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This is in the manual focus mode.

If I focus my lens so that a certain object is clear and crisp, and then do one of the following, will I have to focus again?

  1. Change the aperture
  2. Switch to a different lens that has the same focal length (such as switching from a 19mm prime to a 18-105 and zooming to 19mm)
  3. Change the focal length by zooming
  4. Change the focal length by switching to another lens

Will I need to re-focus? Am I correct in understanding that I have to re-focus in cases (3) and (4)?

Let's assume that the focal point is beyond the minimum focusing distance of the lens.

This is with reference to a Sony NEX-5R, with three lenses (in case it matters): a 19mm f/2.8 prime, a 35mm f/1.8 prime, and an 18-105 f/4 constant aperture zoom. I usually shoot in aperture priority mode.

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Focus is an adjustment to the lens, so any situation where you change the lens will require refocusing.

The other questions are a little more tricky.

Modern cameras (for the last 40 years or so) focus with the aperture wide open, regardless of what you have it set to, and then stop down the instant you take the shot. So, changing the setting doesn't affect focus. Some lenses, do change focus at different apertures, a phenomenon known as "Focus Shift". But, there isn't much to be done about that.

Modern lenses are generally designed so that focus is close to constant at all zoom lengths. These are called "parfocal lenses". Some lenses, however, do not have this property (they are "varifocal“) and require refocusing. Even with nominally parfocal lenses, this is rarely perfect and often very much not perfect, so you should refocus after zooming when focus is critical.

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    To deal with focus shift you basically have two options: Intentionally miss focus in the opposite direction of the known shift when focusing with the aperture wide open OR use the depth of field preview button when focusing manually. – Michael C May 31 '14 at 18:27
  • Third option — if you frequently shoot at that aperture and distance, and your camera supports it, use the AF microadjust to tune the AF for that situation. This may make it miss at other apertures which were previously fine, but if you shoot stopped down usually anyway, it might be the better compromise. – mattdm Feb 24 '15 at 18:44

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