question about lowest f-number lenses
Sorry to say so - but I agree with fmw here ...
However - to answer your question to some extent ~ I have used lenses that went down to F90 and F128- but these were 150mm and 200mm lenses bolted onto 4" x 5" plate cameras ... and it was 50-odd years ago as well.
As you well know, the aperture in use is a fraction of the focal length of the lens ... the diameter of F8 is 1/8 the millimetres of the focal length [tho modern lens design can modify this a bit]. But every lens maker well realises that overdoing the minimum aperture of a lens will cause image degredation, and therefore will cease stopping down before that image degredation becomes an issue - even if the mechanical side of the diaphragm could be made to do it.
It is why so many digital cameras now use an ND filter within the optical path to achieve exposure alterations, rather then trying to overdo the lens's minimum aperture
For your tutorials I would strongly suggest you use readily available commercial materials - with full accreditation to the authors - and save your energies for the easier stuff that your students will probably be more interested in
[ps: I have been an adult education lecturer in photographic techniques in eastern Australia since the mid 1970s and have been thru this 100+ times over the years. I am happy for you to talk to me off-line if you wish to do so]
Phil from the great land Downunder
www.flickr.com/photos/ozzie_traveller/sets/
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