What's going on with "scale" in ACR lens correction?

[AFairley]

If I apply adjustments to correct keystoning with the Vertical slider, at 100% scale, some of the top of the image will be hidden.  I can see the entire image if I decrease the scale with the Scale slider.  For example, putting the Vertical slider to -18, I need to reduce scale to 94% to see the entire image area.  How is ACR determining what is 100% scale?  Is it the original width of the image equidistant between top and bottom (in the case of a Vertical slider adjustment)?  I do see than changing the scale changes the size of the image that is sent to Photoshop, even if the crop area is the same.  I guess the rescaling is not that big a deal because the entire image is being resampled anyway, but I'm curious. 

[madmanchan]

Hi, 100% scale just means that no uniform scaling is applied to the image.

It is easiest to understand this by (for now) ignoring the Vertical, Horizontal, and Rotate sliders. When you leave Scale at 100%, you see the entire image area. When you reduce it to 99%, you start seeing the gray padded border. Similarly, when you expand it to 101%, you start dropping image data as it's cropped off.

Now, when you start using the other sliders (Vertical, etc.), this concept remains unchanged. The only thing that's changed is that the gray padded area is no longer uniform around the image edges, and the original image area is no longer axis-aligned, nor even rectangular.  This is because the perspective projection can be arbitrary. Practically, the Scale slider is used to help you decide how much of this "warped" image area you wish to retain.

[walter.sk]

Hi, 100% scale just means that no uniform scaling is applied to the image.

Am I correct in assuming that when something other than 100% is chosen, that the means of interpolation is determined by the setting in Preferences?  For example, if I have "Bicubic Smoother" as the my default "resampling" method, that Bicubic Smoother will be used in e Scale?

Would the same be true when using Transform or the Crop Tool to resize images?

[Schewe]

Am I correct in assuming that when something other than 100% is chosen, that the means of interpolation is determined by the setting in Preferences? 

Not in Camera Raw or Lightroom...unlike Photoshop, ACR & LR have an adaptive resampling that can use Bicubic Sharper for downsampling and Bicubic Smoother for upsampling. But even then ACR/LR can interpolate between the different Bicubic effects automatically based on how much resampling is being done. For very small changes, it basically simple Bicubic. When going up or down a lot, it's interpolated between Bicubic and Sharper (down) and Smoother (up).

[walter.sk]

Not in Camera Raw or Lightroom...unlike Photoshop, ACR & LR have an adaptive resampling that can use Bicubic Sharper for downsampling and Bicubic Smoother for upsampling. But even then ACR/LR can interpolate between the different Bicubic effects automatically based on how much resampling is being done. For very small changes, it basically simple Bicubic. When going up or down a lot, it's interpolated between Bicubic and Sharper (down) and Smoother (up).

Thanks for clearing that up.