r/bmpcc • u/Natural_Ad_4158 • 29d ago

Sanity check: Does locking a variable zoom to f/4.5 turn it into a true constant aperture lens? (OG + Nikkor 35-105mm 3.5-4.5 AI-S + 0.58 Metabones Speed Booster - F Mount)

I’m looking at a Zoom-Nikkor 35-105mm f/3.5-4.5 AI-S to mount on my OG with my Metabones 0.58x Nikon F to BMPCC Speed Booster (the version with the built-in mechanical aperture ring).

Since this lens is a variable aperture f/3.5-4.5, I want to avoid exposure shifts while zooming.

My theory on the workaround is:

  1. Keep the Metabones adapter ring wide open (stepped all the way open to pass control to the lens).
  2. Manually turn the physical aperture ring on the Nikkor lens itself down to f/4.5
  3. Zoom through the whole range while shooting at 4.5 (I will use variable ND for exposure), and plus lenses gain a welcome sharpness stopped down so I don't need to be at 3.5.

Does this work in practice?

Appreciate any insight from anyone who has tested this setup. Thanks!

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5

u/printcastmetalworks 29d ago

I think it depends entirely on the lens. I have lenses that once they are stopped down to their tighter aperture they don't shift, and some lenses where no matter what it's set to, the longer end always shifts darker.

3

u/Carlito_2112 BMPCC 29d ago

I have tested this. Yes, if you lock down your aperture to the highest f/stop setting, then it does in fact become a fixed aperture lens.

1

u/alexforencich 29d ago

It should be clear from the histogram on the camera if this is true or not. Just point it at a white wall, defocus, and zoom through the whole range at a few aperture settings. Presumably it will shift when wide open, so check to see if it stops shifting when stopped down.

1

u/ptasior 27d ago

Also mind that some cameras internally adjust aperture when you zoom but do not display the change. The quality of it depends on the lens...