How do they do it? (one aperature zoom lenses)

[timmyquest]

If the aperatures physical size is based on the focal length of the lens, then doesnt that mean that when you zoom a lens say from 70 to 200, in order to maintain f/2.8 the aperature has to get larger?

Which would mean that, it could be larger at 70mm thus giving you a wider aperature then f/2.8?
???

[Jon]

The aperture used in f/ number calculation isn't a physical part of the lens; it's what's known as the "entry pupil", a (to simplify) projection of the diaphragm opening through the front of the lens. If you look at the front of your constant-aperture zoom lens as you zoom it, you'll see that the apparent area of brightness will grow as you zoom to a longer focal length. If you think about it, this is actually happening in most zoom lenses. Take the 100-400 f/4-5.6: the 100 mm f/4 aperture is 25 mm. 400 mm f/5.6 aperture is 71+ mm.

[DocFrankenstein]

There's a nice 800 page optics book which answers that question :)

[theflyingkiwi]

There's a nice 800 page optics book which answers that question :)

so who is going to read it and provide a simple answer :) bags not me

[DaveG]

so who is going to read it and provide a simple answer :) bags not me

You take the focal length of the lens and divide it by the exit pupil which is the lens opening without the diaphragm being used at all. If it's a 50mm lens and the exit pupil is 25mm then: 50/25=2 or f2. That's why when you use a 2X converter you lose two stops. 50 would become 100, the exit pupil doesn't change, so you get 100/25=4 or f4.

With a constant aperture zoom the exit pupil must change as the focal length changes so that the math still works. That means that the lenses' diaphragm is closing. And that means that at 70 mm my 70-200 f2.8 is actually a lot faster than 2.8 and is being artificially slowed down to maintain that constant 2.8.

When variable aperture zooms were introduced in the 80's they were and are less expensive than fixed aperture zooms since this "compensation" isn't needed.

[DocFrankenstein]

U sure that the "opening" of the lens is called exit pupil?

[timmyquest]

With a constant aperture zoom the exit pupil must change as the focal length changes so that the math still works. That means that the lenses' diaphragm is closing. And that means that at 70 mm my 70-200 f2.8 is actually a lot faster than 2.8 and is being artificially slowed down to maintain that constant 2.8.

Thats my point.

Why cant they just let us have the extra speed that is "hidden" in that aperature?

[DaveG]

Thats my point.

Why cant they just let us have the extra speed that is "hidden" in that aperature?

Until recently there was a big problem using variable aperture zooms with strobes. You could select f8 as your lens aperture, but you got variation as you zoomed. And you never really knew exactly WHAT aperture you were really getting. Sure you could select f8 but if it was a 3.5/4.5 lens you will get a full stop of variation depending on the focal length chosen. Many people seems unaware that this variation in aperture continues all the way through the aperture range.

So it would be impossible to use studio stobes at say f8 if you couldn't set f8 with any certainty. Althought his doesn't sound like much of a problem it would be if you were doing H&S portraits and were happily zooming back and forth with the exposure varying by a stop.

With a camera like the 20D the camera controls the aperture and it gives you f8 no matter what, even if it a variable aperture zoom. Of course that's not true when you select f3.5 on a 3.5-4.5 lens and then zoom to the maximum focal length, but other than that it takes the pain out of variable aperture zooms and strobes.

Why they don't give us that extra stop in a 70-200 remains a mystery to me. Perhaps it's just that variable aperture zoom lenses were always cheaper and inferior, compared to the constant aperture zooms; and the manufacturers don't want any perception of cheapness when it comes to their premier lenses. Still a 70-200 f1.something to 2.8 could be useful. Ya think?

[karusel]

Until recently there was a big problem using variable aperture zooms with strobes. You could select f8 as your lens aperture, but you got variation as you zoomed. And you never really knew exactly WHAT aperture you were really getting. Sure you could select f8 but if it was a 3.5/4.5 lens you will get a full stop of variation depending on the focal length chosen. Many people seems unaware that this variation in aperture continues all the way through the aperture range.

This was very good, I've never thought of it.

Perhaps the reason in not opening up the aperture all the way at the wide end has something to do with physics that I will never understand, perhaps it's a marketing thing and it could be, that Image quality would become really sucky and besides; when you're shooting something in low light at ISO 1600 you get times of perhaps 1/90 and suppose at 70mm you had f/1.2 for instance, and at 100mm it would become f/1.4 and so forth, so as soon as you zoom, you get a blurry photo. Dang, right? If the aperture is constant you sort of 'get what you see', no confusion, being specially careful about time values, knocking up ISO and so forth...

