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I have my Nikon D7500 set to Manual Mode. I can change the shutter speed and it maintains the setting. When I change the aperture, the setting is not maintained and varies depending on the zoom of the lens. How do I stop the aperture from changing when in Manual Mode?

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Many lower-cost (or even more expensive but high zoom-ratio) lenses do not have a constant maximum aperture. They are constructed so that they have as wide as possible aperture when zoomed out, but the design compromises in their construction mean they are limited to narrower apertures when zoomed in.

I expect that's what's happening here: you have an aperture set which your lens provides at a wider zoom but which is not physically available when zoomed in, so it's doing the best it can.

If this is the case, your lens will have a designation like 18-140mm f/3.5-5.6G. The "G" is a Nikon thing (and irrelevant here), but the other numbers are typical for all makes of lens. The first is the range of focal lengths in millimeters, and the second, f/3.5 to f/5.6, is the maximum available focal length at each end of the range. In between, of course, the limits will be somewhere... in between.

For more on this, see Why do zoom lenses and compact cameras have varied maximum aperture across the zoom range?

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  • \$\begingroup\$ This would be definitively the case if your lens says something like e.g. 100-400mm f/4-5.6 on it - that indicates that f/4 is possible at the 100mm end, but only f/5.6 at the 400mm end. Where the change points along the zoom are highly is dependent on exactly what lens you have, though... \$\endgroup\$
    – twalberg
    Jan 5, 2020 at 19:03
  • \$\begingroup\$ @twalberg Good idea -- I'll add that to my answer. \$\endgroup\$
    – mattdm
    Jan 5, 2020 at 19:05
  • \$\begingroup\$ My lens is 18-300mm f3.5-6.3G. \$\endgroup\$
    – lestat1978
    Jan 5, 2020 at 21:54
  • \$\begingroup\$ @lestat1978 That settles it — 3.5 to 5.6 \$\endgroup\$
    – mattdm
    Jan 5, 2020 at 21:57
  • \$\begingroup\$ (Sorry, 6.3, in your case.) \$\endgroup\$
    – mattdm
    Jan 6, 2020 at 0:44

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