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I have discovered accidentally that if you allow ACR (or Lr) to apply its Lens Corrections automatically, it can change the image completely and not just the geometry. Micro-contrast is destroyed, skies become flat and colourless, colours lose contrast and saturation, shadows lose texture and the whole image dies. Try editing a photograph to its optimal state with Lens Correction turned OFF; then turn it on and select "Auto" and watch your image die on the spot! I experienced this recently when some photographs that I had shot on my D5 with the 200-400mm f/4 VR suddenly exhibited a strange flattening of micro-contrast and saturation when I processed them. When I looked more carefully, I discovered that "Auto Correction" had somehow been activated in my Lens Corrections Panel. I then received some very useful help from one of the Adobe engineers who explained that: Distortion correction attempts to straighten lines with the trade-off of reducing image resolution slightly. If you don't need to correct for distortion, you can leave the corrections off. (He pointed out that the 200-400 lens does not have a significant amount of distortion, so it should be OK to leave Distortion off.) Vignetting correction attempts to make shading/lighting uniform across an image (i.e. it brightens up the corners to be uniform with the centre). Corrections are more apparent with larger apertures. The side-effect to using the Vignetting slider is that doing so reduces over-all contrast. So, if you don't need to correct for vignetting, you can either disable Lens Corrections completely or scale back the settings. As a result of the engineer's comments, I have now tested each of my lenses, in combination with each of my camera models, and have saved my revised settings as my new Defaults. I have found that most of my lenses needed very little correction for Distortion if any at all: far less than the 100% which "Auto" always sets. Ultra wide-angles may need more but unless the distortion is visually disturbing, or you are photographing for forensic purposes, you can probably reduce the amount of correction which you apply. Likewise with Vignetting: I mostly have set my values between zero and 15 for Vignetting but can adjust it on the fly if darkening in the corners becomes apparent. What is great is that now when I change and save a new Lens Profile as a Default for a photograph which was taken with a particular lens on a particular camera body, ACR automatically uses the correct Lens Profile for the lens/camera combination providing that I select "Default" (and not Auto) in the Setup box in the Lens Correction Panel. I have also saved a new over-all Default Preset for ACR so that it opens with "Default" pre-selected in my Lens Correction Panel too. The great advantage of making and saving these new Lens Correction Profiles as my personal "Defaults" is that not only do I now have the benefit of being able to use my new Default Lens Corrections automatically but I have also regained the levels of Resolution and Contrast which I previously lost when my software flipped to "Auto Lens Corrections". [I had recently installed new Upgrades so be aware that "Auto" may be the default installation?] If you choose Custom in the Set-up field, there are Sliders in the Lens Corrections Panel that allows you to scale the lens and vignetting corrections but many people just apply Corrections automatically and probably do not realise how badly they may be spoiling their images by doing so. This option is available in the Lens Corrections Panel in both ACR and in Lr. When you have good Lens Profiles, that Panel can save a lot of time over doing those lens corrections manually for every shot; and the ACR algorithms can be extremely effective in removing the effects of longitudinal Chromatic Aberration.