Be Careful with Scanner Auto Settings

Using scanner auto settings can cause problems when scanning images. Turning off the auto settings, and using manual settings can make image correction easier and more accurate.

The Problem

I was recently scanning some images using my CanoscanFS4000US slide scanner (Canonscan FS4000US ICC profile).  After scanning the images and assigning the custom profile, a number of slides were exhibiting saturated highlights.

autogainonimagezoom1Figure 1 – Saturated highlights

Figure 1 shows the ear of a Koala bear. You can see an area of red pixels around the bright centre of the ear.

I had not seen this in the past and it had me confused for a while, as I thought using a custom profile was supposed to correct this.

With a little investigation I was able to determine my problem was the scanner settings.

Scanner Settings

The Canoscan FS4000US scanner has a number of user configurable settings such as Exposure, Dust Removal, Focus, Colour Matching and Gamma.

I set the gamma to 2.2 and turned off all the auto settings.

gainbutton1Figure 2 – Exposure setting options.

The Auto Gain checkbox was left checked (ON), thinking this was an additional function to the exposure setting.

I then scanned one of my Wolf Faust (www.targets.coloraid.de) IT8 Provia 35mm transparency targets.

autogainonprofile1Figure 3 – IT8 target with Auto Gain ON.

Checking the scan results, a number of the light colored boxes are blown out/bright white (Figure 3).  Deselecting the Auto Gain checkbox (OFF), and using a zero exposure (EV0), rescanning the target produced the same result.

As EV0 was causing saturation, I decided to try EV-1. (Unfortunately the manual exposure setting for this scanner only works in +/- 1 steps.)

autogainoffprofileFigure 4 – IT8 target with Auto Gain OFF and EV -1.

Rescanning the IT8 target with Auto Gain OFF and EV-1, Figure 4 was a bit darker as expected, but the lighter squares showed some colour and detail. I also checked the blacks in the bottom row to make sure each was still distinguishable and they were not saturating.

Rescanning the target with Auto Gain ON and EV-1, I got the same blown out image as Figure 3. From this I can only deduce that the Auto Gain function overrides and disables any manual exposure setting.

Assigning the Profiles

I created separate ICC profiles from the IT8 target with Auto Gain ON and OFF, and assigned them to copies of the scanned image in Photoshop.

preassignFigure 5 – Basic scanned image.

Figure 5 shows the scanned results for the same transparency. The left image was scanned with Auto Gain ON and EV-1. The right image is scanned with Auto Gain OFF and EV-1, and is darker as expected. At this point there are no signs of highlight saturation.

afterautoassignFigure 6 – Applying the ICC profiles.

The ICC profile generated with Auto Gain ON  and EV-1 was assigned to the left image in Figure 6, and the profile generated with Auto Gain OFF and  EV-1 assigned to the right image. Notice that in assigning the profiles to each image, the mid tones and colour casts where also automatically corrected. (There is still a slight red colour cast, but this can be improved later in the workflow process.)

afterautoassign21Figure 7 – Close up view of highlights.

Zooming in on the ear in Figure 7, the area of red pixels formed from saturation can be seen around the bright area in the left image, but the image on the right is showing no saturation and significantly more detail.

compression2d3dFigure 8 – 3D view of compressed and uncompressed ICC profile.

The effect of Auto Gain compression on the pixels can also be demonstrated in viewing the ICC profile with the Chromix ColorThink software in Figure 8. The solid profile is the same for Auto Gain ON EV0, Auto Gain OFF EV0, and Auto Gain ON EV-1. The wireframe shows the profile for Auto Gain OFF EV-1.

Conclusion

A couple of important things I have learned in this little experiment are:-

  1. Turn off ALL your scanner Auto settings when doing a scan. Using manual settings can make image correction easier and more accurate.
  2. Be careful when profiling a scanner. Look for blown out highlights or saturated blacks in your target scans. You may have to fine tune the manual settings to get the best results.

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