On Jan 10, 2010, at 1:10 PM, William Robb wrote: > > ----- Original Message ----- From: "Sandy Harris" > Subject: How useful is SDM? > > >> I have never used an autofocus system. I find myself comparing lenses >> with an SDM motor to ones without. For example, the 17-70 f4 has SDM >> but 16-45 does not. 55/1.4 does but 50/1.4 and 70/2.4 do not. >> >> There are plenty of other differences -- notably price, weight and >> focal length -- and I have some idea how to balance those in relation >> to my usage and preferences. However, I don't have much idea how >> important or useful the SDM motor is, so I am asking here. >> >> Is this something that should be ignored in making a choice? An >> important factor? Somewhere between? > > I think the factor now is can you get the lens you want with screwdrive or > SDM? > I have two SDM lenses, the 55/1.4 and the 60/250 zoom. > I actually did a direct comparison between SDM and screwdrive AF with the 55 > and the FA50/1.4. > The 55 does focus somewhat faster in real life, though if all you do is > compare lock to lock, the 50 is much faster. > One thing to consider with SDM is failure rate. > Over on ForumsNeurotica, there have been a lot of threads dedicated to SDM > failure, especially the 16-50 and to a somewhat lesser extent, the 50-135. > As optically good as these two lenses are (if you can get a 16-50 that isn't > decentered anyway), I'd run like hell from buying either of those lenses > until Pentax fixes the problem. > So far, I haven't read of any SDM failures in the SDM only lenses, so I do > wonder if there is an issue with the dual AF integration. > > William Robb > I have both the DA* 16-50 and 50-135 and have at least a couple thousand exposures on each. No problems. They're both superb. Of course one person's experience proves nothing, but I wouldn't want to be without those lenses. Ditto the DA* 60-250. The three of them make a great kit. Paul
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