[Jon]

What you're seeing isn't a physical change in the diaphragm's opening. It's the apparent diameter of that opening when seen through all the shifting bits of glass. And the changing apparent size of the opening is present in almost all zoom lenses, not just constant-aperture zooms. As I said before, take the 100-400 f/4-5.6: the 100 mm f/4 aperture is 25 mm. 400 mm f/5.6 aperture is 71+ mm. If the physical aperture were constant, it'd be either a 100-400 f/4-16 or a 100-400 f/1.4-5.6, depending on which diameter it stuck with. Take out your zoom and look at the light path as you zoom it; watch the apparent aperture size change as the front elements change the magnification on it.

[roanjohn]

..........my head hurts..........

Ro1

[Jon]

..........my head hurts..........

Ro1 Blame TQ. It was his question!

[timmyquest]

What you're seeing isn't a physical change in the diaphragm's opening. It's the apparent diameter of that opening when seen through all the shifting bits of glass. And the changing apparent size of the opening is present in almost all zoom lenses, not just constant-aperture zooms. As I said before, take the 100-400 f/4-5.6: the 100 mm f/4 aperture is 25 mm. 400 mm f/5.6 aperture is 71+ mm. If the physical aperture were constant, it'd be either a 100-400 f/4-16 or a 100-400 f/1.4-5.6, depending on which diameter it stuck with. Take out your zoom and look at the light path as you zoom it; watch the apparent aperture size change as the front elements change the magnification on it.

Your right in a sense, but i'm not refering to zoom lenses who's aperature changes, i'm refering to zoom lenses who's aperature "doesnt" change.

70-200 f/4
70-200 f/2.8
120-300 f/2.8

All of these lenses have an aperature that could open wider at the shorter ends

Why they don't give us that extra stop in a 70-200 remains a mystery to me. Perhaps it's just that variable aperture zoom lenses were always cheaper and inferior, compared to the constant aperture zooms; and the manufacturers don't want any perception of cheapness when it comes to their premier lenses. Still a 70-200 f1.something to 2.8 could be useful. Ya think?

That was what was floating in the back of my mind, i think it really all may be about perception.

[kawter2]

this was a great thread!!!!


Maybee they could add the extra stop in a switch (like the IS switch) and market it as something like "super-brite" or "ultra-fast"

kind of like a bonus feature 70-200m f/4 "w/70mm Super-brite"

[Jon]

Your right in a sense, but i'm not refering to zoom lenses who's aperature changes, i'm refering to zoom lenses who's aperature "doesnt" change.

70-200 f/4
70-200 f/2.8
120-300 f/2.8

All of these lenses have an aperature that could open wider at the shorter ends

No they don't. The aperture isn't physically changing size. The optics in front of it are changing the magnification on it. The 100-400 was just an example to show you that the same thing happens with variable-aperture zooms as does with constant aperture zooms - the apparent physical dimensions of the apertures aren't constant or consistent. And the physical opening isn't changing size; just the amount of light that's being collected and funnelled into it.

Look into the front of your 70-200 as you zoom and watch the apparent size of the opening change as the inner front (Canon calls them "group 2" in EF Lens Work III) lens group moves back and forth. That lens group is working like a magnifying glass to the aperture. In addition to helping the focal length change, it's also helping affect the aperture.

If you want to learn more about Canon's lens design process and characteristics of the various lenses (both with and without teleconverters) get a copy of EF Lens Work III. Mine was $19.95 at B&H.

[DocFrankenstein]

Here's some math:
Consider 70-200 F/2.8
To get 2.8 at 200 mm, you need an "opening" of d=71 mm. Coincidentally, this is why your front element on the 70-200 is 200 mm apart from the focal plane AND is roughly 70 mm. When the distance from the front element is close to the focal distance of the lens, it's easier to maintain the aperture=focal length/diameter equation. That is why you see MOST of the primes from normal to telephoto with the front element placed focal length apart from the focal plane. (unless it's a speciality or an extremely fast lens)

As we approach the wide angle, the lenses start to violate this rule. Because it's impossible to have the front element that close and still get a fast aperture. The mirror and the SLR camera design doesn't allow that.

Now, if we have a front element 200mm away from the focal plane, but we want the field of view to be 70mm then we are gonna have a problem getting the same aperture=focal length/diameter ratio. It's just impossible to built it like that, because some of the light is going to be trapped in the lens "walls".

