Diaphragm Blades can vary in number, in shape. It directly has effect on the creation of the shape of the opening depending upon the set aperture. We will not go in to details of Diaphragm itself, because that topic; Diaphragm of Digital SLR Camera Lens has been discussed before. This article is specific on the Diaphragm Blades, a part of it. Possibly the most important part.
Basics about the Diaphragm Blades
In old days of SLR or rather Rangefinder cameras (in ’30s or back), the number of leaves were more and were part of an eclipse, that is more roundish. Basically the diaphragm is an adjustable device with a variable opening that is called the œaperture or œiris and without these diaphragm blades, mechanical control of the size of the aperture can not be done. So practically there are two aspects of a diaphragm blades working principle :
- The shape – whether it is a part of an eclipse of part of a straight curved blade
- The Optics – what is the result of the implementation of the shape throughout the range of aperture.
There is an old phrase – The aperture blades are clean, oil-free and snappy. This means, if the mechanical movement is not free, the result will never be optimum. The only absolutely certain way to know the exact number of diaphragm blades that make up the diaphragm is to actually count them yourself, which can be complex to the new users. There are 5 to 9 blades available, price usually increases with the more curved (towards being a part of eclipse) ones and increasing number.
But it is not that, with big number of blades with curved cutting, will suit everyones every need – the price is higher because they are for the professional or serious users. Mechanically its easier to manufacture a lens with straighter blades – its a simple physics. If you try to close the door closer to the hinge, it will be almost impossible to pull. But if you pull from the opening end of the door – it will be very easy to close and open. Thats what we do probably every hour.
Now imagine the door has been an eclipse and kind of straight version of eclipse. In case of eclipse the loss of energy will be less if pulled from the point f hinge but will be more in straighter version. The reason is nothing but your old school physics – the direction of forces. You can make a bow like thing with broom stick and test yourself. In case of part of eclipse, the two directions of force will help to move it in or out from a certain position. In case of camera, the Gravity virtually has no effect. It actually has very minute effect.
So, technically – if the diaphragm blades are towards being a part of an eclipse, more in number, it will :
- Be easy to create a smooth round iris over a long range of aperture
- Be easy to control by the mechanical system
But that is not that easy. There are more factors. One of them is – the reflections by the edges of both the incident and reflected light. Where the blades meet causes a diffraction and has the same effect like that of a star filter. Rounded aperture blades require a more complex opening-closing mechanism than the straight as the naturally the tendency towards center of gravity will be different.
Effects of Number, Shape, mechanism etc. of the Diaphragm Blades on Digital Photography
The effective aperture of a lens coincides with the effective diameter of the entire lens or hole that lets light pass through and actually determines its brightness. There is equation but ISO must be constant. In digital photography it is might not be constant in practical usage. Plus there is the great crop factor. There is some lag too, apart from shutter lag.
More number of diaphragm blades will make OOF highlights less polygonal – more circular. Bokeh will be roundish instead of polygonal. Now the count – odd and even. Even number of blades will in theory eliminate the diffraction effect and the the center will be really in the optical axis of the lens.
It is not known how the minute vendor specific materials are manufactured. That is trade secret. But manufacturers can change the technology without any notice even for the same model.