F<1?

[pit29]

Had to think about the maximal aperture lately (because Leica announced a f=.95 lens). I always used to think that an F number of 1 means that all the available light gets to the sensor / film. If that's true, what does an F number less than one mean, conceptually - or is this just some mathematical thing without any real correlation in the end? Or am I wrong about the meaning of F in the first place?

Help me understand...

 

[swiftaw]

The f number is the ratio of the focal length to the diameter of the pupil. So a lens with a 100mm length at f/0.95 would have a pupil opening of 105.26mm

 

[pit29]

Ok, that makes sense then... Now I also understand why the maximal aperture decreases with most zoom lenses when you increase focal length.

But how does this work with zoom lenses that have the same maximal aperture over their whole range? If it's e.g. 2.8 at 70mm and 200mm, does that mean that the pupil changes when zooming? And if so, couldn't they easily have higher lower apertures at shorter focal lengths?

 

[Buschmaster]

Ok, that makes sense then... Now I also understand why the maximal aperture decreases with most zoom lenses when you increase focal length.

But how does this work with zoom lenses that have the same maximal aperture over their whole range? If it's e.g. 2.8 at 70mm and 200mm, does that mean that the pupil changes when zooming? And if so, couldn't they easily have higher lower apertures at shorter focal lengths?

I'm pretty sure all zooms of this type have internal zoom so the front and back of the lens are always the same distance from each other. Or does that still not make sense.... hm...

 

[seenew]

I'm pretty sure all zooms of this type have internal zoom so the front and back of the lens are always the same distance from each other. Or does that still not make sense.... hm...

I think this is the answer^



Also noteworthy is the f/0.7 lens Kubrick used in Barry Lyndon.
They were designed for NASA's work on the Moon, Kubrick used them to film candle-lit scenes.

 

[pit29]

The f number is the ratio of the focal length to the diameter of the pupil.

So is it about the focal length (which of course can change on a zoom lense, otherwise there wouldn't be any zoom lenses), or about the distance in mm between some lens elements and/or the sensor? The latter of course may not change when you have an internally zooming lens...

 

[anubis]

The f number is the ratio of the focal length to the diameter of the pupil. So a lens with a 100mm length at f/0.95 would have a pupil opening of 105.26mm

Just a little clarification that will clear up a lot of confusion in this thread... the f number is the ratio of the effective focal length (as opposed to physical focal length) to the diameter of the exit pupil, which is the image of the aperture stop in image space. To achieve the same f number while zooming, the stop is placed so that the size of the exit pupil increases the same proportional amount as the effective focal length. This is achieved by having elements "after" the stop that "zoom in" on the stop at the same time other elements are zooming in on the scene, so to speak.

Having the requirement that the f number remain the same throughout the zoom range (a la the canon L series) places an additional constraint on the lens design that requires fancier and more exotic lenses, and thus increases the price

 

[anubis]

So is it about the focal length (which of course can change on a zoom lense, otherwise there wouldn't be any zoom lenses), or about the distance in mm between some lens elements and/or the sensor? The latter of course may not change when you have an internally zooming lens...

The effective focal length is the distance between the object-space principal plane and the image plane.

 

[pit29]

Thanks!