So I started thinking (first mistake) and was wondering if the focus plane on a wide angle (say 12-24 f4) is a straight line or curved to match the outer glass?
Basically if I shoot something that is straight and I'm perpendicular to the camera and focused center should I see a slight blur as I look left or right?
I realize that the image will soften a bit but isn't that do to the sweet spot of the glass?
Is my real focus plane the tangent to the curve of the outer glass at the point I focus at?
It depends on the lens design. Some lenses are designed for "flat field" performance, but most lenses have curvature in the plane of critical focus. The curvature is not dictated by the shape of the front element, rather it's determined by all of the refractive lens elements that the light passes through on the way to the sensor or film. The normal reduction in image sharpness from the centre to the edges and corners is often caused by both optical performance (i.e. sharper in the center) and by plane of focus curvature.
I want to assume that the plane of best focus is a flat plane parallel to the film plane. However, as I've not thought about it or tested it, this would be an excellent item to test.
Though, considering there have been a million and one brick wall tests, it should be fairly easy to look at it.
If you feel compelled, you can always try to test it yourself and it shouldn't be a particularly difficult test either.
Thanks for the responses....I was going to do a test (a la brick wall) but didn't know how to differentiate between the sweet spot being sharper verses the focal plane curve.
jcolwell wrote:
It depends on the lens design. Some lenses are designed for "flat field" performance, but most lenses have curvature in the plane of critical focus. The curvature is not dictated by the shape of the front element, rather it's determined by all of the refractive lens elements that the light passes through on the way to the sensor or film. The normal reduction in image sharpness from the centre to the edges and corners is often caused by both optical performance (i.e. sharper in the center) and by plane of focus curvature.
How can I isolate one? fwiw I'm playing with a tokina 12-24. Is there a lens spec that helps here?
