How do I test the quality of my sigma 10-20?

Started Jul 19, 2011 | Discussions thread
amicah22
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Re: How do I test the quality of my sigma 10-20?
In reply to Gerry Winterbourne, Jul 19, 2011

The sigma 10-20 really needs to be stopped down a bit to get good corners when shooting landscaped because of some moderate plane of focus curvature. The first 6 months I had mine I used it at f5.6 because of the photozone review but now stop down to f8 for landscapes.

Gerry Winterbourne wrote:

ashegam wrote:

I just purchased a Sigma 10-20mm F4-5.6 EX DC HSM Pentax Lens. Having read a little about it, quality control seems to be a known issue with these lenses.

Perhaps I've been reading in the wrong places but I've not seen adverse reports on Sigma lenses for some time. I've owned a few and never had any problems of any kind. I think that reports of QC failure from any maker temd to get blown out of proportion because we take more notice of a few squeals.

Most ownders recommend taking pics at f8 to f11 and then to look for blurred images etc..

I can't think why you'd stop down so far. Like most WA lenses on APS-C best resolution comes round about f/5.6 http://www.photozone.de/pentax/146-sigma-af-10-20mm-f4-56-ex-dc-pentax-k-review--test-report?start=1

DOF is almost infinite at this aperture (from 6 feet to infinity at 20mm

http://www.dofmaster.com/dofjs.html and even deeper at 10mm) so the only practical need for smaller aperture is if you want to increase exposure time.

Using smaller aperture than necessary will either make shutter speed slower or need higher ISO and will always increase the effects of diffraction: all of these things can lead to softer and/or blurred images.

As an amateur who doesn't know much about photography, how else can I tell if I a received a good lens or a lower quality one?

If you want to be sure, set aperture to f/5.6 and find a brick wall that you can get square on to - the easy way to be sure of this is to see that the courses look parallel because with a UWA lens they'll easily taper to one side if you shoot at an angle. Make sure you do for verticals as well as horizontals. Ideally do this on a tripod; if not, use an ISO that gives you a reasonably fast shutter speed.

If your PP skills can do it, take crops from the four corners and merge them into one image; if not, make a large print and cut out the four corners. If one side/corner is noticably softer than the rest your lens is decentred; if not it's a good copy unless the centre of your shot is soft.

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Gerry

First camera 1953, first Pentax 1983, first DSLR 2006
http://www.pbase.com/gerrywinterbourne

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Aaron W.

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