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11-07-2014, 02:19 PM   #1
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Focusing through a window

I have noticed persistent problems with a K3 and the 17-70 DA, at 70mm.

But I have done the focus tests mentioned here
AF microadjustment for the 1Ds mark III, 1D Mk3, 5D Mk2, 7D, 1D X
and I cannot see anything wrong. Occasionally there is a tiny movement in the focus ring between live view and normal mode.

I have tried to do an adjustment in the camera to see what this does and even going -10 to +10 I cannot see any difference! So maybe I am doing something wrong...

But the main scenario where I have focus problems at 70mm is shooting from a light aircraft, through a ~4mm plastic window.

It also seems that using live view solves the problem.

I think this makes sense. LV uses video spectrum optimisation (maximising the high frequency components) so should work through a window or whatever, but maybe the focus sensors used in the normal mode are confused by a window material?

I am happy to use LV; it's just curious whether this is what you would expect.

It does seem a lot worse with the K3 than with the K5.

11-07-2014, 02:23 PM   #2
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That's interesting. I've never had a problem acquiring focus through a window - the image may have sucked because it's soft or distorted but it's in focus.
11-07-2014, 02:24 PM   #3
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Well, I'd expect to have problems from time to time focusing through something... especially if it's that thick...
11-07-2014, 02:55 PM   #4
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It acquires focus allright. It would not take the photo otherwise.

But I have been disappointed with the quality. It is way worse than what I get on the ground.

Another factor is that often one is going through the window at an angle, not square-on.

11-07-2014, 07:02 PM   #5
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Focus to infinity before take off, disable AF and don't touch the lens. That should solve your issues shooting from the plane.

QuoteOriginally posted by peterh337 Quote
It acquires focus allright. It would not take the photo otherwise.
Depending on focus mode, of course.

I am familiar with the focus adjustment protocol from Northlight Images and have used it in the past. The moire' method is useful, but with the K-3 you have better (more precise) options. The key is to use magnified live view with focus peaking.
  • Use center point only
  • High contrast target
  • Camera on tripod with sensor at 20x the focal length from the target
  • Sensor parallel to target*
Now the flow:
  • Turn AF on
  • Acquire focus using PDAF (viewfinder) system
  • Turn AF off
  • Switch to live view and press "OK" to magnify view
  • Note whether the magnified target shows a strong focus peak outline
  • Manually attempt to improve on the AF system focus attempt and note which direction (nearer/farther) was needed to attain true focus
  • Adjust the AF fine focus one way or another and repeat until the two agree
Note that we do not use live view CDAF for this flow, just the true optical focus point vs. the PDAF system.


Steve

* I have also used the printed curved surface of the back of my canned air can. Back focus is readily apparent using that as a target. The challenge is to accurately aim the focus sensor to the nearest point on the can.
11-08-2014, 12:16 AM   #6
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"Focus to infinity before take off, disable AF and don't touch the lens."

That is virtually impossible given that the focus ring will move by itself. Also the 17-70 does not focus at infinity when against the stop, despite having been very recently serviced by Pentax.

I will check your other method - thank you.
11-08-2014, 12:49 AM   #7
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QuoteOriginally posted by peterh337 Quote
Also the 17-70 does not focus at infinity when against the stop
Unfortunately, that is a characteristic of many modern AF lenses.


Steve
11-08-2014, 12:51 AM   #8
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QuoteOriginally posted by peterh337 Quote
It acquires focus allright. It would not take the photo otherwise.

But I have been disappointed with the quality. It is way worse than what I get on the ground.

Another factor is that often one is going through the window at an angle, not square-on.
I've taken pictures through an airliner window and, while the two don't compare (it's - I believe - something like three or four layers of glass and plastic, this last one very far from the glass as I'm sure you know), you've got to expect a massive reduction in contrast, color saturation and rendition of fine detail... Then, if you're shooting through the stream of a jet engine, it gets a blury mess and that's it...

