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03-21-2011, 11:09 PM   #1
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New FA 31mm, couple questions

Quick introduction as I am a new member. I have been playing with cameras for about five years now, just for fun. The first 3 with three generations of point-and-shoots for snapshots, and the last two with a Canon t1i with kit lens, still casual mostly but starting to learn more about it. Recently (couple months ago) I sold the Canon and got the new K5 along with a DA 15mm limited. My interest is primarily travel photography, so more capturing a unique moment than finding artistic perspectives though I do enjoy that as well (even if I suck at it)

I'll be the first to admit it..I'm not that good, but I do take a lot of photos (45,000 so far) so I at least have some man hours behind it if nothing else . I upgraded primarily because it seemed to be "worth it" given how many photos I take and because I had fun with the DOF/bokeh on a f/2.8 lens I rented. I've never owned a fast lens before (f/2.8 as the fastest I've ever shot, on the rental, before buying this lens) so please cut me a little slack if this post seems ignorant.

Long story short, I just purchased the FA 31mm from B&H photo and have 7 days left to try it out before I can return it. I've taken it out a couple times for some real world photos, and in the process a couple questions have come up. It might be that I am not used to the extreme DOF but some portraits I took seemed softer than expected and there was sometimes striking fringing/CA even in indoor lighting and visible in the scaled image.

Here are a few sample photos I took as a result:



f/1.8 center crop

f/2.8 center crop

Now, the two full size photos look okay scaled down but I would say there is a noticeable improvement at f/2.8, and a noticeable "out of focus-ness" to the f/1.8 photo when viewing it at 1920x1080. Both shots were taken on a fixed surface and I tried multiple shots both with manual focus and with AF adjustment and the f/1.8 version did not improve.

All I can compare it with is the DA15 and the kit lens, and while both showed some improvement when stopping down on the same test, the difference was not nearly as drastic across any of the apertures. Neither were as sharp as the FA 31 is at f/4 but I would call both sharp from wide open and onward.

So two questions:
1. Is this normal or does it not match your experience?
2. What aperature do you mostly shoot at with this lens? Is it common to shoot more in the f/2.2-2.5 range? I would say at f/2.8 it's pretty darn sharp.

I do like the way other elements of the photo render, but based on these results I can picture myself keeping it at f/2.8 or higher unless I really need a shallow DOF which seems like a waste. Low lighting is not really an issue with the K5 so I wouldn't say that is a factor.

Here is a photo that I thought turned out okay, but I still think the glasses would have maybe been better at f/2.8? It's been raining all week here so haven't really had a chance to get much variety in shots.

Any thoughts or ideas?

Last edited by mason; 03-22-2011 at 12:45 AM.
03-21-2011, 11:39 PM   #2
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Return it and get the DA35 limited. The FA limiteds are for film. Now this commonsense statement will start a jihad and result in a fatwah being issued against me but its what pentax will tell you if you call them.
03-22-2011, 12:53 AM   #3
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Now that it has been told that one of Pentax main selling point (retrocompatibility) doesn't make sense, you can send me your 31 for cheap and go back to Canon

Otherwise, have you had a look at the multiple existing test of this fine lens? The FA31 produces CA wide open.
03-22-2011, 01:27 AM   #4
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I'd say your results are just fine.
The 31 is not the sharpest lens wide open, both 43 + 77 and FA*24 are sharper in the center wide open.
Good thing with 31 is that the sharpness is very evenly distributed right from f1.8 so off center framing is perfectly fine. The CAs are very visible wide open. But one thing where 31 beats all but 50/1.2 is OOF rendering.
The question then is: do you want really sharp wide open with acceptable OOF or acceptably sharp with really good OOF?
If first, go for 43 or FA35/2,if second be happy with 31.


