How duff is this DA* 50-135mm?


WaypointCharlie

Link Posted 24/04/2013 - 15:20
I bought a used DA*50-135mm recently off eBay. Cosmetically it looks in great condition and came well packed with all the bits.

I tried it out with my K-5 in the garden on some flowers and was impressed by the depth of colour and out of focus bokeh. However, I felt the shots weren't very sharp wide open and it seemed like I was getting a lot of back focus. So I decided to check it.

With a focal length of 70mm at a distance of about 1.3m with no AF adjustment I get a back focus of about 24mm, or an inch in old money. That seems pretty terrible and is about 2% of the 1.3m distance. Setting the AF adjustment to +10 makes matters a lot better but I think there's still a tendancy to back focus. None of my other Pentax lenses have required any AF adjustment.

I also notice from these tests that the lens is a lot softer at f/2.8 than at f/4.0. I know some loss of sharpness is to be expected wide open but I'm thinking the centre doesn't look "very good" to "excellent" as per the Photozone review.

Have a look at these test focus shots:

Picasa link

Do you reckon I've picked up a poor sample? How much might it cost to get calibrated by Pentax and would they be able to improve it much?

Of course, this is the risk you take when buying on eBay rather than through a dealer with a warranty. Sometimes you're lucky, sometimes you're not.

WaypointCharlie

Link Posted 24/04/2013 - 16:14
Looking at it further I think the loss of sharpness at f/2.8 may be a general weakness of this lens at close distances. From a distance of about 6m the DA*50-135mm at f/2.8 is certainly sharper than my old M50mm at f/2.8.

Perhaps I'll just leave the AF adjust at +10 and make the best of what I've got, but I'd still be intrested to see how my findings stack up with other DA*50-135mm owners.

cbrog

Link Posted 24/04/2013 - 17:42
I had a similar problem with a brand new DA*300, using a like target set at 45 degrees the bottom of the page was just a smudge. The top was quite readable although not as sharp as I would have expected from a lens of this quality.

Of course it still took reasonable shots at distance providing I was not spot focussing.

SRS sent it to JP Service Solutions where the AF motor was replaced. Since when everthing has been fine.
Last Edited by cbrog on 24/04/2013 - 17:43

Gamka

Link Posted 24/04/2013 - 18:10
I have been through a recent set of images and pulled out some taken with 50-135 but only one is at f2.8.

My camera does NOT have any focus correction - it may need a little. These are the JPGs straight out of the camera.

I have been through a few recent shots and found some taken with the 50-135 but there are not many at f2.8. A short comment is under each




This was early evening, 95mm f3.5 Autofocus was set for automatic, with Mid-left and Center in focus








The snake is actually in the water. 123mm f4.5 Auto focus set for spot with Center (V) selected







A bright day 115mm and f5.6 - a case of grab it when I could, no time to tweak settings. Auto focus spot - Center (V)








Rather green - but that was the water colour. 63 mm f2.8 Autofocus Spot Center (H) selected which means I was focussed a few inches maybe a foot behind the eyes.

davidstorm

Link Posted 24/04/2013 - 19:36
The only way to really test this properly is to set up the camera and lens on a tripod in decent light and shoot some images head-on at a test chart or another suitable static subject. Alter the AF Fine Adjustment, view the images side by side in Photoshop and continue to adjust until pin sharpness is achieved.

You may need to go into the debug menu to make a wider adjustment than the AF Fine adjustment gives, but this is only if there is a general issue with the camera on all lenses, as opposed to a specific issue with a particular lens. If the lens will not adjust correctly within the AF Fine adjustment setting and your other lenses are fine, the only other option is to get the lens checked and repaired if necessary.

Regards
David
Flickr

Some cameras, some lenses, some bits 'n' bobs

Blincodave

Link Posted 24/04/2013 - 22:07
If you are unhappy with it and purchased it within the last 14 days, why not ask the seller to take it back? It seems to me that the E Bay system is set up much more these days to protect the buyer's interests.

WaypointCharlie

Link Posted 25/04/2013 - 12:01
Thanks guys for your comments.

I've done some more tests and it's blindingly obvious on real world shots that the Phase Difference AF back focusses (prior to adjustment). Contrast AF seems to work fine in Live View. With an adjustment of around +8 to +10 the Phase Difference AF closely matches Contrast AF. With no adjustment you can see the difference in focus between Phase Difference and Contrast AF simply from the distance markers on the focus ring.

Apart from the AF being out I suspect I've got a fairly normal copy. Given that it appears to be at least as sharp as my M50 at f/2.8 I don't think the optics are bad. I just need to learn the good and not so good personalities of this lens...and perhaps work a bit harder on my hand held technique. For instance, I now know not to expect blisteringly sharp results on wide open close ups. However, I don't think that's what this lens is designed for and it's not my intended use, which is mainly for portraits and live events.

I don't intend asking the eBay seller to take it back. I doubt I'd get anything better for what I paid - a case of better the devil you know!
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