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Old 10-01-2016, 05:25 PM
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pdalek (Patrick)
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Join Date: Mar 2013
Location: Melbourne, Australia
Posts: 112
You will find the required size for shorter lens focal length settings will be smaller than available rings.
Use a gel filter holder with squares of metal, platic, cardboard with holes.
The hole size is (focal length)/(required F#)

With a zoom lens you need several apertures to cover the full zoom range in convenient steps.

Alternatively a high quality adjustable iris could be used. These are expensive.

A field stop of this type will give greater vignetting and aberrations than a correctly placed stop.

Note that the size of the front element in a camera lens is always larger than the F# would imply. This is to give a wide angle of view with reduced vignetting.
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