03-22-2011, 01:47 AM   #5
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Doesn't look bad to me, but do a focus chart test to ensure you dont have some BF/FF
03-22-2011, 01:52 AM   #6

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Virtually all fast lenses take a major hit to sharpness once you go wider than f/2, and the FA 31 is no exception. It's the price you pay for speed. Take a look at this chart here: Pentax SMC-FA 31mm f/1.8 AL Limited - Review / Test Report - Analysis. Those crops look about right to me. I also don't see any out of the ordinary CA/PF.

Coincidentally, I also have an FA 31 that I just got from B&H, but unfortunately it's a true dud, and I'm waiting for an RMA so I can exchange it. It's decentered (one side of the frame is significantly worse than the other).

Last edited by Cannikin; 03-22-2011 at 02:07 AM.
03-22-2011, 02:48 AM   #7
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Sadly I don't own the FA31, but I do own some other fast lenses including the A 50 f/1.2.
In my opinion, those results look really quite good for f/1.8, from what I can tell. F/2.8 is much sharper of course, but with a lens of the quality of the FA31 I would expect that!

Enjoy your lens!

Don't be afraid to use a bit of image sharpening in post too, if you need it.
03-22-2011, 02:56 AM   #8
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It's a tough call.
Saying a lens is soft wide open can be as a result of both exquisitely thin DoF and slightly off-target focusing. I've personally been pleased with the 31 ltd wide open, but I qualify that sentiment by admitting that I rarely do shoot with it wide open. The DoF is too thin for most of my applications. So stopping it down makes sense.

But then there are clear indications where f/1.8 would be useful and creatively applicable, so there's its utility, and it doesn't do badly there, despite the comments of softness and CA.

03-22-2011, 03:41 AM   #9
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If its like my fa 31mm you need to check & adjust the focus. Grab a focus chart, set it to f1.8 and adjust accordingly.

Not really sure why but other users report that they back focus on a the K-x body.

Also it is not the easiest lens to use. Focus from f1.8-f2.5 is a bit hit or miss on my K-x in low light. I am getting some very nice results with it but it does require some effort !

Last edited by tux; 03-22-2011 at 03:55 AM.
03-22-2011, 04:19 AM   #10
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Obviously f/1.8 is not as sharp as the other apertures

Image taken from: test report

Do some tests outside to get natural light. Tungsten light might be an issue
03-22-2011, 07:41 AM   #11
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As an owner of a FA31, that looks about right to me (and pretty damn good for f/1.8). It's a keeper.

The FA31 is "dreamy" at or close to wide open, and that's kind of the whole appeal.
03-22-2011, 09:46 AM   #12
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Thanks for all the responses! I had a feeling that is how it is supposed to be (especially based on the photozone tests), just wanted to do a sanity check on it because of all the rave reviews I've heard about it being razor sharp wide open, etc.

So it sounds like most people are shooting at f/2 or higher unless they are specifically looking for the dreamy bokeh in the background / shallow DOF. Seems fair enough to me.
03-22-2011, 10:10 AM   #13
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Of the FA Limiteds, the 77 is sharp wide open, the 43 is sharp wide open at some focusing distances but not as sharp as the 77, and the 31 is probably the softest of the bunch but has the most "character."

In my experience, anyway.

They'll all cut like diamonds stopped down.
03-22-2011, 10:12 AM   #14
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I just got my 31 over the weekend. No FF or BF on my K-x and beautiful sharpness, contrast and bokeh. I think it lives up to it's reputation.
03-22-2011, 10:52 AM   #15
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FA 31 is supposed to be soft wide open.

It looks nice.

Not good for taking pictures of text. But it's good for people.

Best bokeh in the limited line, not the best sharpness. The 43 is sort of the opposite.

You can't have it all (you can see a similar thing going on with the 50mm 1.7 vs. the 50mm 1.4). 1.7 sharpness approaches the fa 43, never reaches it, but it has god-awful bokeh compared to most fast primes. The 50mm 1.4 has really nice bokeh, but suffers in the sharpness department.

Optics optics optics.

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