after distortion correction, a wide angle zoom is not so wide after all

[Aristoc]

I have noticed and read that after distortion correction on a wide angle zoom such as a 10-24 shot at 10mm, your final image is no longer 10 mm but something bigger. So wouldn't it just be smarter to get a wide angle prime with no distortion ?

[viewfinder]

Not really....If you crop out the "distortion" you have merely altered the image to that of a longer focal length lens.

Most of the "distortion" is actually the product of geometry, not optics.  As the angles of incidence to the camera get greater due to the wider angle of view so the "distortion",..actually perspective, gets more extreme.     There is no difference in a zoom lens at say, 10 mm to a prime of similar focal length.

Where a prime might score higher, is in the use of camera movements (rise, drop and shifts) to correct some of the perspective,..provided that the particular prime has sufficient area of coverage to allow it to be moved in relation to the image frame.   Even with camera movements you cannot remove the dictates of geometry entirely.........

.........The classic "distortion" that belies geometry is the shot of columns,...or oranges for that matter, spread out across the field of view,...those on the extremes MUST be viewed 'expanded' by the lens due to geometry.  A simple matter of angles and no lens corrections will help.   

[thierrylegros396]

I noticed that a Powershot S90 produce the equivalent of a 25mm lens without lens correction !

Thierry

[Dick Roadnight]

Not really....If you crop out the "distortion" you have merely altered the image to that of a longer focal length lens.

Most of the "distortion" is actually the product of geometry, not optics.  As the angles of incidence to the camera get greater due to the wider angle of view so the "distortion",..actually perspective, gets more extreme.     There is no difference in a zoom lens at say, 10 mm to a prime of similar focal length.

Where a prime might score higher, is in the use of camera movements (rise, drop and shifts) to correct some of the perspective,..provided that the particular prime has sufficient area of coverage to allow it to be moved in relation to the image frame.   Even with camera movements you cannot remove the dictates of geometry entirely.........

.........The classic "distortion" that belies geometry is the shot of columns,...or oranges for that matter, spread out across the field of view,...those on the extremes MUST be viewed 'expanded' by the lens due to geometry.  A simple matter of angles and no lens corrections will help.   

If is all geometry rather than optics, why does a rectangle (taken square on) look like a rectangle if you use a pin hole camera?

Remember that a lens is just a pin hole with a bit of glass in it to allow you to use a bigger pin hole and still focus a point on a point.

I think you are talking about fish-eye lenses, which hardly attempt to depict reality.

[feppe]

There is no difference in a zoom lens at say, 10 mm to a prime of similar focal length.

While this is true in a perfect world, prime lenses generally have less distortion than zooms at similar focal lengths. I'm sure it is possible to design a zoom which is indistinguishable from a good prime, but the cost would be prohibitive.

Also, zooms are usually slower than primes.

[bjanes]

I have noticed and read that after distortion correction on a wide angle zoom such as a 10-24 shot at 10mm, your final image is no longer 10 mm but something bigger. So wouldn't it just be smarter to get a wide angle prime with no distortion ?

Pincushion and barrel distortion are lens defects, and affect many lenses to some extent but, are most noticeable in wide angle zooms. They can be corrected in software with such programs as Photoshop, Adobe Camera Raw and DXO Optics Pro.

Keystoning is also observed with wide angle lenses and causes buildings appear to lean backwards when the lens is pointed upwards to take in the top of a building. It can easily be corrected in software, but you have to crop off part of the image to achieve a rectangular image.

Volume anamorphosis is a geometric distortion with wide angle lenses and is not a lens defect. This effect causes peoples faces at the edge of the frame to appear fatter than those in the center. It can be corrected with DXO Optics Pro.

See the DXO site for some examples.

Regards,

Bill

[Guillermo Luijk]

Volume anamorphosis is a geometric distortion with wide angle lenses and is not a lens defect. This effect causes peoples faces at the edge of the frame to appear fatter than those in the center. It can be corrected with DXO Optics Pro.

Until now I never read this effect had its own name. Since distortion of elements near the edges is basically and axial enlargement (shapes get more enlarged the farer they are from the centre), I thought a quick solution could be applying deliberately barrel distortion (which although is a tool for solving lens defects, does right the opposite to this anamorphosis, it enlarges elements near the center and compresses those near the edges).

And it works (left the musician's head is distorted and larger than his guitar):

=>

The unavoidable disadvantage is that it bends straight lines not crossing the image centre.
The same or better result could be achieved with a change in the projection from rectilinear to spherical, cylindrical,... using appropiate software.


Regards

[hjulenissen]

As for the geometric distortions:
Is this not similar to people trying to make the sperical globe fit onto a flat map for some hundred years? Or people trying to get the 4:3 evening news to cover their new 16:9 tv for the last 10 years? Is there some "right" solution to the problem, or are there only different angles that make something better and other things worse?

-h