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why smaller maximum aperture at more mm
If, and this is big if, I understand it right, the maximum aperture at longer zoom length (e.g., 200mm) is a smaller opening than the max aperture at a shorter length (say 18mm). Since a smaller opening lets in less light, what is there about a longer zoom that requires LESS light falling on the sensor? I thought you would need MORE light coming down the elongated barrel of an extended zoom. Is it because a zoomed out lens magnifies and thereby gathers more light to focus on the sensor and hence needs a smaller opening? thanks..

Comments (15)

If you go buiy a copy of Digital Photo Pro that says Masters Special ont he cover, you'll find a good article about lens design..

As for maximum and minimum apertures of zoom lenses with variable maximum apertures, you7 still need the same amount of light to expose the sensor properly at the same shutter speed..

The difference between one max, say f5.6 at 200mm, and another max, say 3.5 at 18mm, is that you can't use 3.5 at 200mm because the aperture does not exist..

Lens design is coimplicated, as you'll see from the DPPro article and it's drawings..

BAK..

Comment #1

Tule Creek wrote:.

If, and this is big if, I understand it right, the maximum apertureat longer zoom length (e.g., 200mm) is a smaller opening than the maxaperture at a shorter length (say 18mm)..

There are many different kinds of zoom lenses. A lens that has a CONSTANT aperture size (not the f:#) will exhibit a variable f:#. That's what the f:# is about...the FL divided by the diameter of the aperture. If the diameter of the aperture is constant, then the "speed" of the lens will be a function of the FL..

For example, an 18mm to 55mm lens is typically rated at f:3.5 to f:5.6. Let's do the math....

3.5 = 18 / dia...dia = 18 / 3.5 = 10.3mm.

5.6 = 55 / dia...dia = 55 / 5.6 = 9.82mm.

While the math is a bit off, this is obviously a lens that has a fixed max aperture dia..

Another example, a 14mm to 35mm constant F:2.0 lens. Let's do the math again....

2.0 = 14 / dia...dia = 14 / 2.0 = 7.0mm.

2.0 = 35 / dia...dia = 35 / 2.0 = 17.5mm.

This is obviously a lens that has a variable aperture as the FL is zoomed. This design is expensive and heavy, but it works well, especially if you want fast AF in low light and at full tele FL..

Since a smaller opening.

Lets in less light, what is there about a longer zoom that requiresLESS light falling on the sensor? I thought you would need MORElight coming down the elongated barrel of an extended zoom. Is itbecause a zoomed out lens magnifies and thereby gathers more light tofocus on the sensor and hence needs a smaller opening? thanks.

You thunk wrong. They don't build lenses with smaller apertures at greater FL settings. I think the worst they do is keep the aperture constant. Oh, heck, now that I think about it, some of the really big zoom ratio lenses, like an 18-250 probably have an aperture that gets smaller with increasing FL (because otherwise, the IQ was terrible, the lens was huge, and nobody could afford one)..

It's the cheap way to design a zoom lens. Go price one of those new Zukio constant f:2.0 lenses and you will see how expensive the other way is! Besides, 35 / 14 = 2.5X zoom ratio...the companion is a 90-250 which is a 2.78X zoom ratio. If you build high IQ zoom lenses with constant aperture they will have a limited zoom range...but going up from f:2.0 to something like f:2.8 will allow about 4X zoom....

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #2

Tule Creek wrote:.

If, and this is big if, I understand it right, the maximum apertureat longer zoom length (e.g., 200mm) is a smaller opening than the maxaperture at a shorter length (say 18mm)..

That is often the case. Larger and more expensive zoom lenses can offer constant maximum aperture across the full range. But yes, it is often the case that max aperture is smaller at the longer focal length..

Since a smaller openinglets in less light, what is there about a longer zoom that requiresLESS light falling on the sensor? I thought you would need MORElight coming down the elongated barrel of an extended zoom. Is itbecause a zoomed out lens magnifies and thereby gathers more light tofocus on the sensor and hence needs a smaller opening? thanks.

You are coming at this from the wrong direction. You are assuming that the longer lens "needs less light" which is why the max aperture is smaller. This is not what it's about..

The longer lens needs exactly the same amount of light - or more correctly - the sensor needs exactly the same amount of light (for a given ISO or sensitivity setting) whatever the focal length (zoom) is..

