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01-09-2011, 04:54 AM   #16
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Thank you bdery for the hard work necessary to put this out there. As you say, the number score is the least important part of the review, the photos and review itself are where the importance lies and based on that I think people will be much more able to see if the DA 21 will meet their needs.

01-09-2011, 05:53 AM   #17
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QuoteOriginally posted by ChatMechant Quote
i saw that too, and just figured whoever did the test accidentally swapped something when uploading.... or something. seems to me that what it says is the kit lens, is obviously the 21.
it does look wrong... diffraction at f11 for a f3.2 lens? doesn't sound right
looks to me like a focusing error or wrong file

someone should double check that
01-09-2011, 06:20 AM   #18
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Yeah. There must be something wrong. Kit lens trash both Sigma and DA 21 at all apertures. But I downloaded the images and PhotoMe is correct about the lens (and it corresponds to the sliders), so it may be focusing error?

The results about sharpness are definatelly not in line with the pictures shown in comparison (and with my DA21).

edit: On the other hand if I look at corners, they are way better on DA21 pictures, so it could be just field curvate.
01-09-2011, 06:22 AM - 1 Like   #19
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QuoteOriginally posted by crannogman Quote
Am I going blind or does the kit lens walk all over the DA21 at f8 & f11 in the sharpness comparison? The outperformance of the kit lens against the DA21 at f8 seems to me about the same margin as the DA outperforms the kit lens wide open. Notice I'm not mentioning CA though
QuoteOriginally posted by Pablom Quote
t does look wrong... diffraction at f11 for a f3.2 lens? doesn't sound right
looks to me like a focusing error or wrong file

someone should double check that
I checked and it's accurate, meaning the right files show the right pictures.

"trumps" is maybe a strong word, but I agree that the corners are slightly sharper with the kit. Since it applies to all corners, I rule out misaligned setups.

While it's possible that the camera focused inaccurately, I'd say it's unlikely given the test conditions.

As an optical designer, I can say that it's not unexpected that a lens such as the DA21 will be less stellar closed down. The reasons are:

1-very small physical aperture, meaning diffraction will hit quite fast (that's mainly what we're seeing)
2-optimizing a lens for a wide aperture (as the DA21 is obviously optimized) means that it will not be optimized for smaller apertures. There's no free lunch.

As someone mentioned, that refers to sharpness, not to other optical qualities. And if you compare the two lenses wide AND closed down, you'll see the gap is much stronger wide open than closed down. And for the record, in real shooting situations the 18-55 cannot hold its own, regarding colours, saturation, etc, with the Da21. And that's coming from a guy who is constantly impressed with the kit

I hope this clarifies.

01-09-2011, 07:34 AM   #20
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QuoteOriginally posted by bdery Quote
I checked and it's accurate, meaning the right files show the right pictures.

"trumps" is maybe a strong word, but I agree that the corners are slightly sharper with the kit...

I hope this clarifies.
Thank you for elaborating on that bdery-there are certainly areas in which the kit can't compare, just struck me as a bit funny when I examined the sharpness test (which I like and will try and duplicate at home btw).

Ahh, lenses. Now if someone could just design an 18-400mm f2.8 the size of a coke can for less than...
01-10-2011, 01:11 AM   #21
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QuoteOriginally posted by bdery Quote
As an optical designer, I can say that it's not unexpected that a lens such as the DA21 will be less stellar closed down. The reasons are:

1-very small physical aperture, meaning diffraction will hit quite fast (that's mainly what we're seeing)
Won't the physical aperture be the same size of all the lenses? when they are at the same focal length and f/#?

QuoteQuote:
2-optimizing a lens for a wide aperture (as the DA21 is obviously optimized) means that it will not be optimized for smaller apertures. There's no free lunch.
Why not? Can't you just try to make all the glass good?
01-10-2011, 06:16 AM   #22
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QuoteOriginally posted by sewebster Quote
Won't the physical aperture be the same size of all the lenses? when they are at the same focal length and f/#?
More or less, but not exactly, because it's not the same optical design. The key here is to understand that the aperture is defined by the "aperture stop" inside the lens, and that can or cannot be the smallest diameter that light encounters. With a small lens it often is, but with a more complex design (like a zoom) that's not necessarily the case.

QuoteOriginally posted by sewebster Quote
Why not? Can't you just try to make all the glass good?
It's not as simple as that. Any lens design is about choices and compromises. Simply put, you cannot have a lens that performs well at all focal lengths, all apertures, all focus distances. When designing a lens, you make choices, you accept compromises, and you keep in mid how the lens will be used.

If you take the DA21, it's a wide lens, so the DOF quickly becomes ludicrously large. So you don't really need a small aperture, except to cut down light. And since it's a relatively fast lens, you're probably going to use it at wide apertures, not small ones, most of the time. So you tweak your design according to that. And you also look at CA and other aberrations (things like coma, for instance, are extremely well controlled in modern designs, so we never discuss it, but that's because the engineers do their job well).

Take the kit. It's a zoom, so extreme focal lengths are likely to perform relatively poorly. Mid-range focal lengths will be better, with a sweet spot probably between 20-30 mm. Now, because it's harder to tweak a lens for wide apertures, since it's a cheap kit you optimize it for around f8, because that's easy. You accept that it will basically suck wide open, except maybe around 30 mm or so.

then you put in coatings, glass choices, curvatures of surface elements, focus system (which moves glass elements in relation one to another), and it gets fun...

The engineers at Pentax are probably among the best in the world, if just for this : normally when you make a lens design, you start with what already exists and build from there. That's why all inexpensive fifties look more or less the same : they're based on the same original design. But look at the FA and FA Limiteds, and you see that again and again, the Pentax designers started fresh, and created a new design of their own (there's no existing equivalent to use as a starting point). That's to be commended.
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