Lenses and Aperture Mechanisms

[iND]

Ok I must admit I have not taken a lens apart nor have I looked a schematic.


But I imagine a it consists of a lens or series of lenses ahead of an aperture mechanism.


Now I know this may seem simplified but it is my basic understanding.


So here is my question.


If the aperture of a lens is not a product of the glass but of the mechanical opening and closing of the aperture mechanism.


Why cant all lenses have an aperture mechanism that opens to 1.8 or 1.4.


I know it is a lot more complex than this but see if someone can explain in in simple terms.


"If you can

[neuroanatomist]

Yes, an aperture mechanism is just a set of blades that iris open and closed, so technically the aperture is just empty space. But, that space must be filled with light. So, at wide open aperture, the glass elements of the lens which sit in front of the aperture must be at least large enough to deliver a broad enough circle of light to fill that wide open aperture, and in many cases larger to reduce other optical problems (e.g. vignetting).


The diameter of the aperture is determined by f-number and focal length, where aperture = (focal length ÷ f-number). So, a 100mm f/2.8 lens needs to have an aperture at least 35.7mm aperture, whereas a 400mm f/2.8 lens needs to have 143mm aperture and the much larger glass elements to fill it with light, which is why a 400/2.8 is a big, heavy beast.

[iND]

I knew you could do it.


Thanks