Lightroom: I can't add (certain) custom crop ratios

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  • Problem
  • Updated 1 year ago
  • Solved
  • (Edited)
[This bug is fixed in LR 8.1:]

I'm trying to setup a custom crop ratio, very close to a standard one. Lightroom uses instead a standard (predefined) crop.E.g.: I setup a 15.2 x 10.2 crop ratio, LR (5.5) uses instead 3 x 2 (rounded to nearest standard crop ratio)Please, correct the bug or find a better implementation (e.g. use Alt+click to enforce any custom ratio)

[The official reply below provides an incorrect technical analysis of this bug. See this reply for details of the bug's cause and a simple fix:]
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Ciprian Trofin

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Posted 6 years ago

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Simon Chen, Principal Computer Scientist

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Official Response
Lightroom matches the aspect ratio of the current crop against the existing entries in the aspect ratio popup -- so that the matching aspect ratio entry can be automatically selected. This matching needs to account for some numeric rounding errors (imprecision) involved. For example when the crop is rotated, because the result image dimensions can only be expressed as integers, there will be quantization errors that will lead to inexact aspect ratio computation. The aspect ratio will not match exactly any of the existing aspect ratios in the popup (the magnitude of the rounding errors depends on the rotation angle and the cropped image dimensions). Lightroom has to provide allowance for the rounding errors. Otherwise when you rotate the crop, the aspect ratio would bounce between two aspect ratio entries (known or custom). That is not very intuitive and has been reported previously as a different bug.

So there is a tradeoff to set such a threshold for the rounding errors. It cannot be too tight and it cannot be too loose. Lightroom currently set the rounding threshold as 0.005. And in the case of 13 x 11 aspect ratio, ( 8.5/11 - 10/13 ) = 0.0035, which is less than the preset threshold 0.005. Thus it is considered the two aspect ratio to be the same.

The current behavior is as designed.