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01-21-2012, 12:14 PM   #31
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Depends on how serious the field curvature is. For this shot, the focus can be at the center and the bottom (foreground) will be sharper because of field curvature, while the top (background) will be blurrier. Stop down the aperture may or may not solve the problem depends on the lens design.

Sometimes, field curvature can be used for its advantage. For example, shooting interior with walls on both sides coming toward you. Since the focus on both sides will be closer than the center due to field curvature, the whole picture will appear very sharp. But for a landscape with even distance from left to right, only the center will be sharp.

01-21-2012, 12:59 PM   #32
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QuoteOriginally posted by Ben_Edict Quote
No, it is not about 2 technical differences. These are two completely different lenses.

- The K15 is a super wide angle with a 111 deg angle of view (diagonally)
- the DA 15 is a wide angle with a more common 86 deg angle of view

You can't compare those two lenses usefully.
The original post was asking for an explanation of the comparison.
The question was justified, to the extent that it concerned
two rectilinear lenses of the same focal length,
one covering FF, and the other covering APS-C.
Within the framework of the question,
the different diagonal angles of view
are corollaries of the different coverages.
01-21-2012, 01:19 PM   #33
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QuoteOriginally posted by wlachan Quote
But for a landscape with even distance from left to right, only the center will be sharp.
As is often said,
that kind of wide-angle landscape can
"turn mountains into molehills."

However, many wide-angle landscape photos
work by framing a distant view inside closer subjects,
such as a foreground and overhanging branches.
For landscape shots of this type,
field curvature again becomes an advantage.

Since landscape photography is usually done with narrow apertures,
depth of field means that the sharpness difference is minimized.
If it's still a problem, then APS-C may not be the best medium to use,
so medium format or a view camera might be preferable anyway.
01-21-2012, 09:52 PM   #34
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QuoteOriginally posted by Ben_Edict Quote
- The K15 is a super wide angle with a 111 deg angle of view (diagonally)
- the DA 15 is a wide angle with a more common 86 deg angle of view
??? On the same camera? That must be quite a feat of optical design!

On the same camera, the lenses most definitely can be compared, as they will have similar FOV unless the K15 possesses some magic I am unfamiliar with.

01-21-2012, 11:54 PM   #35
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QuoteOriginally posted by wlachan Quote
Great sharpness near the middle, but not so good toward the top & bottom. Typical field curvature issue.
I don't find it very sharp even dead center - just decent - better than the kit lens, but not by a huge margin. I am well aware of the impressive result numbers on photozone - they are one of the reasons I purchased this lens. But I am not impressed with what results I get in terms of sharpness - the Samyang 14 looks much sharper in the full size images I examined - I avoided it because I wanted something smaller and with less distortion - the DA 15 is good in those areas, but sharpness is just ok. Maybe there are QC issues with it and photozone was lucky to get a perfect sample - it's not the first lens where their results were different from my experience.
01-22-2012, 05:10 AM   #36
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QuoteOriginally posted by Laurentiu Cristofor Quote
I don't find it very sharp even dead center - just decent - better than the kit lens, but not by a huge margin. I am well aware of the impressive result numbers on photozone - they are one of the reasons I purchased this lens. But I am not impressed with what results I get in terms of sharpness - the Samyang 14 looks much sharper in the full size images I examined - I avoided it because I wanted something smaller and with less distortion - the DA 15 is good in those areas, but sharpness is just ok. Maybe there are QC issues with it and photozone was lucky to get a perfect sample - it's not the first lens where their results were different from my experience.
As you said, it could be either optical misalignment or AF issue. What I have found is that AF inaccuracy contributes to a large amount of unsharp images. Like the DA14 which is often said to be soft wide open, but it is indeed very sharp "when in focus". Mine was like that and took me some time to figure out.

The problem with PZ is that their real world sample pics were casually taken while their lab tests were carefully done. The focus on many of their pics are simply not focus properly.
01-22-2012, 12:23 PM   #37
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QuoteOriginally posted by wlachan Quote
As you said, it could be either optical misalignment or AF issue.
With mine, it is not AF. It's not like it's sharp elsewhere than I wanted it - it's just not very sharp in the focal plane.
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