What does a lens spec like "distortion <12~1%" mean?
Asked 1/12/2012
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I’m looking at a vari-focal 10–60mm lens and the spec sheet lists distortion as "<12~1%". I first thought this meant about 12% distortion at 10mm and 1% at 60mm, but I’m not sure that’s correct. How is lens distortion percentage normally measured, and what does this notation and graph likely mean?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
14y ago
2 Answers
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What you are referring to is a chart from Azure Photonics which they call an MTF chart.
It says (see source): Distortion : <12~1% (not <12-1%)
I believe it is saying that within 12mm of the optical centre the distortion is about 1%
<12 I take to mean "within 12mm", and ~1% means "about 1%"
The chart measures the % distortion (of horizontal or vertical lines, how much they would bend as a percentage of the overall image height).
I believe the graph shows the centre of the lens (0mm) at the bottom, moving up to 10mm (or 100% of the lens, I'm not sure) at the top. The x axis is the distortion, so it is approaching 1% at the top of the graph and about 0% at the centre of the lens.
The distortion is negative, which seems to indicate barrel distortion, as opposed to pin cushion distortion
More info on distortion and interpreting MTF charts: digilloyd
Originally by user4191. Source · Licensed CC BY-SA 4.0
user4191
14y ago
0
Generated from our catalog & community — verify before relying on it.
This spec probably does not mean “12% at 10mm and 1% at 60mm.” Based on the chart wording, "distortion <12~1%" is more likely shorthand for something like: within a 12 mm image height, distortion is about 1% or less.
Lens distortion is usually expressed as how far a straight line bows away from where it would be in an ideal rectilinear image, shown as a percentage of image height. In practice, 1% distortion means the bend is about 1% of the frame/image height at that point.
From the chart description in the answers, the vertical axis appears to be distance from the optical center, and the horizontal axis is distortion amount. Distortion is near 0% at the center and approaches about 1% toward the edge of the image circle. If the curve is negative, that generally indicates barrel distortion; positive would indicate pincushion distortion.
So the key takeaway is: this spec is likely describing edge distortion across the image area, not distortion changing from 10mm to 60mm focal length.
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UniqueBot
AI14y ago
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