[le_roi_ponpon]

Hello, I have a DA17-70 that I have sent for repair in Canada almost a year ago, for SDM problems and also sharpness problems. It came back with SDM repaired , but still with the same sharpness problems. I live in Africa so I took the lens anyway because I needed it (I was on vacation in Canada when the lens came back). I have a few months left on the warranty, and I need your advise before I pay big bucks to send it for repair...
My feeling (confirmed by MTF tests) was that the lens was bad between f4 and F8 (inclusively, which is weird) for focal length > 50mm. I did a side by side comparison of DA17-70 and DA55-300, both @ 70mm (so I am comparing two "consumer grade" lenses). DA55-300 is much much better between f4 and f8. The DA17-70 starts to be good at f11 (what lens is not at f11??). I was wondering: is it suposed to be like that, i.e. do I risk to have Pentax repair say my DA17-70 is fine (and therefore loose money and time)? Note that at in the range 17~30mm the lens is very sharp at most apertures (a bit soft at f4 but still fine).
Thanks in advance for you help.
Grégoire.
PS Attached is a 100% center crop.

Attached Images

 

[farfisa]

What camera body are you using it on? It could be a back focus or front focus problem, which would become more evident at the longer focal lengths. (between 17 and 35, the depth of field is so great that it would mostly obscure a focusing problem).

If you have a K20 or K-7, you could try the AF adjust feature. Other cameras have "debug" modes you can search for. Unfortunately, the other option is to send both your camera and the lens in to have the lens calibrated for your camera--then you'd be out big money for shipping and the camera for about a month, I'd guess.

The 17-70 should be able to compete with the 55-300, but it should be close--that's the weakest focal length for the 17-70 and about the strongest for the 55-300, with regard to sharpness. The 55-300 is no slouch!

[builttospill]

That's surprising how fuzzy your images are with the 17-70. My 17-70 took very sharp, excellent photos with my K10D across the zoom range.

[GeneV]

I will admit that I am terrible at doing lens test shots of newspapers taped to the wall and such. However, 100% center crops from my copy of the DA70 prime and from my copy of the DA17-70 are pretty close in a shot of a newspaper at f/4 and ~8-10' away. And that is not because the DA70 isn't excellent. I shoot at F/4 indoors with the DA17-70 frequently, and the results are excellent. I may just have a very good copy, but the results posted here are much worse than anything I've seen with my DA17-70.

I would also wonder if there is a focus error of some sort.

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Last edited by GeneV; 03-03-2010 at 09:40 AM.

[le_roi_ponpon]

Thanks for the insights and for the hint to a possible focus problem. I'll do some tests this wee-end and let you know!

[le_roi_ponpon]

You are the best!! I made the adjustments (I have a K20D) based on test charts (both MTF and moiré), and it happens that I had to adjust focus by +10 for that lens (backfocus, right?). So I tested all my lenses and they also needed some non-negligibile adjustements on the + side. Also it probably explains why my DFA100 macro was a bit softer than I wanted at f2.8-4. I never thought that the focus adjustment would be more than an cool option: in fact it is an obligation once you unpack a new camera or a new lens! I would have probably tried it if all my lenses were soft, it just happens that this one was more obvious than the others. Still, the sharper setting for my DA17-70 is still f11, and the DA55-300 is still super sharp (and now probably sharper at 300). Funny that Pentax reps never suggested I try that.
So thank you very much folks, I am now a happy man (and richer)!
Grégoire