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01-25-2010, 03:47 PM   #1
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K18mm/3.5 vs FA20mm/2.8 - opinions

Anybody had chance to compare these two?
Now, the K lens has 58mm thread going for it (it could share CPL with 31) and fantastic built and 2mm which in this FL do matter a lot.
the FA is obviously AF and AE lens, and ever so slightly faster.
But I think it's bit too close to 24 for comfort.
Mind you, by getting any of these I'd be selling my 10-20... the deal is practically done
any opinions greatly appreciated

TIA
Peter

01-25-2010, 05:40 PM   #2
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I don't think that the auto focus is much of an issue because neither of these lenses would typically be used for candid photography. You usually would have plenty of time to focus and the depth of field will be huge.

The auto exposure is handy because you have a record of the aperture setting on every photo. With manual aperture, you do not. With these lenses, again you would have plenty of time to set up the shot and hypermanual mode works great with manual aperture lenses.

I would tend to go with the wider lens - the 20mm is not going to give you enough wide angle effect on a APS-C camera. 18mm is also going to be a disappointment also if you are moving from a 10-20mm zoom.

Optically, the 18mm has 12 elements in 11 groups. The 20mm has 10 elements in 9 groups. The FA 20mm/2.8 is optically identical to the Pentax-A 20mm/2.8. Compare the lens test results on this webpage. They both have their strengths and weaknesses. Be aware that it is easier to build a 20mm lens than a 18mm one.

http://www.takinami.com/yoshihiko/photo/lens_test/pentax_superwide.html
01-25-2010, 06:05 PM   #3
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QuoteOriginally posted by pentaxman Quote
I don't think that the auto focus is much of an issue because neither of these lenses would typically be used for candid photography. You usually would have plenty of time to focus and the depth of field will be huge.
I will disagree with this. I happen to have the FA20mm F2.8, and the AF is a very nice feature to have on almost any lens. Not only is it good for landscapes and static objects (which would be fine with manual focus), but it is also great for events in tight spaces (indoor or out). I used mine quite a bit this past Christmas to take pictures during my family holiday gatherings. Shooting at f/4 and within 6 or 10 feet, achieving correct focus definitely matters, and manually focusing a wide angle lens under indoor lighting on a human subject is NOT easy. The camera (K-7) does a great job, however . The FA will also give you P-TTL flash (and wireless P-TTL) capabilities for making shooting indoor family events a breeze.

Now if you only plan to use it for landscapes or hyperfocal street shots, then yea it doesn't matter.

The FA is a bit soft (not bad though) at F2.8, sharpens up very nicely by F/4. Here are two good example of why I like this lens, at F4. Autofocus did help here, although I wouldn't say that either shot would be impossible with manual focus.





Here is one at F2.8:


And a couple at F5.6:


01-25-2010, 07:42 PM   #4
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Go with the AF lens... the FA 20mm.
Trying to get precise focus gets to be challenging with wide angles. Sure one can rely on DOF scales or using hyperfocal distance to maximize DOF but from my experience, I'll go with the AF lens hands down.
Plus the K lens limits you to using manual exposure mode only, not the case with the FA lens. Not to mention you'll get full exif info with the FA lens.

01-25-2010, 07:45 PM   #5
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I have the 18 and the M-20 f4. The 2mm does make a difference. The 18 is very well corrected. Reasonably sharp at f3.5 very sharp stopped down. Infinity focus starts around 4 feet so stopped down to say f5.6 most everything will be in focus at Infinity i.e. not much focusing is needed. It really is a gem of a lens.
01-26-2010, 01:16 AM   #6
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thanks guys,
I'll give it a thought or two
01-26-2010, 01:18 AM   #7
axl
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QuoteOriginally posted by pentaxman Quote
...snip...

Optically, the 18mm has 12 elements in 11 groups. The 20mm has 10 elements in 9 groups. The FA 20mm/2.8 is optically identical to the Pentax-A 20mm/2.8. Compare the lens test results on this webpage. They both have their strengths and weaknesses. Be aware that it is easier to build a 20mm lens than a 18mm one.

http://www.takinami.com/yoshihiko/photo/lens_test/pentax_superwide.html
sorry but what do those numbers (59/62...) mean?
01-26-2010, 09:25 AM   #8
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QuoteOriginally posted by axl Quote
sorry but what do those numbers (59/62...) mean?
http://www.takinami.com/yoshihiko/photo/lens_test/procedure.html

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