Silly semantics alert!
I'd always that that casket sets were a collection of cells that could be singly or in pairs as desired. Each cell is claimed to be a fully corrected anastigmat. If I understand Tri Tran's product slate correctly, all of his lenses are single element meniscii and he sells barrels in which meniscii of various focal lengths can mounted, one at a time. This doesn't seem to be quite the same as combining cells to make fully corrected double anastigmats or even switching single multi-element cells in a barre.
One of the basics of photography: the f/stop is determined by dividing the focal length by the aperture's diameter as measured through the front element, (aka "entrance pupil").
When pjd mentioned casket sets (a barrel with multiple interchangeable elements for different focal lengths) being marked in mm, and Pere asked "if the aperture in mm is the real hole or if it is the effective hole size, seen from the front, to give the value that's used in the f/ calculation", I just pointed out that different cells would render different effective aperture sizes, which Pere had said a little less directly.
The manufacturers' aperture scales always give f/stop values for infinity. For those of us who frequently work at close enough distances to need to factor in a longer focal length (bellows extension), having a scale marked in effective aperture diameter would be a more convenient way to figure exposures than converting from infinity-based f/stops.
"I love my Verito lens, but I always have to sharpen everything in Photoshop..."
I mounted a Zeiss Protar VII set and made some measurements. The mm scale on the barrel reflects the physical aperture (how could one scale compensate for different cells, if cells of differing power really make much of a difference to effective aperture). With a 48cm cell up front the aperture apparent through the front element appears less than 1mm different to the physical aperture as shown on the scale. Measuring through the front element is quite tricky, I think this is close enough to be workable.
Zeiss did distribute tables with casket sets showing the aperture in mm required for a given f stop with various combinations of cells, maybe they adjusted these slightly to account for effective aperture rather than actual mechanical aperture. I don't think fractions of a mm make enough of a difference to worry about - my exposures generally come out well enough, and when they don't it's not Carl Zeiss and their mm aperture scale to blame.
"I love my Verito lens, but I always have to sharpen everything in Photoshop..."
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