:35 PM »
for non-constant aperture zooms, you could figure it out based on the max aperture allowed

Oh my, I missed your point people - sorry! I have to check my 70-300L for that because I think/though the min. aperture was static and just the value printed on the lens - but if it would change it would serve as a viable workaround.

a1ex

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Re: Read focal length outside LV and adjust expo accordingly
« Reply #7 on: November 21, 2013, 01:16:18 PM »
IIRC, lens info properties are refreshed when you take a picture. Can you check if this is true with prop_dump?

I believe this info is sent by the Tx19A processor (no idea how to force a request though).