Questions about shutter speeds from a script - Script Writing - CHDK Forum
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Questions about shutter speeds from a script

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Offline zcream

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Questions about shutter speeds from a script
« on: 16 / August / 2009, 00:02:59 »
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Hi! First, I have looked at set_tv and its variants. Looked at scripts and the APEX system wiki.

Here is an extract from the manual
>>
            Factor --- Select shutter-speeds by decimal equivalents. (Extremely valuable for analytic and research photography needs.)

        Value factor = OFF, 1/100K, 1/10000, 1/1000, 1/100, 1/10, 1, 10, 100

        The final shutter speed will be the product of "Value factor" times the "Override shutter speed value".

        Example: If you wish to use a shutter speed of 1/50,000th of a second, you select an Override Shutter Speed Value of 2, and a Value Factor of 1/100K. (2/100,000 = 1/50,000)

>>>

I guess if the menu allows us to set shutter speed in simpler terms i.e. 1/100 is simply factor of 1 and value factor of 1/100, then why do we need to use the TV tables for setting the shutter ?
Could we not put the menu code in a small subroutine and implement it as a set_shutter_value and a set_shutter_value_factor ?

:-( Could not really figure out how to use the set_tv and set_tv96 commands for implementing a fixed shutter of 1/100 ..
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Offline reyalp

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Re: Questions about shutter speeds from a script
« Reply #1 on: 16 / August / 2009, 01:05:12 »
Quote from: zcream on 16 / August / 2009, 00:02:59
:-( Could not really figure out how to use the set_tv and set_tv96 commands for implementing a fixed shutter of 1/100 ..
1/64th  = APEX 6
1/128th = APEX 7
You could just go with 6.5 (actually about 1/90th) and call it close enough. It's within half a stop. If that's not close enough, wikipedia tells us Tv is the time value: Tv = log2 (1 / T)
T is 1/100 so 1/T is 100. logbase 2(100) is about 6.64.

The camera works in the (APEX value) times 96, so it can do fractions of a stop in integer math. So the final value for set_tv96_direct would be ~637

Note that if you are calculating arbitrary Tv values, you should use set_tv96_direct. AFAIK, plain set_tv96 will ignore the value if it isn't one of the pre-defined values used by the camera (it should probably pick the nearest one instead).

Yes, setting the shutter speed in more conventional values would be convenient. However, if your script is actually going to do any exposure calculations, working in APEX*96 values is much more convenient.

As an aside, I've noticed that people tend to worry to much about exact shutter speed values, when they should be worrying about how many stops it is. APEX forces you to think in stops.
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Offline zcream

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Re: Questions about shutter speeds from a script
« Reply #2 on: 16 / August / 2009, 01:19:23 »
This is something that worries me. I come from a video background and am used to specifying aperture and shutter separately. Am I correct in understanding that there is no way to set the independently, for example in A460 I want to set f2.8 (min) and 1/100 shutter ?
Will I only get a shutter speed value and the aperture is corresponds to ?
Quote

As an aside, I've noticed that people tend to worry to much about exact shutter speed values, when they should be worrying about how many stops it is. APEX forces you to think in stops.
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Offline reyalp

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Re: Questions about shutter speeds from a script
« Reply #3 on: 16 / August / 2009, 01:37:29 »
Quote from: zcream on 16 / August / 2009, 01:19:23
This is something that worries me. I come from a video background and am used to specifying aperture and shutter separately. Am I correct in understanding that there is no way to set the independently, for example in A460 I want to set f2.8 (min) and 1/100 shutter ?
Will I only get a shutter speed value and the aperture is corresponds to ?
You can set them individually if the camera hardware is capable of it (Tv = shutter speed, Av = aperture, Sv = ISO). On many low end cameras, including the a460 there is no adjustable aperture, only a ND filter that can be in or out. The zoom setting controls the aperture.

edit:
What I was trying to say about shutter speed is people worry about like 25 vs 30 seconds, when it's a tiny fraction of a stop.
« Last Edit: 16 / August / 2009, 01:39:51 by reyalp »
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Offline zcream

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Re: Questions about shutter speeds from a script
« Reply #4 on: 16 / August / 2009, 07:43:18 »
Thanks for the help reyalp.
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Offline Mac

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Re: Questions about shutter speeds from a script
« Reply #5 on: 16 / August / 2009, 14:00:00 »
Hi reyalp!

Experimentally I can't confirm that setting Tv in fixed steps is a restriction.
Have seen that Canon sets Tv to any Apex value.
Wrote a srcitp to test and confirmed that the actual exposure as measured on the resulting photos do track Tv setting even when they do not correspond to 16 or 32 or 96 multiplrs.  The cameras I tested are the S2is and S5is.
I also found the Tv can be set to '0' without any adverse effect - not necessary to trap this setting.
Has anyone else noticed this?
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Offline reyalp

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Re: Questions about shutter speeds from a script
« Reply #6 on: 16 / August / 2009, 19:31:46 »
Mac: what function are you using to set ? You could post the script. I was looking at:  platform/generic/shooting.c shooting_set_tv96

0 is a valid tv96, corresponding to a 1 sec exposure.
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Offline Mac

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Re: Questions about shutter speeds from a script
« Reply #7 on: 17 / August / 2009, 10:10:47 »
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Offline reyalp

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Re: Questions about shutter speeds from a script
« Reply #8 on: 17 / August / 2009, 17:41:10 »
Ah, you are using _direct, so yes, that will set arbitrary values (and AFAIK does not reject 0)
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Offline Mac

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Re: Questions about shutter speeds from a script
« Reply #9 on: 17 / August / 2009, 17:55:21 »
Thanks reyalp!
Nice to know that I wasn't see something that was be caused by default.
Only problem with my way of setting exposure is that it is too good.
Sunsets and sunrises all look alike except for lights going on or off.
Will have to play around with the published SunSet script to get a nicer effect.
Thanks!
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