When I stop down my EL to f16 the opening is not circular. It's almost in the shape of an eye. Why is this so? Are there advantages and disadvantages to this?

-- J.S (frompartsunknown@hotmail.com), February 10, 1999

Answers

All Minox 35 cameras are like that ! My Minox GT-E diaphram also has <> shape when stop down. As a matter of fact, two leave diaphram is quite common in P&S and APS.

-- martin tai (martin.tai@capcanada.com), February 10, 1999.

Two piece diamond shape diaphram is also called 'cat-eye diaphram'. It was invented by N. Norton in 1855.

Minox 35mm camera has separate shutter and aperture diaphram.

Many P&S cameras have cat-eye diaphram double as shutter. The cat-eye diaphram/shutter of this type of camera is usually closed. When the shutter release is triggered, the electronic control of the camera opens up the cat-eye diaphram and then stop at the set aperture for duration of the shutter, for example, if the electronic sensor of the camera indicates a diaphram of f8 and 1/125 sec, then the cat-eye diaphram will stay at f8 for 1/125 sec, and then close down.

-- martin tai (martin.tai@capcanada.com), August 12, 1999.

To get a near circle aperture, the minimum number of diaphram blades is 7, and the blades must be computer designed to take certain curvature, otherwise it requires 10 blades to make a good circle.

Most 35MM SLR lenses,( including Leica and Zeiss lenses ) use 6 blades diaphram or 5 blades diaphram.

Six blade diaphram forms a hexagonal aperture, 5 blades diaphram a pentagon----in short, it is one blade per side.

Cat eye diaphram uses only two blades, yet provides 4 sides.

From a pentagon shape diaphram to a diamond shape diaphram, only reduces 1 side, but the number of blades is cut down from 5 to only 2, great space saving for compact camera.

How much difference from pentagon to diamond ? 20% difference

The difference from 5 blades to 2 is 250%. Cat eye diaphram is very efficient, great for compact cameras.

-- martin tai (martin.tai@capcanada.com), August 12, 1999.