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FORUMS Photography Talk by Genre General Photography Talk 
Thread started 10 Feb 2013 (Sunday) 02:49
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DoF - straight or curved?

 
Bend ­ The ­ Light
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Feb 10, 2013 02:49 |  #1

I was just looking through some old images, and this one taken with a MF Vivitar 90mm macro, at 1:1.
I noticed the thin "focal plane" is curved.
Is this right, and the focal plane is actually curved, or is it distortion which is not so evident in other lenses or something?

IMAGE: http://farm7.staticflickr.com/6062/6066094063_4a0a93cac5_o.jpg
IMAGE LINK: http://www.flickr.com …end_the_light/6​066094063/  (external link)
Unidentified dragonfly at Ynis-Hir 2 (external link) by http://bendthelight.me​.uk (external link), on Flickr

Thanks.



  
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Kolor-Pikker
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Feb 10, 2013 04:35 |  #2

Yep that's field curvature right there, all lenses have this to some degree, but it usually becomes obvious at macro distances. Macro lenses are typically corrected for this though.

Another example of field curvature is with some wide angles, if you focus the center to infinity, the sides will actually be focused closer.


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Bend ­ The ­ Light
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Feb 10, 2013 04:52 |  #3

Kolor-Pikker wrote in post #15593500 (external link)
Yep that's field curvature right there, all lenses have this to some degree, but it usually becomes obvious at macro distances. Macro lenses are typically corrected for this though.

Another example of field curvature is with some wide angles, if you focus the center to infinity, the sides will actually be focused closer.

Thanks. :)




  
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cptrios
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Feb 10, 2013 19:32 |  #4

Well if that's a flat board, it's definitely field curvature. But are you sure that's not a log the fly is sitting on? It looks rounded to me, which would explain the "curved" focal plane.


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OhLook
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Feb 10, 2013 20:20 |  #5

cptrios wrote in post #15595991 (external link)
Well if that's a flat board, it's definitely field curvature. But are you sure that's not a log the fly is sitting on?

Good point. In the photos just before it at Flickr, the wood looks like a branch.


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Bend ­ The ­ Light
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Feb 11, 2013 00:26 |  #6

cptrios wrote in post #15595991 (external link)
Well if that's a flat board, it's definitely field curvature. But are you sure that's not a log the fly is sitting on? It looks rounded to me, which would explain the "curved" focal plane.

OhLook wrote in post #15596190 (external link)
Good point. In the photos just before it at Flickr, the wood looks like a branch.

The wood is part of a fence along the edge of a wooden walkway through the wetland part of the RSPB reserve at Ynis-Hir, in Wales. It is flat. Here is another shot on the same wood:

IMAGE: http://farm7.staticflickr.com/6074/6066648068_c55c566470_o.jpg
IMAGE LINK: http://www.flickr.com …end_the_light/6​066648068/  (external link)
Unidentified dragonfly at Ynis-Hir 5 (external link) by http://bendthelight.me​.uk (external link), on Flickr

It does seem that some lenses have "field curvature" and that attempts are made (with more expensive lenses) to eliminate this.

So, since the focal plane is not always a plane, it needs another name. So I decided I would call it the "Focus Locus". :)



  
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samsen
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Feb 11, 2013 00:40 |  #7

Nice.


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20droger
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Feb 11, 2013 01:07 as a reply to  @ samsen's post |  #8

It is a plane, assuming you're using spherical geometry.

Plane triangles have exactly 180° as the sum of their vertices, just like you learned in high school. Triangles on the surface of the Earth, however, always have more than 180° degrees. And triangles on the surface of a saddle always have less than 180°.

Geometry is fun, fun, fun when you leave Euclid at home!




  
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Bend ­ The ­ Light
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Feb 11, 2013 06:38 |  #9

20droger wrote in post #15596996 (external link)
It is a plane, assuming you're using spherical geometry.

Plane triangles have exactly 180° as the sum of their vertices, just like you learned in high school. Triangles on the surface of the Earth, however, always have more than 180° degrees. And triangles on the surface of a saddle always have less than 180°.

Geometry is fun, fun, fun when you leave Euclid at home!

Oh, I know that. I am a maths teacher, so spherical triangles and the like are great fun! :)
But how many are using that when they talk of the focal plane? And are they expecting a flat Euclidean plane and getting non-Euclidean?

:)




  
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Kolor-Pikker
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Feb 11, 2013 06:54 |  #10

At first I also thought that the surface might have been curved, but the wood surface shows that this was shot at an angle, for the plane to be curved like that you would have had to shoot straight-on to a cylindrical surface.

Field curvature is fairly annoying as far as MTF figures are concerned, because there's no way to tell whether a low score is acquired via lack of resolution or out-of-focus softness due to this phenomenon. In practice, some lenses may be sharper than graphs imply, because field curvature is less of an annoyance when used on real 3-dimensional subjects instead of a flat plane. In the dragonfly photo, a chart would have shown horrible performance past the center, when it's clearly sharp to the corners (albeit curved corners).


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DoF - straight or curved?
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