From: Milton Aupperle <milton@outcastsoft.com>

Date: April 22, 2012 6:36:54 PM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] figure out the ASA / ISO

Dear Christian;

On 2012-04-22, at 6:00 PM, Christian Humann wrote:

Dear group,

I'm just getting up to speed using the Asrto_IIDC for capturing  

bracketed images of the skyvault for generating HDR images.  In order  

to process the images into HDR however I need to know what the ASA /  

ISO  sensitivity of the camera sensor is.  Does anyone know a way of  

figuring this  out from the settings in the Astro program? I'm using  

the PGR Chameleon USB-2 camera.

ISO is basically the same as Gain (Brightness in Astro IIDC - named that way to match the Brightness control register designation in IIDC) to a digital camera.

See: "Use in Digital Photography"

http://www.ehow.com/about_6736352_difference-between-asa-iso_.html

or

http://digital-photography-school.com/iso-settings

or

http://photo.net/digital-darkroom-forum/00RRaI

ISO or Gain is not specific to a CCD. Gain is applied by whatever internal image processing hardware in the camera after it's read out of the CCD.

Gain for IIDC Camera is logarithmic in brightening the image and varies with the camera model too. The only way to know exactly how much brightening is caused by increasing gain is to measure it yourself using a constant light source and gradually increasing the Brightness control land measuring the change in brightness.

As near as I can tell form looking online, ISO 100 is 1/2 as bright as ISO 200, so basically it appears to be a linear relationship. However I have yet to see anything that says this explicitly. Most times high ISO imeans they use shorten the exposure time to compensate for the same brightness levels.

If you assume that the ISO increase is linear (i.e. ISO 200 is 2 times as bright at ISO 100), you can just use the Exposure time for each image and assign it a "ISO value. So if the first exposure is 10 ms, and the second exposure is 20 ms, then you can say "Exposure one is ISO 100" and "Exposure two is ISO 200".

HTH..

Milton Aupperle