Sigma 10-20 and Tamron/Min 11-18 - difference shown

Started Dec 6, 2005 | Discussions thread
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David Kilpatrick Veteran Member • Posts: 5,436
Sigma 10-20 and Tamron/Min 11-18 - difference shown

These two lenses have entirely different design aims. The Sigma uses curvilinear geometry, like a very mild fisheye, at 10mm. It has a very tight image circle and quite string progressive fall off in illumination. It is however very sharp right to the edge of the usable image.

The Tam/Min design uses orthometric or equidistant projection (not sure which in technical terms), in which equal distances on the subject are shown as much as possible as equal distances on the image, and lines are rendered as straight as possible. It has a larger image circle, with a more gradual deterioration in sharpness (wide open) accompanied by better illumination. There is then an outer image zone which falls off in both illumination and sharpness.

You can see this in a very unscientific test, taken with both lenses held so as to cover as close to the same view as I could manage by eye. Note the curved lines on the Sigma image, and look particularly at the slats of the venetian blind in the bottom half of both pictures. This shows really clearly how the 'drawing' of the Sigma and Tamron/Minolta optical systems differs.

The Tamron/Minolta design has obviously greater compatibility with AS, as a moving sensor will track across a more or less equally 'shaped' image, while on the Sigma design, image scale changes.

This picture was taken on a 9xi full frame camera on an old roll of film (I don't use film now!) and the local minilab has not centered the image circles all that well, but you can also see how much larger the coverage circle of the Tam/Min design is, compared to the Sigma.

I really like the Sigma's HSM focusing and lovely design and build quality, but I do not prefer the optical decisions made in its design, and the geometry of the Konica Minolta 11-18mm will be similar to the Tamron tested here, and very clearly better suited to the AS system as well as yielding straighter architectural rendering.

I converted the image to mono as it makes the differences easier to study.

David
f2photo.co.uk

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