Oly 45 1.8 with Pana GTC1 converter... works !!!

Started May 17, 2012 | Discussions thread
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In reply to kenw, May 20, 2012

Great post!

kenw wrote:

EDIT: I cross posted with your addendum. Good job on the diameter estimate. So yeah, wide open the front element has definitely become the aperture stop of the optical system.

No, the aperture stop stays with the diaphragm, so the f-number stays the same with the host lens. Just think, you view the diaphragm through the front element (of the converter)

and its size is controlled by the magnification of the converter, not the size of the FE.

Only true if the entire optical system is large enough that you can still see the entire image of the iris from the front of the lens. Trivial example to explain why you are wrong - put a lens cap on a lens with a tiny hole in it. The optical system is not a vignetted F/1.8 optic anymore. It is a F/100 optic with the aperture stop in front of the lens.

In fact, you can't vignette a lens on the optical axis - that violates the definition of vignetting which is based the ratio of transmission from an off axis point to an on axis point. As the ratio of the on axis point to the on axis point is always one there is no vignetting by definition.

The fact that someone happens to put an iris somewhere in a lens does not make it an aperture stop at all. If properly designed the iris coincides with the aperture stop, however this is not always true in an improperly designed optical system.

The aperture stop is defined as the diaphragm or element which determines the diameter of the cone of energy which the optical system will accept from an axial point on the object. That could be the iris, the front element, the back element, a lens cap with a hole in it, or an improperly sized baffle, cold stop or glare stop.

In this case - 45/1.8 with the teleconverter on it - the aperture stop of the system is the front element of the teleconverter for wide apertures. At small apertures the aperture stop is in fact the iris of the 45/1.8.

A too small FE will give you vignetting wide open, but it won't change the f-number.

Ah, no. See discussion above. If the front element constrains the on axis light it is by definition now the aperture stop. For more detail see "Modern Optical Engineering" chapter 9 which has an extensive discussion of aperture and field stops, pupils and baffles with good diagrams.
--
Ken W
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