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Archive 2015 · VERY odd depth of field issue...

  
 
jmai86
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p.1 #1 · p.1 #1 · VERY odd depth of field issue...


I've been out shooting with my new Nikon 16mm fish and while it's a great piece of kit, I have noticed one very strange issue. Shooting at or near wide open, up close to subjects, the depth of field goes from in focus at the focus point, to a normal out of focus transition, and then BACK to sharp focus on the edges of the frame!

Attached is an example from a 16x9 crop, at F2.8.

Notice on the left side of the frame, the scene in the background become sharper as it gets closer to the edge! The buildings were parallel and straight, and did not curve towards me at all, and were actually some distance away (the man was a few feet away and is pleasantly out of focus). This effect is somewhat troubling as it makes the image look like I added an artificial background blur. The effect isn't noticeable at F4 or higher though.

Anyone else experience this with rectangular fisheyes in general?







Jul 07, 2015 at 09:22 PM
form
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p.1 #2 · p.1 #2 · VERY odd depth of field issue...


That's very odd, normally I would jump to say decentered element especially considering the jittery nature of the edges, but it's pretty consistent looking on both sides of the frame. Maybe decentering behaves differently with fisheyes...


Jul 08, 2015 at 08:36 AM
panos.v
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p.1 #3 · p.1 #3 · VERY odd depth of field issue...


Field curvature? Bit extreme but then that's a fisheye...?


Jul 08, 2015 at 08:56 AM
Jason_Brook
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p.1 #4 · p.1 #4 · VERY odd depth of field issue...


I thought fisheye's focal planes curved from MFD to infinity just like what we're seeing here?


Jul 08, 2015 at 08:57 AM
MalbikEndar
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p.1 #5 · p.1 #5 · VERY odd depth of field issue...


I have not seen this effect with the 10.5 mm




Jul 08, 2015 at 11:47 AM
Bernie
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p.1 #6 · p.1 #6 · VERY odd depth of field issue...


Could it be one of Nikon's innovations?

The Close-Range Correction (CRC) system is one of Nikon's most important focusing innovations, for it provides superior picture quality at close focusing distances and increases the focusing range.



Jul 08, 2015 at 12:23 PM
jtra
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p.1 #7 · p.1 #7 · VERY odd depth of field issue...


At first it has looked like field curvature. But closer look implies something different to me.

If you look at the wire near bottom right corner - it is completely in focus. Field curvature (implied by what it visible in left side of the picture) would make it out of focus (astigmatism would defocus it too, in one direction) and only background of bank and sea would be in focus.

I have following theory:
Lenses at their widest aperture often have a cat's eye bokeh. You can see it here (top row):
http://jtra.cz/stuff/essays/bokeh/drawing5018g-trace.png
It shows how "lemon" shape or cat's eye bokeh looks and how it is formed.

Now this could happen in your picture too, like I have drawn here:
http://i.imgur.com/KckX83C.png

So sides have more depth of field, but only in one direction. I can see bokeh like that in your picture.

Look at my page on bokeh: http://jtra.cz/stuff/essays/bokeh/index.html



Jul 08, 2015 at 03:23 PM





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