11-08-2014, 09:48 AM - 1 Like   #9
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Let me give you two examples, from this writeup: Flight from Shoreham EGKA to Salamanca LESA, Spain, November 2014

This is a 1:1 detail from file-0025 (the "mystery runway") - http://peter-ftp.co.uk/screenshots/20141108163602390.png
(70mm focal length)

It's hard to say why but I just "know" this is nowhere near as sharp as I saw it with my own eyes at the time.

This is a 1:1 detail from file-0034 - http://peter-ftp.co.uk/screenshots/20141108163926468.png
(23mm focal length)

This is a 1:1 detail from file-0043 - http://peter-ftp.co.uk/screenshots/20141108164117281.png
(48mm focal length)

To be fair I don't have a 70mm ground based example from that trip, so it could be that the lens is simply very bad at 70mm but the reviews don't support such a degradation.

The airborne pics were taken through 4mm of what is basically a single sheet of polycarbonate.

What this suggests to me is that the camera focuses OK in LV but the window material confuses the normal focus mode. Only more tests will tell, and they will be easy enough to do.
11-08-2014, 09:53 AM   #10
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You should also remember that it's not only a "window" you're focusing through, whatever the thickness, but also a few thousand feets of air... which can have its temperature, wind, density, humidity variations etc.
Something we might not perceive clearly with the naked eye, but that will nonetheless disrupt the path of the light and reduce sharpness when we look at 100%...
11-08-2014, 10:24 AM - 1 Like   #11
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These are my thoughts. You can take them or leave them.
  • Beautiful airplane, I am seriously envious!
  • The detail photo of the mystery runway appears to be in focus, but is soft. I suspect that camera motion may be contributing to that. The frequency of the engine vibration may be somewhat higher than what the SR can handle. You might actually want to try shooting with SR turned OFF.
  • There is also a fair amount of atmospheric haze due to scattering of blue light by water vapor, pollution, and/or smoke. All of your aerial shots show this haze. Scattered light is not "sharp" light. It is because of this scattering effect that B&W aerial photography film is preferentially sensitive to the red end of the spectrum with decreased blue sensitivity.
  • Low contrast due to the haze decreases the signal to noise ratio and at ISO 1600 with your K-3, noise is contributing to the softness
  • Your eyes (coupled with your fantastic brain) are a better imaging tool than your camera under these conditions. You see detail...the camera does not.
  • Last but not least, focus calibration to infinity is sort of a wasted effort due to DOF at distance and the mechanical limitations of the system. It is difficult for the system to detect the OOF state and even harder for it to make a fine enough adjustment. The stepping motors have their limits and even a single step may be a 100 meters or so difference in point of focus.
My advice is that you place your emphasis for focus calibration on near (~ 20x the focal length) and near distant (50 meters or less) targets. You might also want to try a manual focus lens with valid infinity stop to tune your expectations for results from in-flight photos. I don't believe that your softness issues are focus problems.


Steve

---------- Post added 11-08-14 at 09:32 AM ----------

QuoteOriginally posted by LensBeginner Quote
You should also remember that it's not only a "window" you're focusing through, whatever the thickness, but also a few thousand feets of air... which can have its temperature, wind, density, humidity variations etc.
Air, the devil that assaults the landscape photographer! In my region of the world, on most days, detail begins to degrade at a few hundred meters simply due to water vapor.


Steve
11-08-2014, 10:43 AM   #12
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QuoteOriginally posted by stevebrot Quote
*snip*



Air, the devil that assaults the landscape photographer! In my region of the world, on most days, detail begins to degrade at a few hundred meters simply due to water vapor.


Steve
And, reading the sum-up again, I see that the OP traveled at FL100 and 110... that's about 10'000 feet (depending on the atmospheric pressure) i.e. 3+kms in the metric system, at massively different temperatures (since it's a vertical column of air and not an horizontal one as it's the case with landscapes)...

Edit: add to that UV radiation, that starts to be an issue at those altitudes, and you've got yourself a pretty picture regarding what can go wrong in those circumstances...

