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Using a 16-85 3.5-5.6 lens. Taking a head and shoulders video at 1/60 and f5.6 in mode "M". This is the exposure I want to lock in. When I zoom out and include some darker areas in the frame, the aperture will change to f4 and f3.5 thereby overexposing my subject. How can I lock my exposure? (Even in aperture priority mode it will do the same thing.)

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  • Does this problem happen only for video? I don't know for Nikon, but for stills on some systems, if you change the aperture while the lens is fully zoomed in (up to F8, then back down to F5.6), it will keep the same setting when you zoom out.
    – xiota
    Dec 23 '20 at 6:30
  • I'm just going to drop some 'buzzwords' to look into, because I don't know enough about it to make it into an answer. T-Stops, aperture ramping, focus ramping. (You could check out parfocal lenses while you're at it). These are what makes the difference between a $1,000 stills lens & a $40,000 cinema lens. Cinematographers need finest control over exposure, stills don't.
    – Tetsujin
    Dec 23 '20 at 9:13
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This happens because you are using a variable max aperture lens. The aperture iris is not actually opening or closing as you zoom out – its actual opening size is staying constant. It's just that the entrance pupil, the apparent size of the aperture opening when viewed through the front of the lens, is changing size due to the movements of the lens element groups as you zoom.

It is for this reason, amongst others, that variable max aperture lenses are not the best choices for video. The only way you can lock your desired exposure is to shoot in aperture priority. If you shoot video in manual mode, you cannot avoid the exposure changing as you zoom.

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  • Seems like this is a video specific issue... For stills, the camera should be able to retain F5.6 through the zoom range because the camera only stops the lens down right before the photo is taken.
    – xiota
    Dec 23 '20 at 6:33
  • @xiota you’re pretty much right. I’d say it’s mostly a video-specific issue. It also comes into play when in live view, most Nikon DSLR bodies can’t change the aperture. So if composing in live view while zooming, the camera will appear to be changing the aperture on variable aperture lenses. Of course, after pressing the shutter button, the camera will come out of live view, reset the aperture linkage mechanism, close down the aperture, and capture the image. As you note.
    – scottbb
    Dec 23 '20 at 15:07
  • @xiota, having said that, it might be 100% a video-only issue. It appears that the more recent “prosumer” Nikon bodies can change the aperture in live view.
    – scottbb
    Dec 23 '20 at 15:24
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    @Tetsujin how is it T-stop? The camera isn't aware of the lens's transmission characteristics. It only knows and controls the lens's aperture linkage, the position of which correlates to reported f-number.
    – scottbb
    Dec 23 '20 at 17:26
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    @Tetsujin ah, okay, thanks for the article. I can see where you're coming from. For a lens that is supposedly controlled for constant aperture, then a reduction in exposure due to focus breathing is focus ramping, which is the T-stop issue you're talking about. However, the 16–85mm lens OP is talking about is not controlled for constant aperture throughout its zoom range. The spec'd "3.5–5.6" is not a measured T-stop exposure change—it is actually observed f/D (focal length over entrance pupil diameter) (at least to the nearest standard f-number)
    – scottbb
    Dec 23 '20 at 20:42

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