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


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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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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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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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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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  • 3 weeks later...

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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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:

 

WorkShoesS.jpg

 

 

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://tonopahpictures.0catch.com/Devil/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://tonopahpictures.0catch.com/4InchGun...val4X5Pics.html

Edited by Christopher D. Keth
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