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Quicktip: Expanding the dynamic range of your pictures with HDR scan mode and Multi-Exposure (Part II Multi-Exposure)

August 14th, 2010 · 2 Comments · Digital Photography, HowTo, News, Processing, Quicktips, Scanner, SilverFast, Tips and Tutorials

In the first part of the article I mentioned the advantages of this procedure and briefly introduced concepts about the bit depth of image files. Also explained the creation of raw files from SilverFast, the different types as well as their differences.

Now it’s time to explain Multi-Exposure (not standard in all SilverFast versions).

This feature is hardware dependent. It means, not all scanners support it. You can find out if your scanner does by checking on the SilverFast website, under the specific scanner model.

What is Multi-Exposure, is it as good as they say and why?

The basic idea behind Multi-Exposure is to expand the dynamic range of a scanner while helping reduce image noise.

Scanners need a specific amount of time to digitize the surface of the image. Multi- Exposure makes two scans of the same image where the first scan is performed at the standard speed and the second one is performed at a much slower speed (up to 4 times slower or 1/4 of the standard scan speed).

As a result, the first image has a normal exposure and the second one will be over exposed revealing more details from the dark areas. Since digital image noise regularly happens at the dark areas, overexposing those areas reduces the production of digital noise.

These two images are combined using highly complex algorithms to keep details from highlights, mid-tones and shadows.

If you choose to have your pictures digitized in 24 bits (8 bits per channel) SilverFast will make a compression of the information in the picture. If you wonder why you do not see any benefits, probably it is because you are not using the right bit depth for this enhancement to be displayed.

Here is where raw files explained in the previous entry come in handy.

Since raw files have a bigger amount of bits, which also means more colors and shadows to display details, the information recorded by the scanner when using Multi-Exposure will also be stored into those files.

Remember that your computer monitor and printer are not capable of displaying that big amount of details. Here, a little post-production is necessary.

However your pictures will show a substantial benefit from Multi-Exposure and raw files when used correctly.

Please bear in mind that although the combination of Multi-Exposure and raw files provide a huge benefit, they can not perform magic. It means they can not create or invent details that have not been recorded in the film base.

So, how does it go?

Easy:

  • Make a prescan
  • Frame the desired area to create your raw file
  • Select one of the raw file types accordingly
  • Activate Multi-Exposure using the button at the left side of the preview window (see below) .

Multi-Exposure button off

Multi-Exposure active

  • Set the  scan resolution
  • Press the scan button and wait until the job is done.

If you thought you could not improve your pictures any further, now its time for you to re-think and start taking advantage of the real abilities of your scanner to improve your pictures.

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2 Comments so far ↓

  • Michael Dickey

    how can one manually control Multi Exposure. For example,
    can a negative be scanned with one histogram specified for the first pass & a second histogram utilized for the second?
    Or, one curve for the first , another for the second.

    Michael

  • amorales

    Dear Michael,

    It is not possible to manually adjust Multi-Exposure.

    Cheers

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