Shutter Speed Scripts for a3300is

JoeNiederriter

5

« on: 17 / August / 2012, 18:52:01 »

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I have been attempting to take a large number of pictures using the script at this link:http://chdk.wikia.com/wiki/UBASIC/Scripts:_Shutter_Speed_Stack.

I have been plotting the digital numbers of the pixels of each color from the set and have noticed that instead of producing a single arc that matches a curve fit it has sudden decreases for several pixel values in a row.

Is there something wrong with this script for some values? can you point me to a better script for varying shutter speed?

Thanks

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reyalp

14118

Re: Shutter Speed Scripts for a3300is

« Reply #1 on: 17 / August / 2012, 21:33:16 »

It's not clear to me what you are actually trying to do, or exactly what you are measuring.

If you explain more specifically what you are trying to accomplish, we might be able to help you find or make a script that is better for whatever that is.

Other potentially relevant information
what camera are you using
what settings are you using with the script
what output values are you getting

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JoeNiederriter

5

Re: Shutter Speed Scripts for a3300is

« Reply #2 on: 21 / August / 2012, 14:37:07 »

My goal with CHDK is to capture a large number of raw images which I can then convert into .pgms. I use Matlab to process the .pgms, picking out the pixel values for each color so that I can get separate averages and maximums for the digital numbers.

My objective is to determine if their is a change in the relative color values, i.e. hue as exposure increases due to saturation of pixels.

I use the Canon a3300is. I set the script to take thirty shots with a step size of 1/10 an EV starting from -3 EV. I believe this should yield shots from an eighth of a second to one second.

The result is the expected curve which begins exponential but which plateaus out at a digital value of around 4500 in this case. However after reaching this plateau around .35 seconds and remaining at this plateau until .5 seconds the digital number then drops to 2500 before rising back to 4500 at one second.

I suspect that the inconsistency in the curve is caused by some misstep in the script as I've repeated the experiment several times with the same result.

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reyalp

14118

Re: Shutter Speed Scripts for a3300is

« Reply #3 on: 21 / August / 2012, 17:22:28 »

To understand what the script is doing, I'd suggest logging the output with http://chdk.wikia.com/wiki/Script_commands#print_screen

You can sometimes see what the camera actually used in the exif. However, this doesn't work on every port, so you should verify first, by taking a single picture with an obvious override and then checking whether the ovrride is recorded in the exif. The Canon "maker note" tends to be more reliable than regular exif.

Since what you are doing seems rather simple, you might be better off writing your own script to do it, e.g. just take 30 shots in a loop, setting shutter speed with set_tv96_direct from 288 to 0.

Note that the camera works in APEX*96 values, so getting exactly 1/10th stop steps is likely not possible. 1/12th or 1/8th might be a better choice.

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JoeNiederriter

5

Re: Shutter Speed Scripts for a3300is

« Reply #4 on: 24 / August / 2012, 15:15:06 »

Thanks for the help. Writing a simplified script worked like a charm. I hadn't understood the values I should be inputting until now.

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