How can a zoom lens possibly maintain a constant f-stop

I know that f-stop is a ratio expressing the effective focal length of the lens and the apparent size of the entrance pupil, but based on this, I have a few questions:

1.) A zoom lens with, say, a maximum f-stop of 1.4, means that at its shortest focal length and absolute widest aperture, the f-stop value comes out to 1.4. But since the aperture can't open up any more than fully open, as the focal length increases, the f-stop value should not be able to remain constant. In most camera lenses, this is the case, and that's why the maximum f-stop is different at the wide and telephoto ends of a zoom lens, but on some lenses the f-stop value is constant throughout the zoom range, how is this possible? If the aperture can't open any wider, and yet the focal length increases, isn't it impossible for a lens to maintain a constant f-stop throughout the zoom range?

2.) Also, I'm just curious, is there a standard place inside of a lens where the aperture goes? I'm sure it isn't just placed wherever there's room, otherwise the size of the entrance pupil for a given focal length would be different on different lenses.

3.) I have been researching why a smaller aperture causes a deeper depth of field, and I understand it pretty well, but what I really don't understand is why only focal length affects field of view and not aperture size. After all, if the size of the aperture determines the amount of light passing through the lens, it should also determine where in the scene that light can come from. If I look through a big hole, I can see everything around me, but if I look through a tiny one, I can only see a tiny part of it. So why doesn't this happen for camera lenses?

Thanks so much, I really value your time and appreciate your enlightenment.

You will notice I am sure that most of the constant aperture zoom lens are the higher priced models, that reflects the amount of optical calculations and type of glass that is necessary to use, more work, better glass = more money !
The aperture placement is part of the above design requirements.

When looking at through a lens at the various apertures you only see a limited area, light is different, at every aperture the whole sensor receives the same amount of light, the aperture size governs the intensity of light falling on the sensor not the area that receives the light.
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peter ellner said:

I know that f-stop is a ratio expressing the effective focal length of the lens and the apparent size of the entrance pupil, but based on this, I have a few questions:

1.) A zoom lens with, say, a maximum f-stop of 1.4, means that at its shortest focal length and absolute widest aperture, the f-stop value comes out to 1.4. But since the aperture can't open up any more than fully open, as the focal length increases, the f-stop value should not be able to remain constant. In most camera lenses, this is the case, and that's why the maximum f-stop is different at the wide and telephoto ends of a zoom lens, but on some lenses the f-stop value is constant throughout the zoom range, how is this possible? If the aperture can't open any wider, and yet the focal length increases, isn't it impossible for a lens to maintain a constant f-stop throughout the zoom range?

Click to expand...

Guesswork only - if the camera changes what portion of the sensor is used between full WA and full zoom, then you could have a situation where the focal length is artificially maintained enough to preserve a constant maximum aperture?

peter ellner said:

2.) Also, I'm just curious, is there a standard place inside of a lens where the aperture goes? I'm sure it isn't just placed wherever there's room, otherwise the size of the entrance pupil for a given focal length would be different on different lenses.

3.) I have been researching why a smaller aperture causes a deeper depth of field, and I understand it pretty well, but what I really don't understand is why only focal length affects field of view and not aperture size. After all, if the size of the aperture determines the amount of light passing through the lens, it should also determine where in the scene that light can come from. If I look through a big hole, I can see everything around me, but if I look through a tiny one, I can only see a tiny part of it. So why doesn't this happen for camera lenses?

Click to expand...

This is due to the behaviour of light. The 'small hole' thesis you describe is so because you cannot physically get your eye right up close enough to the hole in order to see the whole scene. However, if you had a tiny miniature camera, you would indeed see 'the whole scene'. You can test this out for yourself : approach a mirror so your eye is almost parallel to the plane of the mirror surface. You will see a reflection of the corresponding angle of view on the other side of the mirror, but the brain may ask, "how on earth am I seeing that?". It's light, that's how it behaves.

