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Need advice on focus

km6xz

St Petersburg, RU
3574 posts

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"RE: Need advice on focus"

km6xz Moderator Awarded for his in-depth knowledge in various areas, including Portraits and Urban Photography Nikonian since 22nd Jan 2009
Fri 19-Oct-12 03:32 PM

Joseph is right, if you did not know what was beak and what was grass, and think like a camera, you would expect the subject to the what fills most of the focusing point. The camera does not know that a similarly colored beak is not part of the overall scene found under the focus point. Consider also that the focus point for sensitivity is larger than the viewfinder box indicating the focus point. The box is the center, not the limits of the focus point.
For best focusing with AF, give the focus sensor the least ambiguous target, something with distinct color and contrast borders that are not easily confused with the background. That is tricky to provide sometimes, so look for something that looks to be on the same focal plane as the area you want sharp and focus on that. If not too far from the desired subject for focusing, field curvature of lens will not be much of a factor(in other words don't pick a target far to either side of the subject because only some lenses have flat fields, the 18-200 does not) and by focusing on a better less ambiguous target that appears to be on the same plane as the subject the subject should be in focus also.
From the size of the red focus point box, these are still cropped too much for much resolution. Manual focus is the main option when a target is not a good focus target for phase detection AF. Can you post uncropped versions of these images?
I second the recommendation to get a better, longer lens. The 180 2.8 is an excellent affordable lens that will AF better because not only is it far better optically than a 18-200 but since it is a f/2.8 lens, a lot more light is available for the focus sensor and for manual focusing in the VF. The AF and metering occurs with the lens wide open and it stops down instantly after pressing the shutter release. So there is a big difference in contrast, color and light level information available for the focusing sensors with a f/2.8 lens regardless of the f-stop of the actual exposure which in your case was f/11.
The other suggestion, for a little less, but also less sensitive to light, is the popular 70-300. It is better at 300mm than the 18-200 is at 200. Neither gives the advantage of wide base minimum apertures that helps focus so much.
Stan
St Petersburg Russia

Visit my Nikonians gallery.

This is a hot, active topic! Need advice on focus [View all] , burchan Silver Member , Wed 17-Oct-12 10:00 PM
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