Question about prime lens distortion and DX sensor....

Started Jan 15, 2013 | Discussions thread
Leonard Shepherd
Leonard Shepherd Forum Pro • Posts: 13,624
Re: Question about prime lens distortion and DX sensor....

As a general rule with lenses with distortion the amount of distortion increases as you move away from the frame centre. Using DX generally cuts out the worst of the distortion effect.

There is a difference between improving your picture by removing most of a distortion effect and removing all distortion effect in post processing.

When a lens has regular distortion post processing works very well. When the lens has variable distortion removing the regular part of the distortion usually improves the image looks on first inspection though you may still be left with straight lines that are not rendered exactly straight.

An issue with most of the older classic wide-angle primes is, by modern standards, they are optically very poor in the frame corners at or near wide open. Nikons MTF confirms this

For those not familiar with MTF the left hand side of the chart in the link is lens centre quality. The closer the red and blue lines are the top the better the image quality. It is good in the centre

The right hand side is corner quality. In the chart the lines are at the bottom i.e. a very poor optical performance Looking at the numbers on the bottom line, 15 is the edge of DX format and the right hand side is the extreme corners of FX format.

The corner performance of this this prime is so poor that it is easy to understand why Nikon say recent zoom lenses like the 24-70 easily optically outperform it. The prime is a pre telecentric design with more wide open optical vignetting than the zoom. The zoom does cost more and is a lot bigger and heavier.

I no longer own the 20 mm because it does not perform well enough on FX for me. My 35mm f2 has more distortion than the 24-70 at 35mm

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Leonard Shepherd
Many problems turn out to be a lack of intimate knowledge of complex modern camera equipment.

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