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Archive 2017 · Lens Distortion? Help me understand the finer points...

  
 
cdgerston
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p.1 #1 · p.1 #1 · Lens Distortion? Help me understand the finer points...


With many years of reading lens reports and forums like this one, I have a vague understanding of terms such as "Decentering" "Field Curvature" "Wave and Mustache Distortion" and etc...But I'm not sure I could spot any of them and confidently label them in a picture. But, I do sometimes see mysterious (to me) distortions or areas of unsharpness I can't explain. This photo is an example, and I'm hoping someone can help me understand what I'm seeing. I have plenty of experience shooting, and know what I'm doing with apertures and depth of field (but not always lighting with flash, which I was experimenting with in this photo...save that criticism for another time :-) ).

I've attached a fairly large jpg here in hopes that people can see what I'm talking about. But, below is a dropbox link to a full size jpg to see it closer.


I'm shooting the Fuji 23 f/2 here at f/4 on the X-Pro2. Focus is on the smallest blond girl in the middle of the couch. I judged that to be deep enough DOF to get everyone's face in focus, and sure enough, I've got acceptable focus, not only on the people on the couch, but those a little closer on the bean bag chairs and those in the back row standing. In fact, if you look at 100%, even the books in the shelves at the back are sharp. I figure that as I get closer to the edges, and as a subject is closer to the camera, away from the plane of focus, I might have some problems: and sure enough, the boy on the beanbag chair on the left is a little softer, but I would judge it good enough. The part of the photo that mystifies me is the girl standing all the way to the right, with the striped shirt. She's clearly not in focus (or smeared)...really soft/distorted. For the sake of this photo and what it'll be used for, I can live with it, but I can't explain it. Why would that zone, on the same plane as the girl I focused on, and as close to the edge as the boy on the beanbag chair to the left...why would she be so soft? I can't really find another spot in the photo that looks so off. Is this what decentering looks like? Is it field curvature, and if so, why just that spot? And if it is, wouldn't DOF deep enough to include the books in the background keep her in focus as well? This is beyond my ability to explain.

I figure some of you really know how to identify this kind of distortion or defect, and I'd love some input. I'm otherwise pretty happy with this lens, and I know all lenses have their idiosyncrasies. I just want to learn how to spot them better and hopefully avoid them in the future.

Thanks, Curt

Photo Full size







Jun 15, 2017 at 07:27 PM
JohnBrose
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p.1 #2 · p.1 #2 · Lens Distortion? Help me understand the finer points...


It could be that she moved and you have some motion blur. I typically test my new lenses shooting a brick building with camera on a tripod and manually focusing and self timer etc. This will show me how the particular lens does with most of the types of distortion/lens issues that you mention.


Jun 15, 2017 at 07:45 PM





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