Re: [APML] Where does the f stop when scanning?

[Jerry Lodriguss <jerry@astropix.com>]

Hi Jeff,

Yes, it's pretty much the same for scanners. The more detail that you can 
get up out of the noise in the densest areas of the film, the better off 
you are.

Increasing the exposure helps with the shadow detail on slides in deep-sky 
astronomical images but unfortunately these are the very subjects where 
increasing the exposure will hurt the star sizes.  So you are kind of 
caught in a gotcha here.

For slides, you will almost always have some stars that are completely 
transparent on the original. With the correct exposure in the scanner, 
these will always go to 255,255,255, so you won't have any room to push the 
rest of the histogram to the right without bloating the stars. This may be 
an acceptable trade off if you care more about the shadow detail.  You 
start running into bleeding and flare though when you increase the exposure 
of the CCD in the scan.

For neg or slide originals, to make sure you are not blowing out the stars 
and un-necessarily increasing their size,  carefully monitor the pre-scan 
histogram.  It helps to crop and enlarge that part of the image with the 
brightest star and look at the histogram in the prescan.

Depending on the scanner and scan software, you may have to make the scan 
and then enlarge to 100% and examine the histogram in Photoshop.  You can 
also read the pixels there with the densitometer.  You generally want the 
pixel values for the brightest star in the image to be just a tiny bit 
under 255,255,255 to be sure you have not clipped any of the data.

Jerry


At 08:38 PM 7/15/2003 -0400, you wrote:
>Hello all,
>While reading this article on Luminous Landscape, I wondered if any of
>this applies to our scanners.  Should we use an analog gain to push the
>histogram as far to the right while, at the same time, avoiding blowing
>out the highlights?  I guess monitoring the highlights is akin to
>monitoring how far right the histrogram reaches.  How do we look at the
>histogram to monitor if we are blowing out the stars?  I usually do this
>visually because most of our images will never have a histogram that
>comes close to going off of the right side.  I guess this could happen.
>Thanks (Jerry?) for any help on this.
>
>http://luminous-landscape.com/tutorials/expose-right.shtml
>
>Jeff Ball
>www.astro-photography.com
>
>
>
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