I'm considering buying a Tamron 18-250 lens (probably second hand), and as far as I understand most of these lenses suffer from some level of de-centering. For sure I won't purchase a copy that isn't totally free of any signs of de-centering at f/11, but is it reasonable to expect this at f/8.0? If it isn't a pipe dream then this is the benchmark I'm going to set for myself when testing the lens. If it isn't reasonable to expect this, then I may just give up the whole thing.
Thanks!
[quote name='boren' timestamp='1299415742' post='6553']
For sure I won't purchase a copy that isn't totally free of any signs of de-centering at f/11 ...
[/quote]
If you take this serious, you should eventually not buy such a lens.
For most of the focal length, f/11 is not yet 2 stops down ... not much
to let the mentioned centering weaknesses completely disappear.
You buy such a lens for a reason ... that is flexibility an convenience.
There is a price to pay for that.
Just my 2cts ... Rainer
I'm well aware of the compromise, but if I can't get rid of the de-centering by stopping down, then there's really no point in buying it. I don't expect to be able to use such a lens wide open for anything but emergency shots. There's no free lunch.
If the lens is sharp at f/8.0 throughout its zoom range and across the frame then I'd be more than happy. When I'm traveling I usually get by just fine with shooting around this aperture value.
[quote name='boren' timestamp='1299415742' post='6553']
I'm considering buying a Tamron 18-250 lens (probably second hand), and as far as I understand most of these lenses suffer from some level of de-centering. For sure I won't purchase a copy that isn't totally free of any signs of de-centering at f/11, but is it reasonable to expect this at f/8.0? If it isn't a pipe dream then this is the benchmark I'm going to set for myself when testing the lens. If it isn't reasonable to expect this, then I may just give up the whole thing.
Thanks!
[/quote]
Yes, it is reasonable that centering should not play a real-life role when stopping down by 1 f-stop.
From a lab perspective you will, of couse, see it beyond in any case but who cares.
04-05-2011, 06:32 AM
(This post was last modified: 04-05-2011, 06:33 AM by boren.)
Thanks for the affirmation Klaus. If I find it difficult to detect obvious de-centering at f/5.6 for the wide-end and f/8.0 from mid-range on then I'm buying this lens (assuming it's fine in other regards, such as AF functionality, external condition etc).
[quote name='boren' timestamp='1301985171' post='7381']
Thanks for the affirmation Klaus. If I find it difficult to detect obvious de-centering at f/5.6 for the wide-end and f/8.0 from mid-range on then I'm buying this lens (assuming it's fine in other regards, such as AF functionality, external condition etc).
[/quote]
Well, depends. All it takes is flat scene at a longer distance. That's not exactly a shop scenario though.
I will try a little centering experiment this evening with you folks which includes a very basic test. That'll be interesting.
I'm buying the lens from a private person, so hopefully it would be possible to step out and shoot a subject that's far enough to make DOF a non-issue.
04-05-2011, 02:52 PM
(This post was last modified: 04-05-2011, 02:52 PM by boren.)
I checked the lens and decided not to buy it. It was difficult to test for de-centering, but since nothing was really sharp I guess de-centering would have been the least of its problems. To make matters worse, the AF was very slow AF and the MF ring felt as if I attached the lens to a Kenko tele-converter, in other words - it was noisy and difficult to turn.
Maybe this whole ultra-zoom thing is not for me...
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