# What does “distortion <XX%” mean in lens specs?

I'm going to buy a lens and I would like to understand what "distortion <XX%" means.

It's a vari-focal 10-60mm lens, and mentions distortion <12-1%. According to my understanding, at 10mm focal length the distortion will reach 12%, and 1% at 60mm. Is that right?

How do they calculate this percent?

What is the explaination of this graph?

• Hi Raphael, where are you reading these lens diagrams such as in this and your other question here: photo.stackexchange.com/questions/19051/… Are these lenses for photography? Could you show us an example of where you found this lens and this graph? – dpollitt Jan 12 '12 at 13:59
• This is a machine vision lens. – Raphael Jan 12 '12 at 14:18
• @Raphael, this makes the question off topic because it does not pertain to photography. Flagging for moderator to close. – bwDraco Jan 12 '12 at 20:43
• @DragonLord: I'm not sure we should be so quick to call this off-topic. We do certainly discuss optics here, and optics are optics, regardless of the application. That said, we could use some more information Raphael, particularly about the source of your diagram and its context. – jrista Jan 12 '12 at 22:39
• Metadiscussion on topicality here. I don't think we want to say "all optics are on topic". Lasers are pretty much no-go, for example. – Please Read My Profile Jan 13 '12 at 13:24

## 1 Answer

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

• I don't think thats an MTF chart. MTF has to do with lens resolution, not distortion. I think your right in your analysis though, I do believe that the diagram is indicating how much distortion increases away from the center of the lens. – jrista Jan 13 '12 at 7:04
• Yes, MTF refers to contrast based resolution measurements of lenses, not distortion, but the lens manufacturer refers to the above chart, and some others as "MTF curves" so I don't know. I've edited the answer to make it more clear. – MikeW Jan 13 '12 at 7:10