Toothwalker
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Makten wrote:
It has nothing to do with curvature of field. The sharper corners are due to mechanical vignetting. The iris of the aperture isn't circular when looking through it from the corners of the sensor/film plane, and thus its size is also smaller. This makes the DOF larger in the corners, and it is very common with fast lenses. The reason is simply that the front lens element isn't large enough to achieve a round aperture for light hitting the corners of the sensor. Of course, this gives quite alot of vignetting too.
The proper term would be optical or artificial vignetting (in the terminology used by Sid Ray). Theoretically, optical vignetting increases corner DOF, but since this type of vignetting only occurs at large apertures its DOF effect is opposed by oblique lens aberrations which lower the corner DOF relative to the center DOF. The aberrations normally win. Fortunately DOF plays no role in the evaluation of a generously blurred background, and you are perfectly right that optical vignetting results in a reduced corner blur. However, we cannot exclude an additional contribution of field curvature to Lotus' observation, although aberrations are ill defined for out-of-focus parts of the image.
Apart from possible curvature of field, there also seems to be curvature of the dryers themselves. Is the 50/1.0L saddled with such conspicuous barrel distortion?
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