Why does exposure change when zooming video in Manual mode with a Nikon D7500 and 16-85mm lens?
Asked 12/22/2020
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I’m recording video on a Nikon D7500 with a 16-85mm f/3.5-5.6 lens. I want to keep exposure fixed at 1/60 and f/5.6 for a head-and-shoulders shot. But when I zoom out, the camera shows the aperture changing to f/4 or f/3.5 and the image gets brighter. Why does this happen in Manual mode, and is there any way to lock exposure while zooming with this lens?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
5y ago
2 Answers
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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.
Originally by user11924. Source · Licensed CC BY-SA 4.0
user11924
5y ago
0
Generated from our catalog & community — verify before relying on it.
Your 16-85mm is a variable-aperture zoom. As you zoom, the lens’s maximum f-number changes from f/5.6 toward f/3.5. The iris may not be physically opening wider, but the lens optics make the effective aperture change, so exposure shifts during the zoom.
That means with this lens you cannot fully lock the same exposure settings while zooming through the range in video if the zoom moves into a wider maximum aperture. In practice, this is one reason variable-aperture zooms are not ideal for video zoom shots.
If you need exposure to stay constant while zooming, the real solution is to use a constant-aperture zoom lens. Otherwise, avoid zooming during the take or work within a focal-length range where the lens can maintain your chosen aperture.
Aperture-priority won’t solve the underlying behavior of the lens; the effective aperture still changes as you zoom.
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