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Author Topic: Focal plane and focus  (Read 1707 times)

thepancakeman

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Focal plane and focus
« on: December 19, 2011, 04:29:35 PM »
This has come up in a few different threads, and I thought it probably deserved its own.

Am I correct in understanding that the focus is actually to a plane and not a sphere/distance? 

So if I'm taking a picture of a big group of people but they're all lined up along a wall perpindicular to me, they'll all be in focus even wide open?  And conversely, if there are a buch of items equidistant from the lens, most will be out of focus except for the one(s) that defined the plane of focus?   ???

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Focal plane and focus
« on: December 19, 2011, 04:29:35 PM »

neuroanatomist

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Re: Focal plane and focus
« Reply #1 on: December 19, 2011, 04:33:25 PM »
This has come up in a few different threads, and I thought it probably deserved its own.

Am I correct in understanding that the focus is actually to a plane and not a sphere/distance? 

So if I'm taking a picture of a big group of people but they're all lined up along a wall perpindicular to me, they'll all be in focus even wide open?  And conversely, if there are a buch of items equidistant from the lens, most will be out of focus except for the one(s) that defined the plane of focus?   ???

That is how lenses are designed.  How well they actually achieve that design goal varies by lens - when they fail to have a planar focus, it's called field curvature (an issue that the 24-70mm f/2.8L reportedly suffers from, for example).
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Mt Spokane Photography

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Re: Focal plane and focus
« Reply #2 on: December 19, 2011, 06:30:53 PM »
This has come up in a few different threads, and I thought it probably deserved its own.

Am I correct in understanding that the focus is actually to a plane and not a sphere/distance? 

So if I'm taking a picture of a big group of people but they're all lined up along a wall perpindicular to me, they'll all be in focus even wide open?  And conversely, if there are a buch of items equidistant from the lens, most will be out of focus except for the one(s) that defined the plane of focus?   ???

Good answer.  Thats why focus and recompose doesn't work for a shallow depth of field, the distance to the plane is longer at the edges than at the center, so focus will be off and hopefully hidden in the depth of field.

That is how lenses are designed.  How well they actually achieve that design goal varies by lens - when they fail to have a planar focus, it's called field curvature (an issue that the 24-70mm f/2.8L reportedly suffers from, for example).

neuroanatomist

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Re: Focal plane and focus
« Reply #3 on: December 19, 2011, 06:53:24 PM »
Thats why focus and recompose doesn't work for a shallow depth of field, the distance to the plane is longer at the edges than at the center, so focus will be off and hopefully hidden in the depth of field.

Excatly.  If I just set my 85L to f/8, it wouldn't be a problem to use the center AF point of the 5DII and recompose...but that's not why I bought an f/1.2 lens.  Unfortunately, the outer AF points of the 5DII are so tightly clustered in the vertical dimension that there's not going to be a point where I want it, even if they were any good.
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bvukich

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Re: Focal plane and focus
« Reply #4 on: December 19, 2011, 09:42:08 PM »
NA&BS
Chill, or the thread gets locked, and you both get a temp ban to cool off.

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Re: Focal plane and focus
« Reply #4 on: December 19, 2011, 09:42:08 PM »