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The plane that is in focus is an arc and not a straight line?


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#1 weiming

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Posted 19 July 2006 - 09:46 AM

Hi all,

i was browsing through cinematography.net and read that; let's say u focus a camera at a certain distance, the area that is being focused on exists as an arc around the camera.

i always thought it was a straight plane. anyone have any ideas?

thank u very much:)
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#2 Stephen Williams

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Posted 19 July 2006 - 10:33 AM

Hi all,

i was browsing through cinematography.net and read that; let's say u focus a camera at a certain distance, the area that is being focused on exists as an arc around the camera.

i always thought it was a straight plane. anyone have any ideas?

thank u very much:)


Hi,

When testing a lens the lens chart is always flat.
Why do you ask? Are you thinking about focus pulling ?

The only time it could be an issue is with a wide angle lens, by the time arc is big enough to worry about it the DOF will be large enough to forget about it!

Just my 2c

Stephen
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#3 Brad Grimmett

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Posted 19 July 2006 - 04:46 PM

This is mostly true with anamorphic lenses. I've never heard of it being a problem with spherical lenses.
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#4 Max Jacoby

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Posted 19 July 2006 - 05:29 PM

That entirely depends which lens you are using. Different manufactures have different design philosophies and this is one of the elements that contributes to a lense's look. Cooke lenses have a slightly curved plane of focus, while Zeiss lenses are flat. The human eye see in a slightly curved way as well, hence one of the reasons while people find Cooke lenses to look more organic. Hawk anamorphic lenses go more in direction of the Cookes as well.

This is more of an issue with wide angle lenses and also has implications of focuspulling. If someone is on the edge of the frame one needs to take this property into consideration when shooting on a Zeiss lens.
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#5 weiming

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Posted 20 July 2006 - 11:11 AM

Hi,

When testing a lens the lens chart is always flat.
Why do you ask? Are you thinking about focus pulling ?

The only time it could be an issue is with a wide angle lens, by the time arc is big enough to worry about it the DOF will be large enough to forget about it!

Just my 2c

Stephen


hmm... at first i was just curious to know whether this was true or not; but now that u mention it, i think it would affect focus pulling if u did not apply this 'rule'

then again, after reading the other posts, can i safely say that for most of the time, u need not worry about this arc thingy? except maybe when using wide angle lenses?

thanks:)
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#6 Nick Mulder

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Posted 20 July 2006 - 02:43 PM

can i safely say that for most of the time, u need not worry about this arc thingy? except maybe when using wide angle lenses?


and macro
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#7 Joe Anderson

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Posted 07 August 2006 - 02:49 PM

This is an aberration phenomenon known as curvature of field. Image fields are naturally round shaped with respect to the optical center of the lens. This field (called the Petzval surface) can be flattened by a 'corrected' lens that has a negative converging, or convex, lens element in it's design.

It is my understanding that this curvature is more an issue with lenses of higher magnification. The term 'telephoto' was originally reserved for longer lenses that contained this convex element. So wide angle lenses are not as much at risk.

JANDY.
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#8 Chris Keth

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Posted 07 August 2006 - 06:03 PM

Also, just for clarity: If the plane of focus isn't flat (i.e. corrected) it is not an arc, but part of the surface of a sphere. Lens design deals with all three dimensions.

Something else to add but too late to edit:

The petzval surface is actually also a design of lens that has been around for about 150 years. A few modern companies still use variations of it quite a bit. To illustrate the look of a radical curvature of field, check this photo out. It's made with a view camera and a quite old Petzval lens by Jim Galli:

Posted Image


Large format photography actually holds petzval lenses quite in vogue and they're quite popular. Jim has several more shots with that lens at http://tonopahpictur...evil/Devil.html

Here's another page on Jim's site that shows even more radical curvature of field from an old 4 inch gundlach lens: http://tonopahpictur...val4X5Pics.html

Edited by Christopher D. Keth, 07 August 2006 - 06:03 PM.

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