Originally Posted by
reddesert
It does not matter. The main issue is to make sure that when fully open, the shutter does not vignette your lens.
First, the amount of time that the shutter is opening and closing is a small fraction of the total time.
Second, the shutter either in front of or behind the lens is not exactly the same, optically, as the aperture inside the lens. The aperture inside the lens forms the pupil or aperture stop that governs what rays can pass through the lens. The pupil is maximally out of focus. For optics reasons that are too complex to explain quickly, the shape of the pupil determines the diffraction pattern of the lens, which is why for ex a five bladed aperture will give you a different number of light spikes from a point source, or shapes of out of focus blur (bokeh) compared to a four bladed aperture. However, if you stick a pentagonal hole in front of or behind your lens (like using a vignetter in a matte box), you'll get a pentagonal vignette.
There is a lot of optics misinformation or half-information on the internet, and the idea that the shutter has a different effect depending on whether the lens has formed the image already might fall into that category. What matters is whether the shutter is vignetting the lens. Vignetting can change with front or back placement, depending on lens size and angle of view.
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