calibrate/ align a 12-24 Sigma or not?

[photos of hans koot]

<p>I Seemed to have picked up a bad copy of this one (second hand), wich as I found out (to my shame, I should have checked better) seems to occur more often. After some testing: focus/sharpness seems a bit wave like, very soft in the center and noticably , actually quite much, sharper to the corners. Raising the sharpness high in ps raw improves a lot, more then expected, but still makes it not spot on. Curiously enough this seems not to spoil (oversharpen) my photo as it would do with my other lenses (comparison with tokina 12-24). This is on an aps-c sensor. More about the wave form of the sharpness: I asked someone to make some shots on a 5d 'mk II (I ve planned on investing in a 5d mkII, thats why I bought it) and sharpness seems off to the corners again. So here the pattern is: soft in the center, then a quite sharp area, and soft to the corners. It was tested on my 6 kg Gitzo. No stability problem. So what should I do, are there good experiences with calibrating, or aligning a sigma 12-24 or should I better sell it again and take care to get a good copy?</p>

<p>Thanks for your input here!</p>

[colin carron]

<p>Hi Hans. no great words of wisdom from me to cure the problem I'm afraid. My copy of the Sigma 12-24 focused sharper on one side of the frame than the other. The difference was not great and could be improved with selective sharpening but it was annoying. I found the problem almost disappeared when the lens was stopped down a little from max aperture. Of course the lens is already fairly slow so this is not welcome but was the best solution for my lens. Once it was stopped down a bit it was a really good lens and I liked using it very much. Using filters was a bit challenging but I found big handheld square filters were best. I sold the Sigma for a Canon 10-22 for use on APS-C. I use the Canon 17-40L on my Canon 5D but would like to go wider. Occasionally I think about buying another copy of the 12-24 to see if it is better than the first one. If you can get a good one they are very good lens with few practical alternatives.</p>

<p>regards - Colin</p>

[sattler123]

<p>Hans, send it in to Sigma - they have very good customer service and will calibrate your lens back to specs. I have this lens and it is very sharp in the center wide open, while the corners are soft (normal for this lens). When stopped down (as I ususally do for landscapes) the corners get sharp as well. I use mine on a 5D.</p>

[photos of hans koot]

<p>Hi Colin , Thanks very much for your answer, and nice to see you (well, sort of) here again :-). I tested mine at f8 and f11. Because there indeed hardly is any competition for this lens I want it, it also fits well in my photography. Wich filters did you use for it? because its an extra problem indeed with this lens. Good to hear you have better experiences, though the sharpnes from side to side seems bit weird as well.</p>

<p>Thanks Juergen, did you have youre copy calibrated with sigma? Anyway good to hear there are sharp copies around. I just found this one on the web http://www.juzaphoto.com/eng/articles/sigma_12-24_review.htm<br>

I downloaded the raw file and was impressed (no way mine has that sharpness). He also warns about less sharp copies.</p>

 

[colin carron]

<p>Hans, I used only ND grads with it as I didn't have a polariser big enough. I used P sized square filters but they were only just big enough when hand held in front of the lens. Not a very satisfactory solution but it worked if you were careful. If I had it again I would go for some 100mm square filters such as Lee (though maybe a cheaper alternative!)</p>

[photos of hans koot]

<p>Hi Colin, yes, polariser will not be possible, but large nd grads are good idea. I did use them handheld before, and when the camera is mounted on a tripod I often find it much easier to adjust the right position by handheld use! large filters might pe pricy though, I will see and check, or perhaps just autobracket. Though I must say canons autobracketing functions are primitive for a semi pro body, and compared to the competition. In the meantime I've ordered my 5dmkII, quite a steep step up! I am looking forward.</p>