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Samyang 14mm f2.8/t3.1 : edge issues ?

Started Jul 29, 2013 | Discussions thread
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Michael Siemon
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Re: Samyang 14mm f2.8/t3.1 : edge issues ?
In reply to ethern1ty, Aug 3, 2013

ethern1ty wrote:

Michael Siemon wrote:

Are you allowing for the geometry of the 104 degree field of view for 14mm on FX? This implies that if the buildings are along or close to the same horizontal plane, the edges will be about 35% further away from the lens than the center. You need to do some adjustments (e.g. hyperfocal focusing) -- don't focus precisely on the center, but up to halfway between that and the buildings at the edge of the view.

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Michael L. Siemon

The building are at 30m+... Sorry I don't get it. Are you saying that we can't get to hyperfocal with a 14mm ?

Think of a triangle with your camera at the vertex and the row of buildings as the base. For a 14mm lens, the angle at the vertex is about 104 degrees. A line directly from your camera to the center of the line of buildings splits that triangle into two right triangles. The ratio of the distance to the center to the distance to the edge of that line of buildings is the cosine of 90-52 degrees -- about 0.67; or reciprocally, the distance to the edge buildings is 1/0.67 = 1.49 times the distance to the center. In other words, if the focus point is 30 meters from your position, the edge of the frame (again, assuming the buildings are in a roughly straight line perpendicular to your position) is about 45 meters.

However, I just pulled out a depth of field calculator (iPhone app...), and it suggests that at 30 meters, it really doesn't make any difference -- all the buildings should be roughly equally in focus. Nominally, even at f/2.8, everything from 7 feet on "should" be in acceptable focus. So I mostly just introduced a red herring.

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Michael L. Siemon

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