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Author Topic: Some observations about lens quality  (Read 1725 times)

ErikKaffehr

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Some observations about lens quality
« on: September 22, 2007, 02:11:17 am »

Hi,

I just wanted to share some observations about lens quality.

I have only a limited experience with Minolta (KM and now Sony) 135 format lenses and Pentax 67 medium format lenses. I have also studied essentially all MTF lens tests at www.photodo.com. The original MTF tests at Photodo were done at Hasselblad with their MTF equipment. The Swedish journal  "Foto" still does MTF tests at the Hasselblad factory, but their results are available only at a cost (3-4 USD) or to subscribers. (Check http://www.tidningenfoto.se and learn some swedish :-)

Another good information on lenses is http://www.photozone.de/8Reviews/index.html . Photozone tests lenses using Imatest.

This is essentially a summary of my experience of all my lenses:

  Pentax 67 lenses have some lateral chromatic aberration.

  Both wide angles I have are lacking in sharpness at extreme corners at aperture f/8 and above, these lenses are Pentax 67 45/4.5 and Minolta 20/2.8. Both improve if stopped down to f/11.

  Most lenses I have tested were pretty good at the center of the image, with sole exception of Sigma 400/5.6 and Sigma 400/5.6 APO (first version).

  My latest lens is the  Sony SAL 16-80mm F3.5-4.5 Carl Zeiss Lens. This lens is pretty soft on the corners at the wide end. I'm pretty satisfied with it anyway. The test in "Foto" was not very impressive, which agrees with my experience.

One of my lenses has a probable centering error (KM 17-35/2.8-4 by Tamron). I bought another Tamron 17-50/2.8, the first lens I got was horrible but I got a replacement which was quite good. Do some careful experimentation with your newly bought lenses!

Something I have noticed from Photodo and "Foto" tests is that wideangle to short telephoto zooms generally are no great performers. They are not critically sharp in the corner at shortest focal length. Astigmatism seems to poorly corrected at both the short and long end. (Astigmatism means that horisontal and vertical structures do not have the same plane of focus, somewhat simplified).

My experience of my my lenses are in very good agreement with the MTF test by Photodo and Foto. The tests at Photozone are informative, but I have some reservations about their findings on border and extreme border image quality.

Lateral chromatic aberration (often seen as red/green fringing) is quite frequent in wide angles but can be supressed during processing. I'm susing Lightroom and have some presets for my lenses at different focal lengths.

Norman Koren has some excellent writings about Image quality:
http://www.normankoren.com/Tutorials/MTF.html

Norman Koren has also a tool for testing lenses called Imatest
http://www.imatest.com/

Erwin Puts has some very interesting information although his site is much Leica centric:
http://www.imx.nl/photo/site_map.html
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Erik Kaffehr
 
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