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Is it possible to take zero second exposures?

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Offline murk

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Is it possible to take zero second exposures?
« on: 05 / July / 2013, 12:22:53 »
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Hello,

In order to characterize my CCD (bias, read noise) I'd like to take a zero second exposure.

When trying to shoot with the camera (manually pressing button with my hands)

In Extra Photo Operations I've set:
Disable Overrides [Disable]
Override Shutter Speed [0 ]
Value Factor [off]
Shutterspeed enum type [Factor]

I must be confused about something because it seems this should override the shutter speed with 0 seconds. Though I know the range of shutter speeds is dependent of the camera. Does anyone know what is the range of available shutter speeds for a A470


using CHDKptp
I'm using
con> shoot -tv=0.0 -raw=1
but this produces images with 1/48.6 sec exposures for some reason, according to meta data read by drcaw -i -v filename


I'm using
canon a470 1.02c
chdk 1.1.0 2870
chdkptp-r330-linux_x86_64
ubuntu 12.10


Thanks for any help you can give,
-murk
« Last Edit: 05 / July / 2013, 12:25:41 by murk »
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Offline waterwingz

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Re: Is it possible to take zero second exposures?
« Reply #1 on: 05 / July / 2013, 12:58:51 »
Quote from: murk on 05 / July / 2013, 12:22:53
In order to characterize my CCD (bias, read noise) I'd like to take a zero second exposure.
I don't think you are going to be able to do this directly,  but you can force the mechanical shutter to stay closed during an exposure.  Which may do what you want - expecially if you set the Tv value very high.

http://chdk.wikia.com/wiki/Meteor_Intervalometer_with_Dark_Frame_Management

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using CHDKptp
I'm using
con> shoot -tv=0.0 -raw=1
but this produces images with 1/48.6 sec exposures for some reason, according to meta data read by drcaw -i -v filename
I think that you are using Tv values here (more correctly Tv96 I believe) ?   Tv values for shutter speeds can be positive or negative -  a TV value of zero should give you and exposure of 1 second.

http://dougkerr.net/pumpkin/articles/APEX.pdf

reyalp will probably want to correct my explanation - I haven't used chdkptp for shooting
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Offline reyalp

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Re: Is it possible to take zero second exposures?
« Reply #2 on: 05 / July / 2013, 14:46:01 »
The shoot command defaults to "standard" units (e.g. tv=1 is one second, tv=1/100 or tv=0.01 for 1/100th), you can change that with the -u option.

0 is not a valid shutter speed. There is no apex representation of 0 shutter speed. The fact that it lets you do this without complaining is a bug, which I will fix in a future version.

You can request a very short shutter speed, e.g. 1/100000th although there's no guarantee your camera can actually do this. In my testing on a540, shutter speeds down to 1/30000th or so do seem to work

As waterwingz says, you can force the shutter closed before the shot.
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Offline murk

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Re: Is it possible to take zero second exposures?
« Reply #3 on: 05 / July / 2013, 16:24:06 »
Thank you both for your information, taking a bunch of dark shots should do just as well.

I've tried to find the lower limit on my exposure time but it allows me to enter any time. I've tried 10-24s and it didn't complain, and reported that as the exposure time in the meta data, but I'm sceptical... 
Anyway it seems that the read  noise is so large that it begins to dominate very quickly anyway.

If the smallest available clock time is 1ms, via tick_count, how can the camera take sub 1ms exposures accurately? Is it possible to output a time stamp to sub-millisecond accuracy?  Say, If I wanted to now precisely the instant an exposure began or ended?

Thank you again,
-murk
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Offline reyalp

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Re: Is it possible to take zero second exposures?
« Reply #4 on: 05 / July / 2013, 16:41:15 »
Quote from: murk on 05 / July / 2013, 16:24:06
I've tried to find the lower limit on my exposure time but it allows me to enter any time. I've tried 10-24s and it didn't complain, and reported that as the exposure time in the meta data, but I'm sceptical... 
As you should be, "not complaining" does not imply that it worked.

If you want to find out if it actually worked, I'd suggest taking raw (or DNG) of a well lit scene and comparing histograms. Every time you cut the exposure value in half, the histogram should shift accordingly. Of course, once you get up to 1/100000th you are gonna need some bright light.

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If the smallest available clock time is 1ms, via tick_count, how can the camera take sub 1ms exposures accurately? Is it possible to output a time stamp to sub-millisecond accuracy?
The smallest value available by get_tick_count() is 10ms so clearly the camera has a higher precision for exposures, otherwise it couldn't even do the factory fast exposure which is usually around 1/2000th.

This comparison I did shows it goes much further than that https://picasaweb.google.com/115940260949628685898/VenusTransit2012#5832738428087188226


Internally, the camera converts the APEX96 value of the requested exposure time to microseconds. That doesn't mean it has microsecond precision, of course!

Quote
Say, If I wanted to now precisely the instant an exposure began or ended?
That information is not available to us, but as I said, you can work out the effective exposure length by other means.
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