Measuring the entrance pupil

[tao]

F-stop = FL/D where D is the diameter of the entrance pupil. If we assume that the nominal focal length written on the lens is true. How can we measure the D in order to calculate the F?

Let's say I use a ruler. Where should I put it in space? Should it be at the first surface of the front element? And where should I be. At infinity perhaps?

[Oldhand]

The experts will reply in due course, however, here is a reply to a similar question on Flicker:

The following is a method described by Jones in the 1911 editon of his Encyclopaedia of Photography, p. 182, under the "Diaphragms" entry:

While, in the above rough-and-ready system of measuring, the "f" value of a stop may be accurate enough in the cases of ordinary rapid rectilinear and single lenses, extreme accuracy is essential in the case of modern and improved anastigmat lenses; for while a slight error may be of little moment with a small stop, the same amount of error--which error is, of course, proportional to the aperture--becomes serious with large stops, which are a feature of anastigmat lenses. It is necessary in such cases to consider effective aperture. The stops on a modern lens by a good maker are always correctly numbered, and it is only when a worker attempts to check the optician's calculations by dividing the focal length by the aperture that he finds an imaginary error. The division system described above leads to false conclusions when some modern anastigmat lenses are measured by it, because of the great condensing power of the lens in front of the stop, from which lens measurements are taken. To quote an example; one of the most expensive of anastigmat lenses has a stop the value of, and marked, f/8; but the aperture of the stop is one-ninth the focal length, and, according to the rough and ready rule given above, would be f/9; while the stop marked f/5, although accurate, is f/7 according to the division method. The following method of finding the effective aperture of a stop is more reliable with all kinds of lenses. The camera is set at "infinity," or a distant object is focused upon the ground glass. A piece of card is then put in the place of the ground glass, or pasted thereon, so as to entirely cover it. In the center of the card, and on the spot exactly opposite the lens, is made a hole the size of a pin's head. The camera is then taken into the dark-room, and by the assistance of a ruby or orange light a disc of bromide paper is cut to fit inside the lens cap. The cap, with the sensitive paper inside it, is then placed on the lens in the usual way, the sensitive side towards the stop. A lighted candle [Ed- or an LED or a penlight since we now live in the future] is then held against the hole in the cardboard for about half a minute, so that the light may travel through the camera, lens and stop to the bromide paper. The latter, after exposure, is taken out and developed, when a circular black spot will be found thereon, and the diameter of this spot will be the effective diameter of the stop used. If the exposure is made with the largest stop, the developed spot gives the effective aperture at which the lens will work, and the focal length of the lens divided by this, the true aperture, gives the "f" number.

Methods similar to the above are also described in Alan Greene's Primitive Photography: A Guide to Making Cameras, Lenses, and Calotypes, pp 90-91, from 2002. And again in Zakia and Todd's 101 Experiments in Photography, p. 42, from 1969.

Another advantage of calculating aperture using the method described by Jones is that it can be used for determining/verifying the full range of aperture sizes for a given lens and not just its maximum one.

See:
https://www.flickr.com/groups/868027@N25/discuss/72157611521640402/
OH

[ZoneV]

I think the best way to measure the entrance pupil is to take a telescope with macro-focussing capabillity, and if possible cross-hairs.
This should be mounted on a kind of linear stage - optical axis vertical to the moving direction.

The setup is placed before the front lens, with the telescope on the center of the lens. And optical axis from lens and telescope should be nearly the same.
With the linear stage the telescope is moved to the outer edge of the entrance pupil (the bright spot), focussed on the outer edge - likely the iris blades or iris housing.
Then the stage is move to the other side of the entrance pupil, they way between both positions is the entrance pupil diameter.
With fischertechnik construction kit one could build such a device, for example with a simple alidade / diopter / sight. One of the reasons why I love fischertechnik

Another more simple idea is to take a sharp image of the entrance pupil (iris housing free diameter) with a fast lens wideopen, and probably extension tubes. Without altering the focussing (!) one takes a sharp image of a ruler afterwards. With the ruler image one can measure the entrance pupil image.

There are other described ways, like holding the lens as far as possible away, and then judge/measure the bright spot diameter with a ruler in front of the lens. For entrance pupils far inside or behind the lens this results in errors.

[kds315*]

I take a calipher, hold it in front of the lens and close it until it seems to cover the pupil while looking to a bright lightsource (sky).
Rough estimate but good enough for me...

[ZoneV]

[tao]

[ZoneV]