Correct Film Scanner Exposure

This is about how to get correct exposure when scanning with the HP PhotoSmart S20 film scanner. As the manual that came with the scanner is very light on the subject I thought I could write down what I have found so far.

When the scanner has made a preview scan of your film strip it suggests an exposure for each frame. This exposure is based on the assumption that the picture should be medium gray on average. This is often not what you want, especially if you know how to expose your film properly.

Now if you select the exposure tool you will have three slides to adjust, high lights, mid tones and shadows. The first one you should care about is the mid tones slider, with it you can select the pixel value you want to assign to medium gray or zone V (as recored on the film). If a lab has developed your film this value will probably be 0. If you develop your own film, take a picture of a gray card or some other uniformly lit surface, the correct mid-tone slider value is the one suggested by the scanner software for this frame. If you are using a camera with matrix or segmented metering, use a gray card only so you are sure that the metering software does not make its own decisions about the exposure.

I think it's worth pointing out that if you have not exposed a certain frame correctly, you should forget about the correct value for the mid tones slider, adjust it till you get the desired result. However, the value above can be used as a good starting point.

With the high light and shadow sliders you control the contrast of the picture, moving any of them will compress or spread the high lights or shadows from the point of the mid tones (which you decided on above). Start with a value of 0 for high lights and shadows and adjust till you get the result you are after. The histogram tool is very useful for placing the high lights and shadows. Finally as always when working with digital pictures you should have a somewhat calibrated monitor.

Disclaimer! It's not my fault. If you think something is incorrect please let me know. Thanks!

Reference Pictures

Plant 1
This is a test picture that is somewhat properly exposed (the plant has detail on the negative). Click on the link above to see the exposure that the scanner software suggests, probably not what you intended when you took the picture.
Plant 2
This is the same picture as above but with the mid tones adjusted and the high lights compressed so we get some detail in the window as well as in the plant. Okay it's not perfect but you get the idea. By the way you can do these manipulations in PhotoShop afterwards (to some degree), but you might lose some detail as the scanner software only exports 8 bits of data but uses 12 internally (per channel).