Opinions on 18mm 3.5?

[Airy]

Same applies to sharpness or CA charts... on the other hand, I could see with the 20/3.5 UD for instance that there is definitely a correlation between short-and long-range distortion, and I am happy that LR provides a correction profile. It does not take the distance into account, as no such information is conveyed, but it provides a decent correction (calibrated on infinite ?) 100% precision is anyway neither realistic not needed.

I consider all chart info as relative (i.e. easing comparisons), but not telling much in the absolute. In absolute terms, I'd trust some reviewers and not others, after having made my opinion (correlation between their rating and my observations or satisfaction).

[pluton]

I notice most reviewers use a small chart for checking distortion. Isn't this where distortion is at it's worst for most lenses? Why not the old brick wall? Maybe the small chart also. If I'm going to use a lens for photographing architecture do I need to see the distortion it gives when focused to 1.0 meter and less?

Dave Hartman

I have several times wished to do a brick wall distortion demo with a very wide lens at a large distance, like 30 or 40 feet(10-12meters) . However, it has been impossible for me to find a brick wall that can be photographed with the camera dead square to the wall and at large distance and cover the camera frame with the bricks.  Maybe one of those old Gitzo ladder tripods that stands 12 feet tall would be needed.

[Bjørn Rørslett]

Keith, you could shoot at a normal angle of incidence and capture half the frame. Then flip the camera upside down and get the other half

I agree that testing distortion on close-up subjects, like most reviewers do, is a poor substitute for a properly conducted test of geometric distortion. For a lens with CRC, in particular a wide-angle lens, the likelihood is large that you will document barrel distortion that simply does not exist at intermediate to distant motifs.

[David H. Hartman]

Bjørn,

Thank you for confirming my suspicion.

Dave

[pluton]

Keith, you could shoot at a normal angle of incidence and capture half the frame. Then flip the camera upside down and get the other half

Excellent idea.  For distortion, I'll probably leave out the bottom half since distortion is presumed to be symmetrical.  Sharpness would be different, of course.