Sigma 17-50mm f2.8 OS EX on SD1 Merrill - Good and Bad

I understood what you were saying, you copy has high curvature of field as opposed to a flat field. This works fine for interior shots but not so well for infinity landscapes. So you have a specialty lens for very specific purposes.

Scottelly wrote:

mike earussi wrote:

rf-design wrote:

Scott I am not shure which 18-50mm you had used in the comparison. I remember that there exist three versions:

3.5-5.6/18-50mm DC HSM

2.8-4.5/18-50mm DC OS

2.8/18-50mm DC Macro

I onw all three in the past but only the last remain.

The color difference is possible because pretty old lenses are not support by the builtin corrections of the SD1M.

The 2.8/17-50mm EX DC OS HSM is definite defect. My lens show only chromatic aberrations on the edges but not defocus.

Reiner

Sample variation. The 17-50 I owned was sharp in the center at all focal lengths wide open but only at 50mm were the edges ever sharp at any aperture. At all other focal lengths the edges never became sharp no matter how far I stopped down the lens--it got returned, and I've never had an interest in owning another one.

I think maybe you two missed what I was saying. I could get focus right in the middle, and the edges would be blurred (because they are focused approximately the same distance from the lens, presumably), or I could focus a little farther away, and the edges would be in perfect focus, and the middle of the image would be out of focus. This proves that the lens is not faulty or bad, but that the plane of focus is not a plane at all, but a curve, and shaped like a bowl, with the focus point in each direction close to the same distance from the lens, rather than further away from the lens at the corners of the image. In order to have a flat plane of focus, the lens has to be focusing further away, at the edges of the image plane vs. in the middle. This is probably very difficult to achieve, and with my 17-50mm f2.8 OS EX that does not appear to be what is happening. I will shoot a photo of a room with lots of details in it, so you can see how the lens performs, when things on the left and right of middle are closer to the camera than my subject here shows. This subject is actually opposite to what one might normally be shooting, if they are shooing indoors (i.e. portraits in living rooms, architectural photos in hotels, model homes, or office building lobbies, cathedrals, car or boat interiors, or weddings in churches and wedding parties in banquet halls).

Here is a photo I shot that might show you that the lens actually is sharp in the corners (in the bottom right and left corners anyway):

I shot this at the beach the other day.

Settings used to produce the image above.

I should shoot a few of closer subjects at the beach, using wider apertures, so we can see how the the sand is sharply detailed in an arc shape, in focus futher away in the middle of the scene, and seemingly closer (though in probably about the same radius) at the edges of the photos.