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01-19-2012, 10:20 AM   #1
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how does the DA15mm work?

This may seem like a silly question, but how does the DA15 work, and why is it so much more compact than the older Film era 15mm F/3.5 lens?
From the kmp.bdimitrov.de website, this is the old film era K15 F/3.5 lens, which is 81.5mm, and 80mm in diameter and weights 550 grams.

And here is the much smaller DA15, which is less than half the size. Both lenses are Pentax mount, with the same distance to the film/sensor, and both have to use a retrofocus design. How does the DA15 manage to get the same amount of reverse telephoto inverse magnification with so little glass? Its almost the same size as a 28mm film lens, 39.5mm long and weights 212 grams or less than half of the KA15. I'm just curious; I really love the DA15 by the way....
- Sheldon


01-19-2012, 10:24 AM   #2
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Smaller APS-C sensor might have the most to do with it. That and design by computer.
01-19-2012, 10:45 AM   #3
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Smaller sensor and still it's not excellent: only just good when stopped down to f8
01-19-2012, 10:56 AM   #4
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QuoteOriginally posted by Emacs Quote
Smaller sensor and still it's not excellent: only just good when stopped down to f8
Ditto. The old 15 is FF lens while the DA15 is APS-C. The latter sacrifices edge sharpness and field curvature for compact size. Not a smart decision as a modern prime.

01-19-2012, 11:32 AM   #5
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QuoteOriginally posted by wlachan Quote
Ditto. The [DA15] sacrifices edge sharpness and field curvature for compact size. Not a smart decision as a modern prime.
All lens designs are a compromise, but since the DA15
is capable of resolving 35 lp/mm at the edge of the K-5 sensor
according to the Optyczne/Lenstip test,
or reaches an "excellent" level on the borders of the K10 sensor
according to the Photozone test,
stopped down 2 stops,
I would regard it as misleading to quibble
with the DA15's edge sharpness.

If it is to be compared with a full-frame lens,
then the FA*24 would be a better candidate than the K15,
since both are designed for a 75-degree field of view.
According to the Photozone test,
the FA*24 doesn't seem to reach beyond
a "very good" level even on the APS-C borders,
and that only when stopped down 4 stops.

As far as the comparison with the (standard) K15 goes,
the DA15 has the benefit of both
a high-dispersion and an aspherical element,
as well as only having to cover "half-frame" APS-C.
Halving the size halves the radius of an element,
so cuts the area and weight by a factor of one quarter.

Last edited by lytrytyr; 01-19-2012 at 11:38 AM.
01-19-2012, 11:33 AM   #6
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QuoteOriginally posted by K6-III Quote
Smaller APS-C sensor might have the most to do with it. That and design by computer.
Yup, and according to falco's tests it's not an FF lens in disguise, either.

It's remarkable how Pentax was able to create such a superior lens in so small a package.

Edit: also, magic.
01-19-2012, 11:55 AM   #7
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I am afraid the proof is in the pictures. I have yet to see any DA15 pics which are sharp corner to corner due to field curvature (and please no more downsized samples). The fact that it was outperformed by the DA16-45 is disappointment. Also, the FA*24 is really poor on digital although quite alright on film (though still not great). Surely a modern prime can be optically superior. No?
01-19-2012, 12:07 PM   #8
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QuoteOriginally posted by wlachan Quote
Also, the FA*24 is really poor on digital although quite alright on film (though still not great).
For reference I would massively disagree with this statement.

01-19-2012, 12:11 PM   #9
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QuoteOriginally posted by wlachan Quote
I am afraid the proof is in the pictures. I have yet to see any DA15 pics which are sharp corner to corner due to field curvature (and please no more downsized samples).
Take a look at the bottom left sample image on the Photozone test.
You can see the text on the displays
in the storefronts under the Gothic arches.
And that was at ISO 400 in bad weather.

QuoteOriginally posted by wlachan Quote
The fact that it was outperformed by the DA16-45 is disappointment
At 16mm, the DA 16-45 never gets to "excellent" borders
in the Photozone resolution test (on the K10, like the DA15 test).
I'm not sure what aspect of performance you're considering here.
01-19-2012, 12:11 PM   #10
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QuoteOriginally posted by wlachan Quote
I am afraid the proof is in the pictures.
I completely agree with you. Tests and all are nice, but the proof is in real pictures, and there the DA15 shines!
01-19-2012, 12:23 PM   #11
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QuoteOriginally posted by lytrytyr Quote
Take a look at the bottom left sample image on the Photozone test.
You can see the text on the displays
in the storefronts under the Gothic arches.
And that was at ISO 400 in bad weather.
Great sharpness near the middle, but not so good toward the top & bottom. Typical field curvature issue.

QuoteOriginally posted by lytrytyr Quote
At 16mm, the DA 16-45 never gets to "excellent" borders
in the Photozone resolution test (on the K10, like the DA15 test).
I'm not sure what aspect of performance you're considering here.
It's the flat field nature that makes this lens excels. Photozone tests do away the field curvature effect so the DA15 border figures appear better than they actually are in practice.
01-19-2012, 12:35 PM - 2 Likes   #12
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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.
But not so bad, either. In fact, quite good relative to the whole package. it's not a landscape lens per se, but a wide prime. For landscape the 12-24 is better.

QuoteQuote:
It's the flat field nature that makes this lens excels. Photozone tests do away the field curvature effect so the DA15 border figures appear better than they actually are in practice.
In practice as a thread on the forum demonstrates, as a compact WA with stellar flare control, this lens performs extremely well in the field.

the slight (and they are slight as the objective data shows) loss of edge IQ is:

a. not uncommon for a WA....pretty much in the history of lens design
b. acceptable for the price
c. more than acceptable for the compact design
d. well compensated for aesthetically by the flexibility the flare control offers

If you're looking for an edge-to-edge sharp WA, this is not the lens for you.
01-19-2012, 03:49 PM   #13
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QuoteOriginally posted by rob1234 Quote
For reference I would massively disagree with this statement.
+1 to this one.
I was lucky to own great copy of FA*24 and it never disappointend me with sharpness...and for the record I only used it on digital
01-19-2012, 04:15 PM   #14
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The SMC Pentax 15mm F3.5 and SMC Pentax-A 15mm F3.5 (optically identical to its K-series predecessor) were both rectilinear lenses.

So for use In FF format, this might account for some of the extra glass used for optical rectilinear correction (little or no barrel or pincushion distortion), hence making the lenses heavier and bulkier.

PS. I love mine to bits.
01-19-2012, 08:09 PM   #15
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QuoteOriginally posted by Aristophanes Quote
But not so bad, either. In fact, quite good relative to the whole package. it's not a landscape lens per se, but a wide prime. For landscape the 12-24 is better.



In practice as a thread on the forum demonstrates, as a compact WA with stellar flare control, this lens performs extremely well in the field.

the slight (and they are slight as the objective data shows) loss of edge IQ is:

a. not uncommon for a WA....pretty much in the history of lens design
b. acceptable for the price
c. more than acceptable for the compact design
d. well compensated for aesthetically by the flexibility the flare control offers

If you're looking for an edge-to-edge sharp WA, this is not the lens for you.
+1. I have never taken a shot with this lens where corner sharpness really got in the way (even at F4), and JSherman's thread shows just what this lens can do in the hands of many here. I agree about the 12-24, but the times when that difference is needed are fairly rare for me.
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