You would need to study the physics of light to 'get' this - but rest assured, only the focal length can affect the field of view. That's why a tiny camera with a tiny lens can still capture the same FOV as a large format camera with a massive lens, assuming their focal lengths equate. The DOF will vary, and so will the exposure needed, and all kinds of variables, but not where the light comes from.

Sorry I haven't explained the physics very well at all, but I hoep you get the drift.

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Robert

It's past my time to grab my optics diagrams so I'll try the gut feel approach...

peter ellner said:

I know that f-stop is a ratio expressing the effective focal length of the lens and the apparent size of the entrance pupil, but based on this, I have a few questions:

1.) A zoom lens with, say, a maximum f-stop of 1.4, means that at its shortest focal length and absolute widest aperture, the f-stop value comes out to 1.4.

Click to expand...

Just for practicality, I have never held a zoom lens with f/1.4 ability and I doubt I ever will.

peter ellner said:

But since the aperture can't open up any more than fully open, as the focal length increases, the f-stop value should not be able to remain constant.

Click to expand...

You forget that the zoom lens is made of many glass elements. And the glass element move in relation to each other when you zoom. Thus the max f/no can be made to vary or to be kept steady based on focal length.

peter ellner said:

2.) Also, I'm just curious, is there a standard place inside of a lens where the aperture goes? I'm sure it isn't just placed wherever there's room, otherwise the size of the entrance pupil for a given focal length would be different on different lenses.

Click to expand...

These lens designers are not using pen and paper to figure out where to place the iris diaphragm (that is what it is called). They have heaps of calculations on where to best place it.

peter ellner said:

3.) After all, if the size of the aperture determines the amount of light passing through the lens, it should also determine where in the scene that light can come from. If I look through a big hole, I can see everything around me, but if I look through a tiny one, I can only see a tiny part of it. So why doesn't this happen for camera lenses?

Click to expand...

You are thinking of the hole like a pipe that holds a bundle of parallel drinking straws. That's not how it works.

http://electronics.howstuffworks.com/camera1.htm



http://www.rags-int-inc.com/PhotoTechStuff/Lens101/

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Ananda
http://anandasim.blogspot.com
https://sites.google.com/site/asphotokb

'Enjoy Diversity - Live a Little'

If you take a compact camera, such as a Canon S5 (with its 35mm film equivalent 432mm lens at f/3.5,) and you attach the Canon TC-DC58B 1.5X teleconverter, the focal length is now equal to 648mm and the f-stop is now...f/3.5. With front mounted teleconverters the f-number doesn’t change. I would suspect that this is how the constant aperture zoom lenses work. So those 70-200 mm f/2.8 zoom lenses are more like a 70mm f/2.8 lens + a variable front-mounted teleconverter that varies from 1X to 2.86X.

The iris diaphragm (which creates the aperture) can be just about anywhere in the lens, but they’re placed at the rear focal plane because that’s the point where the largest aperture of the lens can be represented with the smallest physical hole. So a 50mm f/2 lens has an aperture of 25mm, but that 25mm can be represented by a smaller hole if the hole is placed at the rear focal plane. This is advantageous as it allows the necessary mechanics for the aperture (diaphragm and control) to be in the lens without blocking light.

Personally, I don’t think about the internals of lenses...optics is a very complex subject, and on top of that lenses are electromechanical devices, which complicates things even further. There’s simply too much there, and knowing it doesn’t help your photography.

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Apeture is the size of the opening at the entrance to the lens (in telescopse there's no variable apeture and minimal optics so it's always just the size of the front element). To get a fixed apeture zoom you make the max apeture opening equal to the fully zoomed apeture and then "close" it as the lens length shortens (note that normally the closing of the lens is an opical rather than physical effect). So rather than physically closing the lens, you'd move it closer to or further from a lens system that results in the apeture changing apparent size.

You might find taking a broken lens apart interesting.