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Trigonometry focus


jbl

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I didn't know what else to title this post, sorry :).

 

If I focus on a something and then reposition the frame so the subject is not in the center of the frame, technically the plane of focus is now in front of the subject if it's in the left hand of the frame and behind the subject if it is in the right hand side of the frame.

 

Does anyone correct for this? I'm more curious than anything.

 

-jbl

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Hi,

 

Using film, I believe that at an aperture of 2, the depth of field covers those differences up, assuming that the image / focal planes are reasonably flat. Additionally the natural vignetting actually means that the depth of field is larger in the periphery of the image than in the center.

 

Photographing digitally with a Noctilux wide open and scrutinizing pixels on your monitor, I presume you may see the effect that you describe.

 

Rgds

 

Christoph

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I didn't know what else to title this post, sorry :).

 

If I focus on a something and then reposition the frame so the subject is not in the center of the frame, technically the plane of focus is now in front of the subject if it's in the left hand of the frame and behind the subject if it is in the right hand side of the frame.

 

Does anyone correct for this? I'm more curious than anything.

 

-jbl

The plane of focus would be behind the subject after recomposing wouldn't it? So you'd need to sway back sightly when using a very wide aperture close to the subject to keep the subject in the plane of focus. With a subject straight in front of you, imagine that the plane of focus is a large, flat board (ignoring curvature of field for convenience) with a rod from its centre to the centre of your lens. You focus by placing the 'board' in the plane of the subject's eyes, then swivelling at the hips either way to recompose will cause the board to 'travel' behind the subject's eyes won't it?

 

Pete.

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The problem with applying the theory to practice is that many lenses don't have a flat plane of focus. My favourite lens, the 35 Summilux, has a pretty barmy plane of focus that varies in shape considerably from aperture to aperture.

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