Strange OOF issue

[todd_caudle]

<p>Hi Gang,<br>

I'm posting this on photo.net because I think this forum has the most technically savvy visitors. And now that I've buttered you up...<br>

I shot this morning out at Garden of the Gods, where it was an amazing 14 below air temp when I started, and conditions were quite extraordinary. An hour after sunrise some clouds started to form, making for a very dynamic scene. I've shot from this overlook many times, but this was some of the best conditions I've ever encountered here.<br>

When I got home and started looking through the images, to my horror, I noticed that I was properly focused on the red rock ridge, but that the distant mountains looked totally out of focus. This seemed quite unlikely to me, as I was shooting my 70-200 f4L lens at f14, which I've found to be its sweet spot, and the point of focus (the red rock) was a smidgen over half a mile away as the crow flies. I've been doing this long enough to know that if I focus on something roughly 2700 feet away and stop the lens down at all, it should focus to infinity. Still, I went to an online DOF calculator, punched in the details of the shot posted here (5D Mk-2, 70-200 @ 150mm, 1/20th sec., f14, iso100, etc.), which confirmed what I thought. The shot should be in acceptable focus from 162.3 feet to infinity (and BEYOND!), yet here the ridge was nice a crisp, but the mountains were fuzzy.<br>

I'll follow this with a crop to demonstrate.<img src="http://www.toddcaudle.com/focus-test-1.jpg" alt="" /></p>

[todd_caudle]

<p>And here's a crop from the image. Note that the OOF appearance goes all the way up to the summit of Pikes Peak, and side to side. The length of the ridge is sharp. I'm beginning to think it's an atmospheric effect from the ice crystals and light fog obscuring or defracting the light. Anyone else experienced something like this?<img src="http://www.toddcaudle.com/focus-test-1-inset.jpg" alt="" width="700" height="479" /></p>

[hawkman]

<p>Todd,<br /> Focus seems to be good this is an atmospheric effect, cold temperatures cause condensation in the air which changes the index of refraction in the path that light needs to reach you from the distant peaks. I have seen this in telephoto landscapes many many times. Not much you can do about it, try to process the image and pop the contrast a bit, image is still nice.</p>

[bobatkins]

<p>You're shooting though several miles of turbulant air which tends to blur out detail.</p>

[tudor_apmadoc]

<p>I agree, it's not a focus issue but atmospherics. The visible part of the clouds is the only visible part of the level of mositure in the air. That's what's causing the sharpness issue. Think of it as stages.... </p>

<ul>

<li>very dry, crisp clear air</li>

<li>humid air</li>

<li>fog</li>

<li>clouds</li>

</ul>

<p>The longer the distance between you and the subject (the distant mountains in the case) the more the moisture in the air can affect how sharp your photo will be.</p>

<p>Same thing with dust in the air. </p>

<p>Think of it like shooting through a single pane of glass - very clear, no problems. Now stack 25 layers of glass together, not so clear..</p>

[paddler4]

<p>Yup. I shoot mostly in the northeast, where humidity is higher, and this kind of thing happens much of the time.</p>

<p>One PP trick that was recently suggested to me is a weird sharpening adjustment that is in effect a strong local contrast increase: set the threshold (or clipping, depending the software you use) to 0 or at most 1, set the radius to a very high level, such as 30-35, and try an amount like 40-50. You'd have to play with the values, and you might not want to do this to the already clear areas, such as the sky. Someone did this to a shot I took in the Adirondacks, and it helped a good bit. I think he got the suggestion from Tim Grey's newsletter.</p>

[john_a5]

<p>I thought it was pretty obviously atmosphere as soon as I looked at the first crop and think there is enough throughout the scene behind the main subject that I wouldn't expect sharp results in any of it. I might also add that I see in shots on the 1dsmkIII that critical focus/detail suffers when things like trees, small details, are pretty far off. So, what you notice is atmospheric on top of a softer detail inherent in the process.</p>

[bjscharp]

Isn't it great to live in a time where a man can shoot a picture in a place like that in the morning and be online the SAME AFTERNOON to talk about it?

 

Just sayin'.... ;-)

[g dan mitchell]

<p>The vast amount of air between the red rock and the distant mountain will also cause "blur" - can't blame everything on the lens! :-)</p>

<p>Dan</p>

[todd_caudle]

<p>Thanks all. I guess I was so shocked because I shoot in conditions that at least <em>look</em> like this as often as I can find them. Apparently the atmospheric conditions were just different from what I'm used to.</p>

<p>And B.J., yup, it is an amazing modern world we live in!</p>

[Robin Smith]

<p>Todd</p>

<p>I agree with the others. Also remember that diffraction with small aperture openings will tend to limit detail (which one naturally tends to look for in distant objects) and also that an object simply "within the depth of field" will not appear as sharp as that object absolutely on the plane of focus. You might actually have got a more punchy shot by shooting at f8.</p>