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11-06-2010, 12:50 AM   #1
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Please help me to understand what with my 31mm/1.8

I have just bought 31mm/1.8, but I am unhappy with quality of images that I get with it. I see some blur and aberrations on aperture less than ~5-8.
For example this photo is taken with my kit 18-55/3.5 (35mm/f5)

18-55 (35mm/f5) - Pentax User Photo Gallery


And following pictures are taken with 31mm/1.8

f1.8

31mm/1.8 (f1.8) - Pentax User Photo Gallery

f2

31mm/1.8 (f2) - Pentax User Photo Gallery

f3.2

31mm/1.8 (f3.2) - Pentax User Photo Gallery

f4

31mm/1.8 (f4) - Pentax User Photo Gallery

f8

31mm/1.8 (f8) - Pentax User Photo Gallery

the most shock i have got when took this photo:

lights in blur - Pentax User Photo Gallery

and this looks not bad for first view

almost not bad - Pentax User Photo Gallery

but if I zoom a light

blur again - Pentax User Photo Gallery
I see the same story.

There is always some blur directed to right and up. And also there is some chromatic aberrations directed to left and down.

Could those people who have 31mm/1.8 please take some similar pictures and check if you have same problems? I am almost sure that there is a bug with this lens, but I thought that build quality of this lens should be good enough for this money.

11-06-2010, 06:15 AM   #2
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What you see are chromatic aberrations. This are natural occurences in big aperture and specially visible in wide-angle lenses. This is a inherent characteristic of the lens and will happen mostly when wide open or almost open, between 1.8-2.8 normally. Fortunately it can be corrected rather easily in post-processing, or so I've read.

Take a chance this weekend and go out to take pictures with the lens, try to shoot at the aperture you want and if possible one at f/4 or smaller. You've got a great lens, just remember that that happens mostly when you have high-contrast/extremely bright areas in a photo and you'll do fine. With practice you'll be able to identify the subject and adjust your settings accordingly.
11-06-2010, 06:20 AM   #3
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It looks to me like it may be mis-focusing, either front or back. You might want to test it on a simple focus chart and see.

I do not have that lens, and if those are representative samples, I won't anytime soon. I would guess that something is wrong.
11-06-2010, 09:33 AM   #4
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QuoteOriginally posted by summonbaka Quote
What you see are chromatic aberrations. This are natural occurences in big aperture and specially visible in wide-angle lenses. This is a inherent characteristic of the lens and will happen mostly when wide open or almost open, between 1.8-2.8 normally
according to this (and many other tests)
Pentax SMC-FA 31mm f/1.8 AL Limited - Review / Test Report - Analysis
this lens should have very little CA and level of CA should be stable.

Also if I understood correctly CA should be simmetrical to center of picture. So if purple color appears on the left side of an object that is on left side of picture, the same quantity of purple should be on the right objects. But I do not see any simmetry on current my photos.

Also here I see strange blur that has the same color as object. This blur is blue for "WR" letters and white for white letters. As far as I know CA do not do that.

And finally if I can do better pictures on my kit lens (for $150) with f5 than on this lens (for $1000) with f8 what is the point to call this lens one of the best lens for pentax? (((

QuoteOriginally posted by summonbaka Quote
With practice you'll be able to identify the subject and adjust your settings accordingly.
I have limit of time to return this lens back to a seller.

QuoteOriginally posted by Nick Siebers Quote
It looks to me like it may be mis-focusing, either front or back. You might want to test it on a simple focus chart and see.
I tried to use autofocus and manual focus. I think I was very accurate when I used manual focus.
But anyway I will try to do some tests in this side. Thank you for the advise.

Additional questions to 31mm/1.8 owners - do you have any noice when you shake the lens? I have some noice and other time I would not worry about this but now I think if maybe some elements are not fixed well.

11-06-2010, 09:42 AM   #5
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For me, this is a defective lens. I do own a FA 31.
100% crop of a shoulder portrait @ f2:

Last edited by Zav; 11-06-2010 at 09:53 AM.
11-06-2010, 11:17 AM   #6
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I have a 31mm ltd. and it's my sharpest lens. Unless I'm doing something wrong, it would never give me pictures like you're getting. The blurring you're getting doesn't look like camera shake and not even a problem with focusing. My opinion (non-expert mind you) is that the optics are very soft. Something within the lens is not right. I would return it for repair or return.

