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I have a Canon EOS 2000D camera with Canon EF 50mm f/1.8 lens that can achieve shallow depth of field. If I adjust the aperture settings of the lens on the camera, I'm not seeing any change of depth of field on the LCD preview or the traditional viewfinder. However, after I have taken a picture with large f-number (small aperture), the depth of field will be deep in the JPG/RAW file although I didn't see the deep depth of field on the LCD preview or viewfinder.

Now, I understand that my eye has also autofocusing feature, so if I look at the image I'm going to take through the traditional viewfinder, the depth of field will be deeper than it would be when looking at the LCD preview. The eye autofocus is messing with the camera's focus.

But why can't I see any change of depth of field on the LCD preview? I'm using the "AF quick" mode that flips the mirror, uses phase detect autofocus and flips the mirror again, so the LCD preview goes black when focusing. It's phase detect autofocus (PDAF) if I understand correctly.

Is the camera only changing the aperture when I'm actually taking the image?

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    There are several existing questions here that point out that most cameras do metering and AF with the lens wide open and only stops the lens down to the selected aperture the instant before the shutter curtains open. – Michael C Feb 9 at 10:53
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Ok, now I found how to do what I want to do. The camera instruction manual has instructions for depth of field preview.

  1. Enable depth of field preview to be used
    • Under "3: custom functions", set "9: assign SET button" to "4: depth of field preview"
    • For details, see page 258
  2. Exit the menu
  3. Press the SET button, works both with optical viewfinder and with the LCD preview

Apparently, the camera is using the maximum aperture before taking the picture. The depth of field preview with small aperture (large f-number) will darken the optical viewfinder, of course, as the smaller aperture allows less light.

There is no separate depth of field preview button on this camera, so the SET button has to be assigned for this purpose.

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