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Message: <blockquote data-quote="Woodyg3" data-source="post: 669198" data-attributes="member: 24569">I was wrong. <img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7" class="smilie smilie--sprite smilie--sprite1" alt=":)" title="Smile    :)" loading="lazy" data-shortname=":)" />So I took both the 100-400 and 150-600 outside and tried focusing at the extreme edges on the D500 in single point mode at max focal length. You are right, it will not focus. Why? Because these focus points are not able to get enough light at f/6.3, I assume. It is very usual to have the center focus points better able to focus in lower light than the ones on the edge, so this isn't too surprising. With my Nikkor f/4 and a 1.4X TC, effectively f/5.6, these focus points work fine. That tells me that the focus sensors on the edges have a "break point" beyond f/5.6. That seems to be indicated by the chart you posted in the first post. Lenses that Nikon makes and that are normally used on the D500 are only slower than f/5.6 if used with a teleconverter, so this limitation seems pretty reasonable within the Nikon product range. I suspect that ANY lens put on the D500 with a minimum f-stop smaller than f/5.6 will have this problem. I don't think the manufacturer or any kind of firmware has anything to do with it. The fact that I have used both lenses a good deal and never noticed this issue tells me that it is not a limitation in my photography. Obviously, it might well be a problem for others. To sum it up, I suspect ANY lens that is slower than f/5.6 is going to encounter single point focusing problems on the extreme sedges with the D500.</blockquote>

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