Hope it works out for you.
alex
11-06-2010, 11:25 AM   #7
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Well...
Today i started doing more photos and some of them are not bad. I started thinking that as Nick said it could be a problem of focusing. Probably this lens really does not like when I miss focus and make that blur around of lights. I will try again today evening.

these are pictures:

https://www.pentaxforums.com/gallery/photo-text-f1-8-22294/


https://www.pentaxforums.com/gallery/photo-cube-22295/


https://www.pentaxforums.com/gallery/photo-just-picture-22296/

Maybe summonbaka is right and all I need is just more practice with this lens. But I really afraid that it is something wrong with it and it will be too late when I realize this.

Zav, could you please show some of your pictures that contain high contrast elements, lights in the night and some lights that are out of focus?
If you have something black with white letters on it it would be great if you can do some pictures of it.

Actually I suppose that even if a light is out of focus it should be blurred to all sides uniformly (with the same value) - without any accent to one side.
11-06-2010, 11:40 AM   #8
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I had a focusing problem with my 31ltd (on a K20D) until I switched to a auto/matrix(?) from center or select which I normally use. Now it is mostly in focus.

11-06-2010, 05:43 PM   #9
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QuoteOriginally posted by Neco Quote
Zav, could you please show some of your pictures that contain high contrast elements, lights in the night and some lights that are out of focus?
If you have something black with white letters on it it would be great if you can do some pictures of it.
Not really the kind of pictures I fancy! But I hope those two will helps:


By xiaozav at 2010-11-06



By xiaozav at 2010-11-06


full size jpegs there: Free - Envoyez vos documents
11-06-2010, 06:03 PM   #10
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The first set of images looks terrible, and it could be focus issues, but that image of the bus is beyond miserable and could such an image be because of focus issue????????. I can't completely rule out mis-focus or other issues, but for certain this is not at all indiactive of the FA31.

A quick shot taken at f1.8... indoor light and so the WB is not quite right
Full shot


100% crop

Last edited by pcarfan; 11-06-2010 at 06:18 PM.
11-06-2010, 06:09 PM   #11
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QuoteOriginally posted by pcarfan Quote
The first set of images looks terrible, and it could be focus issues, but that image of the bus is beyond miserable and could such an image be because of focus issue????????. I can't completely rule out mis-focus or other issues, but for certain this is not at all indiactive of the FA31.
I agree. The first set of shots could be out of focus, but the bus shot just looks off. My copy of the 31 is pretty sharp, even wide open at f/1.8.
11-06-2010, 07:34 PM   #12
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Thank you a lot guys. Now I almost sure that there is a bug with my copy ((
I do not see on your photos that I see on my.

Very sad for me because I had been going to buy tamron but changed my mind because had found information that tamron has very many bad copies. And for buying online it is not the most funniest exercise in the world - to send lens back and forward until you get your good copy.
And I was pretty sure that pentax provides stable quality with their lens. I was wrong.
And I bought on adorama - and now I checked and found on ebay that they ship only to US. But this is my last week in US and I am not able to exchange it. Only return my money back (I hope).

And now it is dilemma for me - return it and forget about 31mm or keep it and create some rules when i use it. Because I see that some pictures could be not bad like this one

target - Pentax User Photo Gallery

or this

birds - Pentax User Photo Gallery

but I really must avoid to take pictures on aperture less than f8 if there are bright lights in the scope

purple lights - Pentax User Photo Gallery

katy fwy - Pentax User Photo Gallery

So I must forget about night photography with this lens (fast lens, that is purposed for low light/night photograhy!!!)

What would you do if you were me?
11-06-2010, 08:07 PM   #13
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Dumb question, but you don't, perchance have a "protective filter" on that lens do you?
11-06-2010, 08:10 PM   #14
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QuoteOriginally posted by Wheatfield Quote
Dumb question, but you don't, perchance have a "protective filter" on that lens do you?
Not a dumb question.... maybe dumb that no one else thought to ask it first
11-06-2010, 09:21 PM   #15
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QuoteOriginally posted by dgaies Quote
Not a dumb question.... maybe dumb that no one else thought to ask it first
I have to prove my worth occasionally or you lot won't put up with me.
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