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08-28-2011, 05:58 PM   #1
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f/2.8 = Blue Lines K7

I was reading on ken rockwell that small apertures of f/8 and smaller blur the image due to diffraction. I decided to take a few test shots with myK-7. I used a SMC Pentax DA Star 16-50 mm lens. What I found was that with the aperture open to f/2.8, I get blue lines around high contrast parts in the photo.

I took several shots with a tripod, and a remote control,changing only the f-stop, and the corresponding shutter speed. I uploaded the test pictures to my Flickr site, and put it in a set called “Test Shots”. You can view them here.

I am not sure about the diffraction issue as of yet, but I am puzzled on why I get those strong blue lines at the lowest f-stop. On my photos, look around the windows and thero of line of the house. You will need to select “original size” on the size options page. Does anyone have any ideas?

Incidentally, I do not have this problem if I shoot with a RAW file.

Last edited by Adam; 08-28-2011 at 06:05 PM. Reason: typos
08-28-2011, 06:08 PM   #2
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That's not diffraction, but rather chromatic aberration (CA). Almost all lenses suffer from it, although higher-end lenses usually do a better job of mitigating it. CA typically shows up around bright edges in out-of-focus areas and at large apertures because of shallow DOF.

Speaking of diffraction- it's really hard to observe, but basically it means you don't want to be shooting at F22+ if you can help it.

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08-28-2011, 08:27 PM - 1 Like   #3
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QuoteOriginally posted by Desertdog Quote
I was reading on ken rockwell that small apertures of f/8 and smaller blur the image due to diffraction
diffraction limits lens resolution at smaller apertures - the camera sensor can be a high resolution, but it is the lens that puts a limit on the total system resolving power. Correct me if I'm wrong but at f/8 the diffraction limit for average visible light is around 120 line pairs per millimetre and as far as I know there aren't many digital sensors that can record detail at that level of resolution so it's safe to say you are worrying about nothing. That's not to say diffraction doesn't exist but it's effect isn't a hard limit as most people perceive it to be.

incidentally, I would advise you to not take anything Ken Rockwell puts on the internet seriously - there are more educated people who have put decades of time and effort into studying technical aspects of photography than that dilettante.

Last edited by Digitalis; 08-28-2011 at 11:49 PM.
08-28-2011, 08:59 PM   #4
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For my K20D's sensor, the diffraction limit is about f/9. That does NOT mean not to shoot at f/11. It means that if you pixel-peep shots taken in lab conditions (tripodded with mirror-lock-up etc), f/11 will be almost imperceptibly less sharp than f/8. For handheld shots, ever with SR on, diffraction just won't be noticeable even at the lens' smallest aperture.

Smallest apertures of some of my lenses:

f/45 - DA18-250
f/40 - DA18-55
f/38 - FA100-300
f/32 - DA10-17, F35-70, A70-200

Note that even DA's meant for APS-C sensors have tight apertures. I don't often have reason to shoot there -- but I *can*. For handheld shooting, I have concerns other than diffraction limits. The only times I necessarily stay at f/16 or wider is with studio-type work.

Last edited by RioRico; 08-28-2011 at 10:11 PM.
08-28-2011, 09:29 PM   #5
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Good Evening and Welcome to the Forum,

If you take a look at some reviews for your lens, you will find that the sweet spot for it is somewhere between f5.6 and f8 when considering resolution and image quality across the entire face of the lens (center, edges and corner). To be worried about diffraction at f8 is not something to really be worried about. Most lenses will have their best image quality near f8. As Digitalis and Rico posted, diffraction for all intense purposes starts in at around f11 to f16, and you will really have to look for it. If you want to try, just take a shot at every f stop up to f22 or the smallest f stop the lens supports. Then go pixel peeping - if you are really that interested.

As Adam observed the outlining you see is CA and it can be easily removed in any of the post processing utilities. You can google CA and the package you are using to get the instructions on how to remove it. Here is a bit of information on CA....
08-30-2011, 04:27 PM   #6
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If you actually look at the shots it is not CA (there being no corresponding yellow fringe) but blue fringing. CA would also not appear in the centre of the frame as this does (window panes etc.) Software CA correction will not cure it, desaturating the cyan shade of the fringe might, but you'd have to be careful not to kill the sky colour at the same time.

Am I missing something or is the DA* 16-50mm really this cr*p ?
08-30-2011, 06:25 PM   #7
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I personally don't worry about diffraction and often shoot at f11 (sometimes f16)
I worry a little bit more about CA... But to be honest, not much worrying there either...
If you nail your composition and your subject (whatever that may be) is sharp, only pixel peepers will notice (and IMO they usually make bum photographers)... Everyone else will enjoy a beautiful image...

Ps... If it doesn't happen when you shoot RAW... Shoot RAW! There are a more than a few (as you will find if you search 'jpg vs raw') good reasons for doing so anyway...
09-14-2011, 04:53 PM   #8
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Thank you all for your help. I thought CA happened with the longest and shortest wave lengths, there by the red and blue fringe. Since it did not happen in RAW, and only the blue fringe is showing, I figured that it must not be CA. At least I am sure that nothing is failing in my camera.


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