The fact that zoom lenses (often) offer a smaller maximum aperture at the longer focal length is NOT because the lens "needs" to close down at that length, or because not as much light is "needed". It is - put very simply - because:.

The aperture (f/5,6 or whatever) is a ratio of the physical width of the aperture (the opening in the lens) to the focal length. Without going into gruesome technical detail, it simply means that the longer the lens, the wider the aperture needs to be to let in the same amount of light. It's a bit like the "elongated barrel" thing you mentioned..

So, for example (and these are very crude examples, not 100% accurate in terms of the actual optics of lenses, just for illustration of the point at a very simple level):.

A 200mm lens at f/4 will have (need) a (200/4=) 50mm opening (aperture).A 100mm lens at f/4 will have (need) a (100/4=) 25mm opening (aperture)..

Both lenses (f/4) are letting in the same amount of light, we know this because f/4 is f/4 is f/4, whatever the focal length. But the 200mm lens needs a wider opening to achieve the same result - basically because it's a "longer tube" if you like..

So to open up as wide a, say, f/2.8, a 200mm or 400mm lens needs to be really big and fat and heavy - and expensive. f/4 or f/5.6 is a lot easier to achieve..

So, for example, the Nikon 18-200 f/3.5-5.6 lens will open to f/3.5 at the 18mm end, but only to f/5,6 at the 200mm end. If it were bigger and fatter and heavier, you could get f/3.5 or more at the 200mm end..

Now, not everything I have said above is technically 100% accurate. But I have stayed away from the devilish detail in an attempt to clearly illustrate the underlying principle (and in particular where you are coming at it from the wrong direction)..

I hope that helps..

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window..

Comment #3

Some excellent and detailed descriptions above..

Bottom line... a longer focal length doesn't 'need' less light. In an ideal world you'd want to get *more* light in at longer focal lengths, so that you could use a faster shutter speed to eliminate the camera shake that becomes more pronounced at greater magnification..

Getting less lighjt as you zoom in is just something you have to put up with if you want a compact, affordable zoom lens. E.g. Sigma 55-200 f/4 - f/5.6: 100. Sigma 50-135 constant f/2.8: 450, and many times more weight.Mike..

Comment #4

Chuxter wrote:.

You just answered all my zoom questions before I had to ask...

Comment #5

Tule Creek wrote:.

If, and this is big if, I understand it right, the maximum apertureat longer zoom length (e.g., 200mm) is a smaller opening than the maxaperture at a shorter length (say 18mm). Since a smaller openinglets in less light, what is there about a longer zoom that requiresLESS light falling on the sensor? I thought you would need MORElight coming down the elongated barrel of an extended zoom. Is itbecause a zoomed out lens magnifies and thereby gathers more light tofocus on the sensor and hence needs a smaller opening? thanks.

To understand what's going on, let's use a simpler model..

Grab a toilet paper tube (the cardboard center), and a paper towel tube.Same diameter, different lengths. If you look through the short tube,you see MORE of the subject, and MORE total light gets to your eyes.If you look through the long tube, you see LESS of the subject and LESStotal light gets through your eyes..

Because there is no lens in these cardboard tubes, the subject in front ofyou does not seem to get dimmer as the tube gets longer. However,the measured f/stop does change, because f/stop is the ratio betweenthe diameter of the hole (same in both cases) and the distance betweenthe hole and the eye. Many zooms are like this: the aperture doesn'tchange but the distance does, so it's a "variable f/stop." To maintain thef/stop, the lens must either (1) hold the aperture at the same distance,or (2) make it GROW as it gets farther to maintain the same ratio..

Now, with focusing optics, there is a correlation: smaller f/stop meansless light, which DOES make the subject dimmer. This is not intuitive butit is borne out with a second experiment. Suspend a smallish black spot infront of your lens with a bit of cellophane tape (careful not to scratchthe glass). Depending on the lens, the spot may be as big as a pennyand still apply! Look through the camera with the aperture wide open.Your whole scene will get dimmer and you might still see a fuzzy spot butyou will still be able to see everything in the scene, even through theusually opaque spot. Light from the center of the scene hits ALL spotson the lens (except where blocked by the spot) and is recollected intothe center pixels. The aperture limits light collection in the same way asthe spot, and the whole image will get dimmer..