Last edited by LensBeginner; 11-08-2014 at 11:27 AM.
11-08-2014, 11:31 AM   #13
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QuoteOriginally posted by peterh337 Quote
Let me give you two examples, from this writeup: Flight from Shoreham EGKA to Salamanca LESA, Spain, November 2014

This is a 1:1 detail from file-0025 (the "mystery runway") - http://peter-ftp.co.uk/screenshots/20141108163602390.png
(70mm focal length)

It's hard to say why but I just "know" this is nowhere near as sharp as I saw it with my own eyes at the time.

This is a 1:1 detail from file-0034 - http://peter-ftp.co.uk/screenshots/20141108163926468.png
(23mm focal length)

This is a 1:1 detail from file-0043 - http://peter-ftp.co.uk/screenshots/20141108164117281.png
(48mm focal length)

To be fair I don't have a 70mm ground based example from that trip, so it could be that the lens is simply very bad at 70mm but the reviews don't support such a degradation.

The airborne pics were taken through 4mm of what is basically a single sheet of polycarbonate.

What this suggests to me is that the camera focuses OK in LV but the window material confuses the normal focus mode. Only more tests will tell, and they will be easy enough to do.
Really enjoyed reading about your trip to Salamanca, was well written and the photos looked great to me, could see any real glaring problems with photos in the write up. Did not check the other links only the write up. Thank you for sharing. Cheers
11-08-2014, 03:25 PM   #14
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"The detail photo of the mystery runway appears to be in focus, but is soft. I suspect that camera motion may be contributing to that"

I have tried as fast as the camera will go (1/2500? - I don't have it handy) and it doesn't make any difference. Surprising, sure. But it doesn't, once you go past about 1/1500. I also have loads of really sharp pics done at say 1/400 F8 with the K5 (and I think the same 17-70 lens) which I am uploading here https://www.dropbox.com/sh/dga689r1dbv5rde/AACkxDvmhAOUr1X6nEJt-Op-a?dl=0 (give it an hour or two) and these prove that vibration or handholding is not an issue. Well, not with the K5, anyway.

The method I use to avoid vibration is to totally avoid the camera touching any part of the aircraft. I hand hold it, but using finger(s) against a window to support it. Also touching a window with it scratches the window and those windows are awfully expensive...

"There is also a fair amount of atmospheric haze due to scattering of blue light by water vapor, pollution, and/or smoke. All of your aerial shots show this haze. Scattered light is not "sharp" light. It is because of this scattering effect that B&W aerial photography film is preferentially sensitive to the red end of the spectrum with decreased blue sensitivity."

Yes - I struggle with this always. On that trip, all airborne (and most ground) shots were taken in raw, edited etc and then converted to jpeg in Lightroom. Haze is a tough one, not least because it is worse as you go higher (in elevation) in any photo because you are looking through more air. There are various variously involved techniques for reducing it; when shooting raw and wanting a quick hack I use the Blacks feature in Lightroom (ACDSEE PRO 7 has a similar one) and that is all I used in this case. Perfection is not going to be achieved... The way to minimise haze is to fly just after a really nasty cold front (preferably a hurricane) has moved through the area, but those chances are rare.

"Your eyes (coupled with your fantastic brain) are a better imaging tool than your camera under these conditions. You see detail...the camera does not."

I don't find that at all on the ground - the K3 way outperforms my eyesight. But maybe not in haze...

I think what I need to do is try to take airborne pics in clear-air conditions and at a low altitude and see if I can get sharp ones. Obviously I used to be able to get sharp ones in the past - see the above dropbox link. But those pics didn't involve haze - at least not in front of the stuff I was focusing on. There is haze before the distant ones.

It could simply be that I need to use live view in these hazy conditions...

Thanks for the input on the trip report
11-08-2014, 08:38 PM   #15
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I think the camera sees haze much better than our eyes. I don't know if it's sensitivity or our brain ignoring it or a function of magnification. I have many images of aircraft in focus but hazy. With that, magnification shows what our eyes simply cannot.
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