[Laurentiu Cristofor]

These tests do seem to suggest that you may have a bad copy,

For the DA 15, at f/8, though the resolution improves and gets closer in the green MTF, it shows more signs of astigmatism as well. Anyway, my expectation from a Limited lens is that I don't have to use it at f/8 to get its best performance. I don't think my copy is bad, it just is what it is.

EDIT/UPDATE 6/17: After using the DA15 on my EPL2, on which I can more easily check focusing, I revised my opinion on its sharpness in this post.

or perhaps that the Samyang wouldn't suit you any better anyway.

I am not considering the Samyang - it is just the subject of this thread and I am surprised to see someone complaining about its sharpness.

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Last edited by Laurentiu Cristofor; 06-17-2012 at 07:43 PM.

[causey]

For the suitability on landscape, let's take the green MTF at the two-third field position at f/8.
The 50% figures (radially) are 24 lp/mm for the DA 15 and 27 lp/mm for the Samyang.
However, this comparison disregards the 5.5% distortion exhibited by the Samyang on APS-C.
Correcting that level of distortion typically degrades the MTF by an order of 10%

Do you mean, distortion correction in raw or jpeg? I use raw, and I have't noticed any deterioration. And I'm a pixelpeeper--if I weren't, I would have bought the DA 15mm.

[lytrytyr]

Do you mean, distortion correction in raw or jpeg?

The link I referenced was examining in-camera correction on the Samsung NX 200.
So far, that's the only place I've found this issue discussed.

[lytrytyr]

Anyway, my expectation from a Limited lens is that I don't have to use it at f/8 to get its best performance.

For overall flat-field sharpness with a reasonably small wide-angle retro-focus lens, f/8 seems to be needed.
All my K-mount wide-angle primes: DA 15, M20/4, K24/2.8, and ZK 25/2.8, are like that.
Wider apertures may be usable if the field curvature matches the subject
(when Mr Petzval and Mr Scheimpflug cooperate!).

[Marc Sabatella]

Now, we can throw all these tests away - the bottom line for me is that I don't find my DA 15 sharp. It may be a dud, but I don't see obvious signs of a defect. And if reviews are oscillating in their numbers so much, perhaps it's a sign of poor quality control.

Perhaps, but how can you prove that it *lens* quality that is the variable here rather than the *test* quality? I suspect two different reviewers comparing the same exact two lenses on the same exact two camera would often obtain very different results, just from different (and in adequately controlled) testing methodologies.

Anyhow, if your DA15 is not he sharpest lens you own, I'd like to see the controlled test comparisons that have led you to believe that.

[lytrytyr]

The Samyang does not meet my personal practical criterion for being "reasonably small":
Mounted on the K-x, will it fit in a bridge camera waist pack?

The other issue with the Samyang is the concern
that the loss of resolution entailed by correcting 5.5% distortion negates
"Its primary strength [of] an extremely high resolution across the image frame"
as measured relative to the DA 15.

As the new Zeiss Z*15/2.8 shows, a big lens can give you the resolution without the distortion,
but the costs are unacceptably high: 6 times the dollars, and migration to Canikon.

[Laurentiu Cristofor]

Perhaps, but how can you prove that it *lens* quality that is the variable here rather than the *test* quality?

How picky we are when we disagree. If I would have said that the DA 15 is an excellent lens, you wouldn't have asked me to prove that it actually is.

I stated my opinion, based on my experience. For example, I think that the FA 31 is an excellent lens. but I think that the DA 15 is just ok. These are my impressions, based on my experience of using both those lenses. I can ellaborate on what made me feel this way, but I don't intend to try and prove anything.

We already have a thread going on the DA 15, so we shouldn't derail this one.

EDIT/UPDATE 6/17: After using the DA15 on my EPL2, on which I can more easily check focusing, I revised my opinion on its sharpness in this post.

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Last edited by Laurentiu Cristofor; 06-17-2012 at 07:44 PM.

[Matchete]

Back on topic, now that I have practiced focussing with it for a few days, I can say that it is an optically amazing lens.

It is indeed already very sharp in the center AND the corners at F4, and the extra 2mms really help outside and especially inside.

It is quite heavy, but not more than say the DA 16-50 2.8, so I can live with it.

[Marc Sabatella]

How picky we are when we disagree. If I would have said that the DA 15 is an excellent lens, you wouldn't have asked me to prove that it actually is.

Well, true, because most objective tests and my own experience show it is - it is "generally known" to be true, so there is no real point looking for verification every time someone posts something most people already believe or know to be true. It is when individual results vary from what is generally known to be true that one rightly *should* question those outlying results. If, upon careful examination of the test procedures and results, it is found that the outlying tests results really are valid for the particular copy of the lens, *then* one might start to wonder about it being a bad copy. And only if that possibility is ruled out - if one finds a pattern of several objective tests of multiple copies that stand up to close scrutiny all challenging the conventional wisdom - only then does it becomes worthwhile to challenge the conventional wisdom. I would consider these to be generally true statements of how the scientific method is usually applied, not anything specific to lens testing.

The thing specifically relevant to lens testing, though, is that it is easy to stuff up a test to make a lens look worse than it actually is. Whereas it is next to impossible to stuff up a test to make a lens look *better* than it actually is. If one given controlled test shows great sharpness, then that lens *is* capable of great sharpness, period, and no further tests will cause that to not be true. But if a lens appears in one test to not be sharp, it remains quite possible that this is due to inaccurate focus, insufficient camera stability, subject motion, flare, use of a UV (= "Unnecessary and Very bad") filter, etc, and that re-testing might lead to a re-evaluation.

And I'd say you have succinctly demonstrated this:

We already have a thread going on the DA 15, so we shouldn't derail this one.

EDIT/UPDATE 6/17: After using the DA15 on my EPL2, on which I can more easily check focusing, I revised my opinion on its sharpness in this post.

Noted on both counts, so perhaps there is no need to further continue this tangent.

Still, when someone posts a thread expressing disappointment with one given ultrawide, discussion of the merits of other similar lenses the OP might then consider instead. In my opinion as a moderator, this isn't sufficiently off-topic to be worth moving, but I suppose if we do wish to continue this particular aspect, it will be better to focus our comments in a more appropriate thread.

[lytrytyr]

Whereas it is next to impossible to stuff up a test to make a lens look *better* than it actually is. If one given controlled test shows great sharpness, then that lens *is* capable of great sharpness, period, and no further tests will cause that to not be true.

I see several ways this might break down.

First, some lenses are good at a particular distance range, but less so in others.
A standard lens review might show a lens to be sharp
when focused on a test chart at a few meters,
but that lens could be much worse at infinity or in the macro range.
I see your point about the lens always being good at the intermediate range,
but that doesn't help a user who may be looking for sharpness at infinity.

Another case involves (typically wide-angle) lenses with strong field curvature.
A tester may refocus the lens between the central and the edge resolution measurements.
The lens will then look like it gives great sharpness,
but users will be disappointed if they're photographing flat objects.

A third case might arise with lenses that exhibit astigmatism,
where a test is picking up the "good" orientation, and disregarding the "bad."

There are yet other ways a lens could be made to look good,
for example post-processing the test image,
or Sigma's practice of showing 30 lp/mm MTF curves,
where other manufacturers show 40 lp/mm.

PS: Sorry if this is rambling off-topic too much.
They are called "threads," implying they can weave around a bit.

[Laurentiu Cristofor]

Well...

Continuing this discussion on the DA15 thread:

https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/189130-15-ltd-7.html