Summicrons F-value?

[HuubL]

A question for the experts. I understand that a lens f-value equals the focal length divided by the diameter of the lens opening. To accommodate a larger lens opening a lens needs a large enough front lens and that's why the front lens of an f/1.4 50mm lens is larger than an f/2. Why then is the diameter of the front lens of my v4 35 mm summicron 35% larger than the one of the v1 summicron, roughly 27 mm vs. 20 mm?

 

[Sam N]

It's the lens aperture area that matters. The size of the front element could change depending on the design of the lens. The Sigma 50/1.4's front element is MUCH larger than that of the Nikkor 50/1.4 LTM, but the aperture openings should be the same size.

 

[HuubL]

Thanks Sam. I understand. That poses new questions, like why a bigger front element would be better. But I guess I would need a degree in optics to follow the argumentation for that 🙂

 

[usagisakana]

Thanks Sam. I understand. That poses new questions, like why a bigger front element would be better. But I guess I would need a degree in optics to follow the argumentation for that 🙂

At a guess, it might be something to do with the centre of a lens having less distortion than the edges. Although I could be way off.

 

[spyder2000]

the f number of a lens is a function. That function is defined as the focal length of a lens divided by the diameter of the entrance objective. It is the focal length that causes the confusion when comparing lenses. Focal length is defined as the distance from the lens nodal point to the focus point (film or viewing plane.)

It's the lens nodal point that causes the confusion. For some lenses it is at or in front of the iris. For others it is behind. In other cases it can be in front of the entrance objective (I think this is the case for the Nikkor UW lenses). The lens design determines this.