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Message: <blockquote data-quote="Dave_W" data-source="post: 109741" data-attributes="member: 9521">Well, the second issue is not coming from your lens, it's your body. Your camera will use an area surrounding your focus point in order to obtain a good focus and focus is dependent on contrast, which is very much lacking at night. This can be address by having a flashlight and illuminating the area you want the camera to focus on and then switching off your auto-focus once focus is obtained. The first issue is much more critical. There should not be any change in focus from wide open to stopped down. If anything, the focus plane should increase and be more inclusive rather than less. I suspect your lens may suffer from "focus shift" and that's generally a function of the lens design and not something I believe they can fix. The hall marks of Focus Shift is a lens that is sharper wide open than stopped down. And in fact will tend to back focus when stopped down. Here's a snipped on focus shift from Nisam MansurovSince cameras acquire focus with lenses at their maximum aperture (in this case f/1.8), stopping down the lens to f/4-f/5.6 will put your focused subject slightly out of focus, moving the focus plane back – result of focus shift. In addition, the area that appears sharp in the center will not distribute evenly across the frame – result of field curvature. This is generally not a huge problem for landscape photography (most of these issues are gone by f/8-f/11), but could definitely be problematic for other uses.Focus shift explanation <a href="http://photographylife.com/what-is-focus-shift" target="_blank">here</a></blockquote>

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