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When shooting landscapes, a single image (say 1 of 200) is totally out-of-focus.

A typical setup might be: 

focal distance = 7.1 (x 3.9)  
f-number: 5  
exposure time: 1/800

According to extended EXIF info, hyperfocal distance was 1.32 m.
There was no high single straw, no low hanging branch, no finger in front of the lens etc.

It was focused to infinity and yet, the result was like this (it's a crop):
crop

I suspect an unnoticed small flying bug might appear in front of the lens while auto-focusing...?

How do I prevent these spoiled images?

Notes:
The camera in question is FujiFilm X-30. (Firmware has been upgraded to 1.01 which fixed another auto-focusing issue.) 

Focus mode: S (front dial)  
Macro: Off  
Focus area: unchanged (in the centre)  
Release/Focus priority: AF-S: Release; AF-C: Release  
Instant AF Setting: AF-S  
AF Mode: Area
Pre-AF: OFF  
AF Illuminator: Off

Should I change some of those advanced settings to prevent this problem? My only basic requirement is to keep the ability to point on something, then press the trigger halfway, compose the image (move the subject out of the centre) and take a photograph.

EDIT (> 1 month later)
I've been playing with my camera settings such as turning OIS completely off etc. but no help. The problem is appearing usually when shooting panoramas (not in a camera built-in panorama mode, but in the A semi-auto mode - to be stitched in Hugin). It's especially annoying when one of 10 or 20 images is spoiled, thus the intended panorama is spoiled too.

The only remedy seems to be shooting with the manual focus. However I want to use this camera in a point-and-shoot mode. When I go primarily photographing, I take a bigger camera with me and cay play with a manual mode, focusing and other settings. This camera is used when stopping at the roadside for a few seconds to take a few pictures or a panorama so I expect a reliably working auto-focus mode.

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    \$\begingroup\$ For a landscape, I'd not use continuous autofocus, but rather focus on the initial shutter release press. From the EXIF data, "Release/Focus priority: AF-S: Release; AF-C: Release", I'm not sure if AF-C (i.e. continuous) is active. \$\endgroup\$
    – DrMoishe Pippik
    Jun 8, 2015 at 23:10
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    \$\begingroup\$ I don't even use autofocus for landscape shots. Manual focus usually produces better results if you use sufficient magnification to check for correct focus. \$\endgroup\$
    – Count Iblis
    Jun 9, 2015 at 13:16
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    \$\begingroup\$ @user681768917: please show us a complete, high-res, blurred picture with the complete EXIF of that file. \$\endgroup\$
    – TFuto
    Jun 9, 2015 at 15:47
  • \$\begingroup\$ @user681768917: also, was your camera focus-hunting before taking the blurred picture? \$\endgroup\$
    – TFuto
    Jun 9, 2015 at 15:54
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    \$\begingroup\$ @TFuto: Here you have a correctly focused DSCF3044 and blurred DSCF3045 image with the same settings and EXIF info. Click "Show original" or something similar above too see them in the original resolution. \$\endgroup\$
    – user681768917
    Jun 9, 2015 at 16:09

3 Answers 3

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It took me almost 4 years to come to the following conclusion: It is a camera issue (or rather a camera model issue) and it definitely isn't rare. Many reviewers call it erratic autfocus or just autofocus issues.
Looking closely at the pictures reveals that even seemingly focused ones while fitted to screen ("in the print size") are slightly out-of-focus while others are perfectly focused.
FujiFilm XF10 compact camera and Pentax DA-17-70mm-F4-SDM lens are good examples of an erratic auto-focus.

How do I prevent these spoiled images?

You can't. But there are some mitigation tips.

  1. Repeat shooting the scene (if applicable). Ideally try to focus different objects in the same distance and then recompose. Choose the best.
  2. Avoid focusing on glittering objects.
  3. Back up with manual focus. You'll probably lose to perfectly auto-focused image (especially with short focal lengths) but you won't probably get a completely spoiled image.
  4. While shooting panoramas, focus on the object that's known as the most reliable for your autofocus, then switch to manual focus. Check the last image after shooting.
  5. Know your camera (or lens). Spend some time training with various focal lengths, apertures, focused objects, light conditions etc. to find out strengths and weaknesses of your autofocus.

Possible causes of erratic autofocus
Disclaimer: This is a speculative part, based rather on my observations.

  1. Glittering objects. Even when out-of-focus, as they're "blown", these cause contrast enough to satisfy the automatics.

SMC PENTAX DA L 18-50/4-5,6 DC WR RE

  1. "Noisy" bokeh. It adds enough frequencies for the auto-focus to snap while better focusing is still needed.

SMC PENTAX DA L 18-50/4-5,6 DC WR RE

Are there any lenses completely free from that problem?

I doubt that. There are excellent lenses with a very low rate of unfocused shots but even here you can't be 100% sure that there won't be a failure when you need it the least. Reshoot the very important scene even while using these reliable lenses.

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Did you manually set the focus by rotating the focus point all the way left\right? If so that could be your problem. On many modern lenses infinity is not actually at the end of rotation, but a little bit before, I.e. you can focus past infinity (and thus out of focus). I believe this is due to the way autofocus mechanisms work. The auto focus needs to go past focus and then back into focus. If you don't know this setting focus manually to the extreme end can lead to out of focus shots.

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    \$\begingroup\$ This is a good thing to check in general, but we see from the info given that the camera was in autofocus / single mode. \$\endgroup\$
    – mattdm
    Jul 11, 2015 at 9:15
  • \$\begingroup\$ @mattdm Agreed. 'Display Name' made a good point but it's indeed an autofocus issue. \$\endgroup\$
    – user681768917
    Jul 13, 2015 at 6:40
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One out of 200 shots, statistically would mean that either your shutter button was pushed all the way down before auto focus had a chance to acquire or something in the shot changed the focus. I suggest using back button focus if possible on your camera to separate the shutter button from the auto focus, it will also be helpful when recomposing.

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  • \$\begingroup\$ Thanks but I don't think the shutter button pushed all the way down is the case. A flying bug in front of the lens might change the focus but I'm not sure if AF would respond on it...? \$\endgroup\$
    – user681768917
    Jun 11, 2015 at 5:52

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