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03-31-2015, 05:15 AM   #1
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K-50 focus issue

My daughter borrowed my Pentax K-50 and DA*16-50 to take pictures at two of here photo shoots last week. After looking at come of them , i noticed that they were not very sharp. I did a quick test last night, taking some pictures of a water bottle. I set the DA*16-50 at 50mm, the picture was not sharp, i adjusted the fine tune from +1 to +10 and -1 to -10 and the picture never got clear. I attached my DA70 Limited and did the same test and the results were about the same. I then tried my DA 18-135, these were the worst of the three. I decided to try my recently purchased Tamron 90, OMG! the bottle jumped off my monitor and i did not knot there was so much detail on the label. Any idea why i cannot get this level of focus with my Pentax lenses

03-31-2015, 06:33 AM   #2
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can you post some example pictures with exif
or explain some more details about the circumstances the pictures have been taken:
Available light, distance to object, settings on camera

The 90mm Tamron is a macro, the others are not, could it be that the minimal focus distance was not reached?
Meaning you can get much closer to your subject than with your other lenses... just a thought

Edit: it seems you're now in the LBA club, nice lenses!

Last edited by max_pyne; 03-31-2015 at 06:53 AM.
03-31-2015, 07:46 AM   #3
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Distance from object 8ft, light from overhead chandelier. No fine adjustments on AF
Image 1314 is DA* 16-50
Image 1319 is Tamron 90
Image 1320 is DA70 Limited
Image 1324 is DA18-135
Attached Images
View Picture EXIF
PENTAX K-50  Photo 
View Picture EXIF
PENTAX K-50  Photo 
View Picture EXIF
PENTAX K-50  Photo 
View Picture EXIF
PENTAX K-50  Photo 
03-31-2015, 08:18 AM   #4
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have you shot them hand held? the ISO is not the same, I'm guessing you are using Av mode?interessting is that you have different ISO levels for every shot
first (50mm): ISO 640
Second (90mm): ISO 125
Third (70mm): ISO 250
Fourth: ISO 400

the higher the iso the more noise.

03-31-2015, 08:26 AM   #5
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Yes the pictures were taken hand held and the camera was set on Av. I will try again, this time keeping the ISO the same.
03-31-2015, 08:38 AM   #6
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They're all in focus, more or less, considering the slow shutter speeds. Are they hand-held? The lighting is soft. Soft low light will make images soft. Now try using a flash. I think you'll see sharp.
03-31-2015, 08:39 AM   #7
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for such kind of comparisons it is important to put your camera on a desk/or whatever steady surface or directly on a tripod and using the remote / 2 sec. delay, turn stabilization off.
otherwise you never know if it was you or the camera / lens causing the blur.

Last edited by max_pyne; 03-31-2015 at 08:45 AM.
03-31-2015, 09:20 AM   #8
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Yup, definitely use a tripod. Also, I would recommend using a different test subject since your water bottle is translucent and can thus easily lead to focusing errors.


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03-31-2015, 10:15 AM   #9
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Which one is out of focus?


Steve
03-31-2015, 11:02 AM   #10
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QuoteOriginally posted by stevebrot Quote
Which one is out of focus?


Steve
#2 is the only one in focus. If you zoom in you can see the shading detail around the letters
03-31-2015, 11:46 AM   #11
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QuoteOriginally posted by kjphilippona Quote
#2 is the only one in focus. If you zoom in you can see the shading detail around the letters
There is no way to zoom in, but I will take your word for it.

That sort of makes the point of several of the above comments. The notion of missed focus is highly dependent on subject, magnification, and final viewing distance. For example, in #1 the area available to the center focus point covers a broader range of arc across the face of the bottle than in #2 and presents a more ambiguous target*. The amount of data for the pertinent section is also much reduced. Changing the distance to provide equivalent framing would level the playing field as would focusing on a flat target rather than a curved surface. I would also suggest a best-of-ten approach for the comparison.**

Continuing on, the reason why all of your example photos appear equally in focus as posted is because the area of interest is adequately small on each photo. This is DOF in action where acceptable focus is determined as much by final magnification and viewing distance as by the focus accuracy and taking aperture. Missed focus is a relative thing and in many ways is a fairly recent concern born of digital photography and high performance AF systems. Prior to digital and the ability to evaluate the image at maximum resolution, if it looked OK, it was OK.

Sorry about being pedantic I suspect that if you repeated your tests with equivalent framing for all lenses that the results would be much more even.

Steve


* This is an issue of common complaint on this site. People post wondering why their camera misses focus on a face in oblique profile where the area available to the focus point sensor spans portions of the eyebrow, cheek bone and eye. The expectation is that the camera would magically know which feature is important when in reality, the system locks on whatever feature comes sharp.

** Focus sensitivity on the K-50 is limited to f/5.6 meaning that precision (chance of same focus point twice) falls with lenses faster than f/5.6. This limitation is common to the great majority of dSLR camera models with the exceptions being high-end cameras. Those generally feature a focus precision of f/2.8 for one or more of the central focus points and f/5.6 for the remainder. The K-5 has a single f/2.8 focus point while the K-3 has three in the center column.

Last edited by stevebrot; 03-31-2015 at 11:53 AM.
03-31-2015, 02:19 PM   #12
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QuoteOriginally posted by stevebrot Quote

** Focus sensitivity on the K-50 is limited to f/5.6 meaning that precision (chance of same focus point twice) falls with lenses faster than f/5.6. This limitation is common to the great majority of dSLR camera models with the exceptions being high-end cameras. Those generally feature a focus precision of f/2.8 for one or more of the central focus points and f/5.6 for the remainder. The K-5 has a single f/2.8 focus point while the K-3 has three in the center column.
I did not know this. makes buying faster lens's a bit of a waste?
03-31-2015, 03:16 PM   #13
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QuoteOriginally posted by nonstopnick Quote
I did not know this. makes buying faster lens's a bit of a waste?


Yep i now have around $1500 i wasted on lenses

03-31-2015, 03:46 PM   #14
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QuoteOriginally posted by nonstopnick Quote
I did not know this. makes buying faster lens's a bit of a waste?
This is where live view/focus peaking comes into play.
03-31-2015, 04:19 PM   #15
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QuoteOriginally posted by paulh Quote
This is where live view/focus peaking comes into play.


I have neve used live view on my pentax. Tried it a few times on the X's Nikon, it took way too long to take a picture, i was not sure what all the clicking and ckacking that went on.

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