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Question about taking aerial imagery.

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Offline waterwingz

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Re: Question about taking aerial imagery.
« Reply #10 on: 21 / June / 2013, 15:35:19 »
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Quote from: ThePara on 21 / June / 2013, 15:19:55
I'm doing aerial shoots with a CHDK'd A2200, using USB trigger and the live video feed via a wireless transmitter. I've tried to upgrade to an A4000, I've modified an HTC 11-pin USB connector to fit the Canon
http://chdk.setepontos.com/index.php?topic=9610.msg99056#msg99056
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Offline peabody

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Re: Question about taking aerial imagery.
« Reply #11 on: 03 / July / 2013, 18:29:08 »
I'm confused.

As soon as my kite arrives, I'll be doing some KAP - kite aerial photography - using my A590is.  I've done alot of time lapse with CHDK, and understand why it's necessary to lock down everything when doing that.  But for aerial stuff, I'd like to make use of the camera's ability to get the right exposure.

So I had planned to use manual focus at or near infinity, daylight white balance, manual ISO, manual zoom, and Tv set at 1/1000 sec., and then let the camera determine the correct aperture to get the right exposure.  But here I see that I should be setting a shutter override.  Is that different from shooting in Tv?  Or is that recommendation based on the fact that most Canon P&S cameras no longer have Tv?  I'm hoping that's the explanation because otherwise it means I don't understand.  :-)

By the way, it's a 7-foot Rokkaku, which should be awesome.
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Offline andrewhazelden

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Re: Question about taking aerial imagery.
« Reply #12 on: 03 / July / 2013, 18:52:59 »
Hi Peabody.

The suggestions were aimed at people shooting low altitude photos or aerial mosaics from a model airplane traveling forward at 50 km/hour.

If your kite is stable and has a Picavet suspension you might be completely satisfied with photos taken using the TV mode.
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Offline waterwingz

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Re: Question about taking aerial imagery.
« Reply #13 on: 03 / July / 2013, 19:21:55 »
Quote from: peabody on 03 / July / 2013, 18:29:08
So I had planned to use manual focus at or near infinity, daylight white balance, manual ISO, manual zoom, and Tv set at 1/1000 sec., and then let the camera determine the correct aperture to get the right exposure.
Be aware that the A590's aperature range is only f2.8 to f8.0.   You might find this is not enough range to get the correct exposure depending on where you fix the ISO & Tv values.  One way around this is to use a CHDK script to read the current brightness level and set the exposure to maximize the shutter speed at lowest aperature & ISO combination the camera can manage.

Or,  as Andrew says,  try it in full Auto first and see what happens.  On a brightly lit day,  I would expect the shutter speed to end up well over 1/200 sec anyway.
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Offline peabody

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Re: Question about taking aerial imagery.
« Reply #14 on: 04 / July / 2013, 12:09:26 »
Waterwingz, thanks for reminding me about the aperture limits in the A590.  You can take it to F11 as an override, but that would work only in Av or M, not Tv.  The Auto ISO limits are pretty limited too.  It looks like it only goes between 80 and 200.

If you know of a script that does the exposure reading, I'd like to look at it.
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Offline waterwingz

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Re: Question about taking aerial imagery.
« Reply #15 on: 04 / July / 2013, 12:23:46 »
Quote from: peabody on 04 / July / 2013, 12:09:26
If you know of a script that does the exposure reading, I'd like to look at it.
Here's a Lua function that demonstrates the concept :
Code: [Select]
function check_exposure()
    press("shoot_half")
    repeat
     sleep(50)
    until get_shooting() == true
    local tv1=get_tv96()
    local av1=get_av96()
    local sv1=get_sv96()
    local bv1=get_bv96()
    release("shoot_half")
    repeat
      sleep(50)
    until get_shooting() == false
    return tv1, av1, sv1, bv1
end
In your case, you'd do something like this to prevent the shutter speed going below 1/200 by locking the exposure at 1/200  and recalculating the ISO value when it does.

Code: [Select]
tv96, av96, sv96, bv96=check_exposure()
if (tv96<752) then
    tv96=752
    sv96=tv96+av96-bv96
end
set_tv96_direct(tv96)
set_av96_direct(av96)
set_sv96(sv96)
shoot()
« Last Edit: 04 / July / 2013, 12:33:23 by waterwingz »
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Offline peabody

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Re: Question about taking aerial imagery.
« Reply #16 on: 04 / July / 2013, 14:40:51 »
Thanks very much for the script.  I'll study it this afternoon.

Meanwhile, I've found a possible alternative which still lets the camera do its thing:

1. Enable Custom Auto ISO in CHDK and set the target Minimal Shutter Speed to 1/1000 sec., and the minimum and maximum auto-iso values to 80 and 800.

2. Set the camera to P, and set ISO to Auto.

So what appears to happen is that P sets both aperture and shutter speed, but the Auto ISO values in CHDK make it select at least 1/1000 second if it possibly can.  But within that limitation, it will set ISO to the lowest value consistent with a middling aperture value (it appears to like f/4 to f/5.6).  But if it just can't get a proper exposure at 1/1000, it will open the aperture all the way, set the ISO to maximum, and use whatever shutter speed is needed to get the picture, even if slower than 1/1000.  So the 1/1000 is a preference, but not an absolute limit.  This seems to work pretty well in limited testing in my back yard, and has a good bit of lattitude.

P stands for Professional, doesn't it?

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Offline msl

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Re: Question about taking aerial imagery.
« Reply #17 on: 04 / July / 2013, 14:51:16 »
P stands for 'Program Mode'.  ;)


@ww

Nice script idea.

Maybe better for all cameras:
Code: [Select]
if get_nd_present() ~= 0 then set_av96_direct(av96) end
msl
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Offline waterwingz

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Re: Question about taking aerial imagery.
« Reply #18 on: 04 / July / 2013, 15:07:16 »
Quote from: peabody on 04 / July / 2013, 14:40:51
So what appears to happen is that P sets both aperture and shutter speed, but the Auto ISO values in CHDK make it select at least 1/1000 second if it possibly can.  But within that limitation, it will set ISO to the lowest value consistent with a middling aperture value (it appears to like f/4 to f/5.6).  But if it just can't get a proper exposure at 1/1000, it will open the aperture all the way, set the ISO to maximum, and use whatever shutter speed is needed to get the picture, even if slower than 1/1000.  So the 1/1000 is a preference, but not an absolute limit.  This seems to work pretty well in limited testing in my back yard, and has a good bit of lattitude.
Thanks !  I think you are the first person to try AutoISO and report back here on how it seems to work. There is not a lot of documentation about the function beyond what you just wrote and this : http://chdk.setepontos.com/index.php?topic=7284


Quote from: msl on 04 / July / 2013, 14:51:16
Maybe better for all cameras:
Code: [Select]
if get_nd_present() ~= 0 then set_av96_direct(av96) end
@msl :  you've lost me here.  Why do you want to only set the Av value if the camera has a Neutral Density filter? ( or a Neutral Density and variable aperature) ?   If it has neither,  then set_av96_direct() just gets ignored  AFAIK.   And I don't believe any value used with set_av96_direct() will cause the ND filter to swing in?
« Last Edit: 04 / July / 2013, 15:10:46 by waterwingz »
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Offline msl

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Re: Question about taking aerial imagery.
« Reply #19 on: 04 / July / 2013, 15:44:27 »
'if get_nd_present() ~= 0' => all cameras with iris diaphragm.

Are you sure that the command is ignored when the camera has no iris diaphragm? I have only cameras with iris diaphragm.

msl
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