I received an enquiry from someone who scanned a bunch of slides using the scanner settings for a negative. So, therefore all the digital images of the slides, which are positives, were imaged as negatives.

Having never done this myself, I wanted to see the impact. So, we scanned a slide as both negative and positive. We used ACDSee to edit the images. We took the positve and brightened and dehazed the image. We took the negative and applied "special effect" "negative". Here's how the results compared:

Compare negative (inverted to positive) on left to positive plus editing on right

The negative definitely suffers from a blue cast and a lack of depth. The positive has a blue tint in the sky whereas the negative is washed out. The negative blacks are also crushed. Comparing the histograms (at the bottom of the images above) of the RGB and luminence there is an obvious improvement for the positive scan!

Thoughts: A negative has much larger range of darks and lights than a positive. So, when setting to negative the scanner expects the images to have this large range. In our case the positive does not have this large range and it shows in the resulting image above. This confirms that negative setting should be used for negatives and positive setting for positives.

However, if the someone really does not want to re-scan everything, they could invert the image digitally and it will work...just not the best result.