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On the Sony A6400, when using the "S" mode - Shutter Priority - I am expecting the F-number to go down (larger aperture) as it gets darker outside.

However, the quirk is that the aperture closes as it gets darker. I'm trying to do a timelapse starting at about 7:30pm when it is still light outside that should go on until about 9:30pm.

I can do this just fine in manual mode but I was thinking that Shutter Priority can help me speed things up a bit and not have to fiddle with the camera in the middle of the timelapse. Also processing would be a lot easier with frames taken in S-mode.

Settings:

  • Mode: S (Shutter Priority)
  • Shutter speed: 8s
  • ISO: 800
  • Interval shooting: on, 10 seconds.
  • Metering mode: Tested with Multi and Full frame average, but I think all have the same result.

I'm expecting the aperture to start high and go down to f/1.4 and stay there until I stop the interval shooting.

It goes from f/2.8 (7:30pm) step by step to f/16 and stays at 16. Each frame it adds another 1/2 stop (1 step)

Lenses:

  • Sigma 20mm F1.4 DG DN
  • Sigma 50mm F1.4 DG HSM

Video of the issue: https://youtu.be/Gyo8oZdt9Bg

I am able to reproduce this at home too, as follows:

  1. Go to a room where you have drapes that you can cover the windows with. Start with drapes fully open.

  2. Point the camera at a piece of furniture or something far from the window. Set:

    • Mode: S
    • ISO: 400
    • Shutter: as many seconds as needed to give you an f/8.
    • Metering: I tried Multi and Full screen average, both fail.

  3. Start interval shooting, making sure the interval is more than the shutter speed, plus 3-4 seconds is enough to let you see the settings on the screen before the each frame.

  4. Confirm that 3-4 frames are done at the same settings and the camera doesn't just jump around

  5. Cover 1/4 of the windows.

    • Expected result: Aperture goes from f/8 to f/5.6 or something
    • Actual result: Aperture goes to f/9

  6. Cover 1/2 of the windows.

    • Expected result: Aperture opens up.
    • Actual result: Aperture closes even more, f/10.

You can go further by covering more of the window and observing the aperture change.

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  • \$\begingroup\$ Is any type of "safety shift" mode enabled? Safety shift allows the camera to override the manually entered settings to get a "proper" exposure. Are the values recorded in the EXIF info for shutter duration and ISO what were manually selected or something different? \$\endgroup\$
    – Michael C
    Commented Oct 18 at 6:04
  • \$\begingroup\$ I don't know of such a setting. In manual mode everything works fine. The only thing that changes in the S mode is the aperture. However, the S-mode works fine on the A7IV, only a small issue there - I get a frame at f/1.6 then the next one at f/1.8, then another one at f/1.6. It's ok though, I can fix that in DaVinci Resolve. \$\endgroup\$
    – Astro Darius
    Commented Oct 21 at 23:20

1 Answer 1

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That is an odd behavior... most Sony's lock the aperture from the first interval exposure in S priority mode. Interval shooting isn't particularly well implemented in Sony cameras IMO.

The best method for transitional timelapses with Sony's is to use manual mode and auto ISO... unless something (firmware) has changed in the last year or so regarding interval shooting that I'm not aware of.

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  • \$\begingroup\$ Interesting, but I'm not seeing that behavior on the A7IV. On that one, the aperture opens up as expected. The only small issue is that after a while going down from f/8, I get frames at f/1.6, then f/1.8, then f1/6 again, so it oscillates for a while, even if the scene doesn't change. However in my use case I think this can be fixed by choosing "full frame average" as the metering mode. Either way, I can fix it in DaVinci Resolve too, no problem with the A7IV. \$\endgroup\$
    – Astro Darius
    Commented Oct 21 at 23:25

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