[ e d @ h a l l e yc c ] http://www.halley.cc/pix/..

Comment #6

BA baracus wrote:.

Chuxter wrote:.

You just answered all my zoom questions before I had to ask..

Gosh, I hope that's a good thing? Hey, if you really wanted to ask and now feel cheated, go ahead. I'll ignore them... .

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #7

To understand what's going on, let's use a simpler model..

Grab a toilet paper tube (the cardboard center), and a paper towel tube.Same diameter, different lengths. If you look through the short tube,you see MORE of the subject, and MORE total light gets to your eyes.If you look through the long tube, you see LESS of the subject and LESStotal light gets through your eyes..

Because there is no lens in these cardboard tubes, the subject infront ofyou does not seem to get dimmer as the tube gets longer. However,the measured f/stop does change, because f/stop is the ratio betweenthe diameter of the hole (same in both cases) and the distance betweenthe hole and the eye. Many zooms are like this: the aperture doesn'tchange but the distance does, so it's a "variable f/stop." Tomaintain thef/stop, the lens must either (1) hold the aperture at the same distance,or (2) make it GROW as it gets farther to maintain the same ratio..

Excellent analogy!Best wishesMike..

Comment #8

Chuxter wrote:.

BA baracus wrote:.

Chuxter wrote:.

You just answered all my zoom questions before I had to ask..

Gosh, I hope that's a good thing? Hey, if you really wanted to askand now feel cheated, go ahead. I'll ignore them... .

It's a good thing, I was looking for exactly that info before I posted about it..

I'm kinda confused at the math though. A variable zoom at 18mm f3.5 would have 18/3.5=Aperture, right? I get 5.14mm...

Comment #9

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window.

F= focal lenthN= f number.

???Now I'm more confused than I was before...

Comment #10

Mike703 wrote:.

Grab a toilet paper tube (the cardboard center), and a paper towel tube..

1) Would you believe that the tube in a toilet paper roll, also that in a paper towel roll, is called a "trollo"? [Yup. No kidding. Why would I lie to you?!].

2) Would you also believe the people behind the internationally famous Oxford English Dictionary do NOT know that the cardboard centre of a toilet roll is called a "trollo"? [Yup. No kidding... etc.]Regards,Baz..

Comment #11

Just got back from another day at school and youngest daughter's swimming lesson and am trying to wrap my head around your detailed and wonderful answers to my question. Thanks BAK, Charlie, Arrowman, and Edyou people are great. Wish I could repay the favors. Maybe, someday, long, long ways down the road I'll know enough to help novices as you do. Mike..

Comment #12

BA baracus wrote:.

I'm kinda confused at the math though. A variable zoom at 18mm f3.5would have 18/3.5=Aperture, right? I get 5.14mm..

Yes....

F# = FL / diameter, thus, 3.5 = 18 / diameter or diameter = 18 / 3.5 = 5.14mm..

Note that this diameter is NOT the diameter of the glass, but rather the diameter of the aperture. Look into your lens and you will see this hole..

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #13

BA baracus wrote:.

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window.

F= focal lenthN= f number.

???Now I'm more confused than I was before..

The way to understand this is to simplify....

Area = pi * radius^2 ("pie are squared").

By inspection, it seems that (focal length / (2 * F#)) = radius...right?.

Hey, that's the same as (focal length / F# = diameter) which you already knew..

Math is simple....

Charlie DavisNikon 5700 & Sony R1HomePage: http://www.1derful.infoBridge Blog: http://www.here-ugo.com/BridgeBlog/..

Comment #14

Barrie Davis wrote:.

Mike703 wrote:.

Grab a toilet paper tube (the cardboard center), and a paper towel tube..

1) Would you believe that the tube in a toilet paper roll, also thatin a paper towel roll, is called a "trollo"? [Yup. No kidding. Whywould I lie to you?!].

Hi Baz,.

But they have a constant aperture all along the tube, so how come they are so cheap?.

2) Would you also believe the people behind the internationallyfamous Oxford English Dictionary do NOT know that the cardboardcentre of a toilet roll is called a "trollo"? [Yup. No kidding...etc.].

And the spelling checker threw out both "centre" and "trollo". Are you sure it isn't a troll joke?.

Regards, David..

Comment #15

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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