Question about variable zoom lenses and aperture

[stevet1]

As I understand it, with a variable zoom lens, as you zoom in, your available aperture gets smaller..

As you zoom in, the amount of light hitting your sensor is reduced. If your aperture gets smaller, that reduces the amount of light hitting your sensor gets reduced even more.

That seems to be counter intuitive to me.

What am I missing, or don't know?

Steve Thomas

[SignifDigits]

Now I'm curious about this too. 

There are (I own two) fixed aperture zoom lenses, and I'm wondering how they achieve that, as well.  I'm guessing that the (typically less expensive and lighter) zoom lenses really have a fixed aperture and the smaller field of view delivers less illuminance/lux/foot candles to the sensor, and that the f-numbers are really just the loss of illuminance (effectively smaller aperture) by that design. If so, then a fixed aperture lens would have to somehow have an expanding physical aperture relative to the narrowing field of view as you zoom in.  That would explain the heavier and more expensive nature of them.  I'm sure someone here will know and tell us.

[TomRamsey]

Think about if you are standing in a tunnel, if you are close to the end you are hit by a lot of light, go deeper in the tunnel and there is less.  In a lens it is about getting light to the sensor and it should fill the entire sensor.  That gets more difficult as the lens get physically longer, which is why fixed aperture zooms are bigger around.  This is the very simple version, the lens elements are also involved in this.  

[normadel]

What do you mean by "As you zoom in, the amount of light hitting your sensor is reduced"? This is what happens when the maximum aperture is reduced  at long focal length. This is what "f/4.5-5.6" in a lens name means. This is what a variable aperture lens means. In order for a lens to keep its maximum aperture (f/number) throughout its zoom range, it becomes a heavier, more expensive lens to design and build.

So YES, the amount of light hitting the sensor at max aperture  gets reduced by zooming in....because the maximum aperture gets smaller. Your autoexposure system takes care of this, by slowing the shutter, increasing ISO, etc.

[stevet1]

What do you mean by "As you zoom in, the amount of light hitting your sensor is reduced"?

As you zoom in, your field of view gets smaller. Ergo, less light is coming in.

It's got nothing to do with your aperture per se.

Steve Thomas

[AtticusLake]

It's important to understand that YOUR F-NUMBER IS NOT YOUR APERTURE. Yes, I know, many people will tell you that it is, but many people are wrong. If you look at the definition of F-number, for example on Wikipedia, you will see that F-number is FOCAL LENGTH DIVIDED BY APERTURE (where aperture is the diameter of the entrance pupil).

In other words, the F-number depends on BOTH focal length AND aperture.

The F-number is actually a measure of the amount of light coming in through the lens. As you reduce the aperture, this is obviously reduced; but as you zoom in, it is also reduced. Zooming in means you're selecting a smaller part of the scene, so obviously you're getting less light.  The intensity of light in a given part of the scene isn't changing, but you're choosing less of it.

This is why, in general, zoom lenses have a variable maximum aperture.

Some zoom lenses have clever optical engineering to compensate, so that they can keep a constant maximum aperture. If "clever optical engineering" sounds like "big, heavy, and expensive", then you're dead on. In other words you can have a constant aperture zoom if you're willing to pay for it (in money, size, and weight), though generally this only works on lenses with smaller zoom ranges, like a 3:1 zoom. A 24-70 can do this, for example.

Hope this helps.

BTW, F-numbers are dumb. They go the wrong way because they are focal length divided by aperture; it should be the other way round. And they go in stupid steps because they are based on aperture diameter, when the amount of light you get actually depends on the area.

[SignifDigits]

And all these years I thought I was dumb because the f-numbers were so counterintuitive!  I'm glad to hear the f-stops were really the dumb ones!  Seriously, I'm glad to hear someone else thinks it.  Who thought "lets make this really confusing and use these numbers that go the opposite way of what the person using the camera"?  Probably someone before cameras decided this was the convention for optics and it got carried over to cameras.

So my first guess was correct in that most lenses have a fixed aperture and f-numbers rise zooming due to the smaller FOV/light.  I will have to find an article on the "clever optical engineering".  I assume the geometry of the optical elements move around and the aperture really just is still physically the same.  That is just basic optics/physics, but still beyond my abilities.

And there ARE a LOT of sources that say they are the same!

[ebiggs1]

Some good info here and some not so good. A single lens, just the piece of glass, has a focal length and that FL is used to calculate its f-number. The FL is the focus point where the parallel lines of light converge. The very same method is used for a complex camera lens. The same principle of f-number is consistent. A way to measure the light gathering ability. 

In a complex camera lens, the elements in front of the actual diaphragm magnify or unmagnify the arbitrary size of the lens and f-number.  Two  main things come into play here. First, and most important, the cost and will anybody buy the lens at that cost. And secondly, how heavy will it be as magical elements are heavy. If the designers had carte blanche there isn't anything they couldn't design and build but would it be practical and/or would anyone buy it. Much more would they be able to carry and use it.

[AtticusLake]

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@SignifDigits wrote:

Probably someone before cameras decided this was the convention for optics and it got carried over to cameras.

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I think that's more or less it... I think it was from when you would set the aperture by measuring it with a ruler, so the formula probably made sense then.  But today, in modern photography, it's really just not helpful.

[AtticusLake]

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@AtticusLake wrote:

This is why, in general, zoom lenses have a variable maximum aperture.

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Ugh, I meant "variable maximum f-number".  See, even I'm doing it.

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... so that they can keep a constant maximum aperture.

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F-NUMBER!!!!  <smacks forehead>