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05-16-2016, 01:45 PM   #1
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D FA 15-30 corner softness/DOF

Has anyone else who owns the 15-30 experienced awful corner sharpness and poor DOF?

I understand an UWA will always give more sharpness in the center of the frame than in the corners. I also understand that in general a prime will outperform a zoomable lens.

However, my Samyang 14mm obliterates my Pentax 15-30. I thought the 15-30 was based off the Tamron 15-30 which received stellar reviews. So, maybe my expectations were just set too high but I have been severely disappointed and am trying to figure out if I have a bad copy or what. Corner sharpness lags far behind the Samyang but perhaps the more intriguing thing is the smaller DOF on the Pentax. I understand the FOV plays into DOF and as such the Samyang would be expected to have a slightly wider DOF but the difference between the two at comparable F stops is remarking.

Here are a couple test shots. The first shows the corner sharpness and the second the DOF. These are 2:1 crops. I have produced similar results with multiple tests both inside and out but these are just a couple examples.

Shot 1: Focus was set on the same object in the center of the frame which was a comparable distance from the purse in the crop. The purse was in the far corner and is clearly sharper in the Samyang. Pentax left and Samyang right.

Shot 2: Focus was set at infinity and the F stop at 2.8. The crop of this picture was certainly not focused properly but on an UWA I generally want a wide DOF for landscape purposes to avoid focus stacking whenever possible. So, this test was designed to test the DOF. Again, Pentax right and Samyang left. I zoomed in on the object at the center of frame which was more or less at infinity to ensure the focus was actually at infinity in case the lenses were calibrated improperly. I then zoomed in on this crop of a foreground item. Again, Pentax left and Samyang right. How is it possible the DOF varies this much between the two?


Has anyone else noticed poor performance with this lens? For a $1500 lens I was expecting it to be closer to the $400 Samyang but am currently left very disappointed. AF and the ability to zoom were both important factors for me in this purchase but I am not sure having those two abilities can justify me keeping it with results like this. Do note that in the center of the frame the Pentax does have a slight advantage in terms of sharpness over the Samyang, but only by a small margin.

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05-16-2016, 02:03 PM   #2
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QuoteOriginally posted by turff Quote
However, my Samyang 14mm obliterates my Pentax 15-30. I thought the 15-30 was based off the Tamron 15-30 which received stellar reviews. So, maybe my expectations were just set too high but I have been severely disappointed and am trying to figure out if I have a bad copy or what.
There was a review of this lens recently that compared it to Samyang 14mm. They said that the Samyang has better corners, but the 15-30mm is sharper. Keep in mind that the corners might not just be soft - maybe it is the DoF that is highly curved, at least that is what the photo you posted seems to show.
I'd say do a couple more tests, like taking photo of a newspaper near the minimum focusdistance, at 45 degree tilt so you can see the whole DoF, as it curves towards the edges
05-16-2016, 02:08 PM   #3
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DOF is dependent on Aperture, focal length and subject distance. DOF should be broadly similar between the 14 and the zoom at 15mm at the same aperture and the same subject.
05-16-2016, 02:12 PM   #4
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QuoteOriginally posted by Pål Jensen Quote
DOF is dependent on Aperture, focal length and subject distance. DOF should be broadly similar between the 14 and the zoom at 15mm at the same aperture and the same subject.
But what about curvature? Doesn't that depend on corrections? For example, dedicated macro lenses have a very flat field of focus. So it looks a little different from a zoom lens at the equivalent focal length and aperture.

05-16-2016, 02:20 PM   #5
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When evaluating corner sharpness, you should focus on the corners using live view. Anything else isn't an apples-to-apples comparison.

The 14mm should have a greater DOF since it's wider. And with these ultra-wides, even while almost everything from up close to far away is in focus, the corners can still be really picky.

All this aside, in practice (when focusing in the center) I think soft corners are common with the 15-30mm @ 15 due to curvature and DOF. That's been my finding so far, but they're still better than what my legacy primes can pull off. Plus, looking back at the center of the frame and even close to the edges, the 15-30mm is so insanely sharp that I think it's worth every penny. The angle of view you're getting (10mm on APS-C) isn't easy to pull off in a wide-aperture zoom!

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05-16-2016, 03:03 PM   #6
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I had not really considered the DoF itself may be curved but that would completely explain what is going on here. I will have to mess around with it a bit more and see if I can adjust knowing its limitations. Thanks guys!
05-16-2016, 03:58 PM   #7
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Yeah, a curved DoF can cause problems in "brick wall" tests. Adam's tip is spot on.
But when you shoot f8 to f14 and use a little bit of zone focusing / hyperfocal focusing, you will get really wide DoF and this will not be a big problem anyway.

I have Samyang 14mm, but that 15-30mm is really tempting. Seems like a very comfortable zoom range, good aperture, sharp optics.
Once you shoot raw, place some lens profile corrections, it should get even better

05-16-2016, 09:15 PM   #8
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I do not have an example at 2.8 but attached is my brick wall with the Pentax 15-30 at 15mm f6.3 iso 400
Corners looks good to me.
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05-16-2016, 09:29 PM   #9
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QuoteOriginally posted by turff Quote
I had not really considered the DoF itself may be curved but that would completely explain what is going on here. I will have to mess around with it a bit more and see if I can adjust knowing its limitations. Thanks guys!

Not kidding ... it's what the brick wall test is for, Turff!


Hope it shows your lens is fine ... make sure you set up absolutely parallel and level on the tripod and use Live View to rule out the PDAF system.
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