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10-14-2018, 08:36 PM   #1
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What's up with my lens?

I took my 14 2.8 out to take some shots of the sky and noticed that on the right side the stars don't quite look in focus. This was shot at f2.8 at 30 seconds

https://puu.sh/BLesZ/30889cbfa4.jpg



I don't know if it's decentering, I never noticed this before. Plus the building on the right looks tack sharp. Any ideas?

10-14-2018, 08:46 PM - 1 Like   #2
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Nothing in the sky is in focus, so you're well forward of infinity.
The flare in the bottom left suggests light pollution might be at least part of the problem.
10-14-2018, 08:47 PM   #3
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QuoteOriginally posted by Sandy Hancock Quote
Nothing in the sky is in focus, so you're well forward of infinity.
The flare in the bottom left suggests light pollution might be at least part of the problem.
So the problem is my lens isn't hitting infinity then. I need to adjust that then and try again.
10-14-2018, 10:43 PM   #4
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QuoteOriginally posted by ZombieArmy Quote
I took my 14 2.8 out to take some shots of the sky and noticed that on the right side the stars don't quite look in focus. This was shot at f2.8 at 30 seconds

https://puu.sh/BLesZ/30889cbfa4.jpg
Shooting any lens wide open will not give you optimum sharpness across the field, but I do think it's decentered. Try shooting a flat brick wall at f/2.8. Try the same in the day of a city scape at f/2.8. Of course things will improve at f/5.6 or f/8, but if you're trying to shoot stars, you probably don't want too high ISO.

10-14-2018, 11:17 PM   #5
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QuoteOriginally posted by Alex645 Quote
Shooting any lens wide open will not give you optimum sharpness across the field, but I do think it's decentered. Try shooting a flat brick wall at f/2.8. Try the same in the day of a city scape at f/2.8. Of course things will improve at f/5.6 or f/8, but if you're trying to shoot stars, you probably don't want too high ISO.
Well if it's decentered I'm basically out 200 bucks so I hope it isn't that.
10-15-2018, 12:33 AM   #6
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Are there any good guides to help calibrate this lens so it can actually focus to infinity? Thanks in advance.
10-15-2018, 12:38 AM   #7
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Yeah, for your sake I hope I'm wrong, but I'm a bit confused.

The metadata says this was shot with a 15mm Pentax with your K-01. Did you shoot this with a 14mm? And if so, the Rokinon, Samyang, or Pentax 14mm f/2.8? New, the first two are $300 lenses and the Pentax is about $650 new. Your lens may have been only $200 because the previous seller knew about this issue. A decentered lens can be fixed at a price, but at mid-apertures, the decentering would less of an issue.

10-15-2018, 09:06 AM   #8
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QuoteOriginally posted by Alex645 Quote
Yeah, for your sake I hope I'm wrong, but I'm a bit confused.

The metadata says this was shot with a 15mm Pentax with your K-01. Did you shoot this with a 14mm? And if so, the Rokinon, Samyang, or Pentax 14mm f/2.8? New, the first two are $300 lenses and the Pentax is about $650 new. Your lens may have been only $200 because the previous seller knew about this issue. A decentered lens can be fixed at a price, but at mid-apertures, the decentering would less of an issue.
It wasn't 200 new, it was an "excellent condition" used lens. But returning it has sailed unfortunately.

But yeah it's a Samyang 14 2.8, as per my sig.
10-15-2018, 09:13 AM   #9
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This may not have anything to do with anything, but sometimes lens focus does not stop at infinity but keeps going past infinity. Or conversely, the lens has a mount problem and can't reach infinity (less likely and harder to fix) Perhaps do a test to see if infinity on your lens is actually infinity with perhaps a distant billboard or something else regular that you can judge. Long shot, but it may be worth a bit to check this out.
10-15-2018, 09:15 AM   #10
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QuoteOriginally posted by ZombieArmy Quote
It wasn't 200 new, it was an "excellent condition" used lens. But returning it has sailed unfortunately.

But yeah it's a Samyang 14 2.8, as per my sig.
Here's a guide on how to check for decentering:
How to Check Your Lens for Decentering - Articles and Tips | PentaxForums.com

As for the focus, you may need to tweak it manually because the infinity mark on the lens is often just an approximation. One trick I use (if the moon is out) is to focus on the moon using live view first, then adjust the composition as desired. Also, stopping down even by half a stop could be worth it, IMO.

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10-15-2018, 11:55 AM   #11
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What did you focus on in this photo? If you focused on something 12 feet away, then you're at the hyperfocal distance and the stars shoud also be in focus. Maybe it isn't that your lens won't go to infinity, but your lens might be back-focusing (?) and you therefore aren't getting the stars in the DOF.

(As an aside, it looks like Pentax exif doesn't contain the focus distance?)
10-15-2018, 02:21 PM   #12
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https://puu.sh/BLCka/4dc1a2d27d.jpg


https://puu.sh/BLCgQ/123ce64004.jpg



First one is top left of the frame, second is top right where the softness was. What do you guys think? I think I'm expecting the worst here.
10-15-2018, 02:46 PM   #13
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The top image is back focussed - the clouds and bottom right roof line look reasonably sharp.
Nothing in the bottom image is in focus.

An image of a flat planar subject, with detail from edge to edge, focussed carefully in live view, would give us something to work on.
10-15-2018, 03:01 PM   #14
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QuoteOriginally posted by Sandy Hancock Quote
The top image is back focussed - the clouds and bottom right roof line look reasonably sharp.
Nothing in the bottom image is in focus.

An image of a flat planar subject, with detail from edge to edge, focussed carefully in live view, would give us something to work on.
The top and bottom picture are the same exact tree just moved a bit on the tripod.

Also what do you mean by flat planar subject? I'll try to find something to do that with.
10-15-2018, 03:08 PM   #15
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I still think it's a decentered lens. I know many have praised the quality of their Rokinon/Samyang primes, but that has not been my experience. At the extremes, such as wide open at f/2.8, it does not have edge or corner sharpness and there is greater sample variation in quality from lens to lens. At f/5.6-f/8 they are fine, but not at max or min aperture.

The Pentax 14mm is more than double the price of Rokinon/Samyang for various reasons:
a) Pentax field of view: 90 degrees vs. RokSam 94 degrees
b) Pentax 12 elements in 11 groups vs. RokSam 14 elements in 10 groups
c) Pentax 420 grams vs. RokSam 530 grams

But beyond the tech numbers there is also quality control and overall IQ. I think your lens will be fine in the mid-aperture ranges, but noticeably problematic f/2.8-4.5 and probably f/13-22.

---------- Post added 10-15-18 at 04:10 PM ----------

QuoteOriginally posted by ZombieArmy Quote
Also what do you mean by flat planar subject? I'll try to find something to do that with.
Like a flat brick wall where you can fill the frame from corner to corner.
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