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fixing an f value with no Av control

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Offline cwylie

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fixing an f value with no Av control
« on: 02 / April / 2014, 18:18:53 »
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Is there any way to control the F stop value? I have shutter speed and ISO locked while shooting on bright and sunny days (aerial photos), but every 6th picture or so the F goes from 2.7 to 8 and the picture is very, very dark.

I know there's no Av control, but is there maybe some trick to stop this from happening all together? Maybe drop my ISO even lower or something? I have beginner/intermediate knowledge on cameras, but can't seem to figure this one out.

Thanks,
-Colin
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Offline waterwingz

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Re: fixing an f value with no Av control
« Reply #1 on: 02 / April / 2014, 19:29:39 »
Please don't post the same question in multiple threads.  The few people who answer questions on this forum monitor all posts.  Asking the same question twice mostly just annoys them.

Thank you for your cooperation.
 
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Ported :   A1200    SD940   G10    Powershot N    G16
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Offline paul.pelletier.771

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Re: fixing an f value with no Av control
« Reply #2 on: 06 / April / 2014, 19:45:14 »
Same problem I have with the Ixus125 HS. I wnat to fix exposure time at 1/1000 s but don't seem to see where we can fix aperture.... It would ideally be f2.7 with ISO varying to compensate different illumination...
Anyone have an answer?
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Offline waterwingz

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Re: fixing an f value with no Av control
« Reply #3 on: 06 / April / 2014, 19:49:59 »
Quote from: paul.pelletier.771 on 06 / April / 2014, 19:45:14
Same problem I have with the Ixus125 HS. I wnat to fix exposure time at 1/1000 s but don't seem to see where we can fix aperture.... It would ideally be f2.7 with ISO varying to compensate different illumination...
Anyone have an answer?
Most of the less expensive Canon P&S cameras - including all the IXUS models  - do not have a variable aperture that the user can control.  The f-stop reported is only a function of the zoom lens position.  You can't change it as it's physically fixed - there is no mechanism in the camera to vary the iris size.

As a small consolation,  all the camera with fixed aperture have an Neutral Density filter that can be inserted, giving about the equivalent of 3 f-stop decrease.  That's what was causing the occasional dark pictures noted in the original post in this thread.

Here's a script that will let you play with various priority modes ( Tv, Av, M, AutoISO ) if you want to fool around a bit :  CHDKplus

I recall from your previous post that you fly UAV's.  Perhaps this will help : KAP UAV Exposure Control Script  as it manages exposure for precisely your situation.
« Last Edit: 06 / April / 2014, 19:54:11 by waterwingz »
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Ported :   A1200    SD940   G10    Powershot N    G16
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