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09-07-2019, 09:04 AM   #1
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Pentax 28-105mm DFA resolution and fringing

Just bought a used Pentax 28-105mm DFA. Looks in very good condition. This has had some amazing reviews in terms of good image quality and low CA. Took the lens out for a the first time yesterday (on a Pentax K1) and was a bit disappointed with the images sharpness particularly towards the edge of the frame and the large amount of CA, again at the edges of the image. Played around with the K1 focus adjustment and when on +10 the focusing is a lot better but still back focuses a bit. Also tried it on a K3 and similar back focus but correction of +5 sorted it out. Don't think it is the K1 at fault as most other lenses have no correction or very slight (for sigma 70-200 Mk II). Has anyone else had similar image quality issues?
Lens has a guarantee so should be able to send it back if it proves to be a bit of a "duff" copy.
Thanks

09-07-2019, 05:58 PM - 1 Like   #2
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I would recommend the good old brick wall test to see if the lens has decentering issues. I suspect that it does. My copy is very sharp and has very little in the way of CA.
09-07-2019, 07:03 PM   #3
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I am absolutely delighted with mine.

Show us some images - you may well have a bad/de-centred copy, which should be repaired or replaced under warranty.
09-07-2019, 07:04 PM   #4
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QuoteOriginally posted by darylk Quote
I would recommend the good old brick wall test to see if the lens has decentering issues. I suspect that it does. My copy is very sharp and has very little in the way of CA.
This ^^

The 28-105 has received near universal praise as a sharp, colorful lens and a great value for the received IQ. The main objection seems to be susceptibility to shutter shock between 1/60 and 1/125 when used on K-1.

09-07-2019, 11:51 PM   #5
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QuoteOriginally posted by monochrome Quote
The 28-105 has received near universal praise as a sharp, colorful lens and a great value for the received IQ. The main objection seems to be susceptibility to shutter shock between 1/60 and 1/125 when used on K-1.
... and pretty strong vignetting (easily fixed in pp). But I agree with the others; its image quality is excellent across the frame. Let's see some examples.
09-08-2019, 05:35 AM   #6
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Thanks for the feedback.
As suggested tried a few test shots and results attached. Hopefully the filenames explain the camera settings. (focal length +fine focus correction). All images taken at either f3.5 for 28mm and F5.6 for 105mm. 100 ISO and shutter speeds of 1/1250s or about 1/650s. Wall photos at 28mm +0 (image 1), edge a lot worse than centre but (2) 28mm +10 (image 2), centre gets worse, edge gets better. 105mm - centre and edge both improve with fine focus on +10 (image 4) rather than 0 (image 3). Images are all raw files lightened a bit and saved as jpegs. No sharpening or other changes in photoshop. All full frame images.
Just to clarify, I am not one of these pixel peepers, its just that the images from the first outing with the lens were really disappointing and not what I expected from a lens with such good reviews. I have looked at some sample photos (high res) on various review sites and the detail at the edges is amazing and very little different from the centre but seem to be much better than my copy.
Attached Images
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PENTAX K-1  Photo 
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PENTAX K-1  Photo 
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09-08-2019, 02:48 PM   #7
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QuoteOriginally posted by Richard1956 Quote
As suggested tried a few test shots and results attached
I am no expert but this does look de-centred to me. Especially with the 105mm shot the edges do not have equal sharpness. I would take it back for a replacement.

Best way to test for these things is to plonk the camera on a tripod and use LV, zoomed 100% and manually focussed. Introducing the AF/FA variable when testing a lens for decentre-ing is only going to complicate matters.
09-08-2019, 04:00 PM   #8
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QuoteOriginally posted by pschlute Quote
Best way to test for these things is to plonk the camera on a tripod and use LV, zoomed 100% and manually focussed. Introducing the AF/FA variable when testing a lens for decentre-ing is only going to complicate matters.
In addition to the above, it is essential to have the camera's sensor perfectly parallel to the brick wall.
I would also take the same shots from a little further back for comparison.

09-09-2019, 12:19 AM   #9
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Thanks. I'll try a few more as advised by you and post them soon.
09-10-2019, 08:51 AM   #10
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Pentax 28-105 test

Bad weather meant that wall photos were not practical - I know the K1 is waterproof but ....

So used a lens test chart which is A1 in size. Spent ages making sure the camera was level in both directions and exactly in the middle of the test chart suing huge set square tape measures etc to that it was exactly parallel to the chart.

The result was that there was quite a difference in clarity between the bottom left (1st image), bottom right (2nd image), top left (4th image) and top right (5th image). Fine focus set to +10 to get the centre focused as well as possible (3rd image)
As a result of this test decided to send the lens back and will look for another.
Attached Images
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PENTAX K-1  Photo 
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09-10-2019, 06:36 PM   #11
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Yes, just send it back, Richard, you've taken those photos as evidence if the vendor decides to 'pushback'.
09-17-2019, 12:02 PM   #12
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Thanks again for the advice. Sent the lens back which was not a problem and got a full refund. Found another used one for just 20 more (349 inc free delivery). What a difference! Much sharper and consistently sharp across the frame. Bit softer at the edges compared to the centre at maximum aperture but that's to be expected but not really noticeable at normal image viewing distances. Far less CA at the edges than the first example - in fact hardly any at all. The performance of this one seems to match the excellent reviews it has received. Problem solved!
09-17-2019, 02:26 PM   #13
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That's how it should be, Richard, glad it worked out!

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