Imagine you're an engineer, having to build a 70mm lens that:
focuses from 0 to infinity
is sharp
lets in lots of light
has to be some space apart from the focal plane so that the mirror doesn't hit it
is a part of a 3x zoom
AND THE FRONT ELEMENT IS 200 MM AWAY FROM THE FOCAL PLANE

It's IMPOSSIBLE from the point of view of optics to get that 70/70= f/1 aperture... at least not with current technology and required image quality.

It's better not go any deeper, because then you'd have to spend at least a few months digging in optics books.

[DaveG]

Here's some math:
Consider 70-200 F/2.8
To get 2.8 at 200 mm, you need an "opening" of d=71 mm. Coincidentally, this is why your front element on the 70-200 is 200 mm apart from the focal plane AND is roughly 70 mm. When the distance from the front element is close to the focal distance of the lens, it's easier to maintain the aperture=focal length/diameter equation. That is why you see MOST of the primes from normal to telephoto with the front element placed focal length apart from the focal plane. (unless it's a speciality or an extremely fast lens)

As we approach the wide angle, the lenses start to violate this rule. Because it's impossible to have the front element that close and still get a fast aperture. The mirror and the SLR camera design doesn't allow that.

Now, if we have a front element 200mm away from the focal plane, but we want the field of view to be 70mm then we are gonna have a problem getting the same aperture=focal length/diameter ratio. It's just impossible to built it like that, because some of the light is going to be trapped in the lens "walls".

Imagine you're an engineer, having to build a 70mm lens that:
focuses from 0 to infinity
is sharp
lets in lots of light
has to be some space apart from the focal plane so that the mirror doesn't hit it
is a part of a 3x zoom
AND THE FRONT ELEMENT IS 200 MM AWAY FROM THE FOCAL PLANE

It's IMPOSSIBLE from the point of view of optics to get that 70/70= f/1 aperture... at least not with current technology and required image quality.

It's better not go any deeper, because then you'd have to spend at least a few months digging in optics books.

In large format a lens requires it's focal length to focus at infinity. A 90mm lens requires 90mm of bellow extension and a 210mm lens 210mm of BE. Lenses longer than normal in large format are called long focus. Now there are some TELEPHOTO lenses in large format too, and these are lenses that do NOT need their focal length to focus at infinity. They tend to be expensive, large and have small image circles, but they also can be used with cameras with shorter bellows.

My point is that we are are using nothing BUT telephoto lenses in 35mm and current digital photography so I have used many lenses whose front element is nowhere near it's focal length since tricks of optical design reduces the need for it.

I also don't see where the light is getting "stuck" inside of the lens barrel. My formula for aperture is focal length/exit pupil = fstop number.

This is just arithmatic and has nothing to do with the glass. F-Stops are inexact enough that professional movie lenses are tested individually and then marked not in F-Stops but in T (transmission) stops!

If we change the focal length either the exit pupil OR the f-stop changes with one remaining constant. Then we plug in numbers: 200/exit pupil (71mm) = f2.8. 70/70=f1. If we had a 70 mm lens with an 70mm exit pupil we'd have an f1 lens and we should with the zoom too!

[DocFrankenstein]

If we change the focal length either the exit pupil OR the f-stop changes with one remaining constant. Then we plug in numbers: 200/exit pupil (71mm) = f2.8. 70/70=f1. If we had a 70 mm lens with an 70mm exit pupil we'd have an f1 lens and we should with the zoom too!
Whoops... sorry. I had a misleading conclusion. I wrote:

It's IMPOSSIBLE from the point of view of optics to get that 70/70= f/1 aperture... at least not with current technology and required image quality.

What I meant was:
It's IMPOSSIBLE from the point of view of optics to get that 70/70= f/1 aperture in the 70-200 2.8 lens that satisfies the above design requirements... at least not with current technology and required image quality.

so I have used many lenses whose front element is nowhere near it's focal length since tricks of optical design reduces the need for it.
I'm not sure of the point of the statement. Need for ... ?

No "tricks in optical design" will allow your front element be smaller in diameter than the focal length/aperture. Or did you find a lens like that somewhere? HEHE :D

In fact, tricks are needed to keep the front element from being too big. IE: Canon's 50mm f/1.0 has a front element of around 70mm...

Where do you suggest the light goes after entering the lens? If all of the light that came in the 70mm front element was put on the sensor, it would have an aperture of f/0.7... Somehow it loses that one stop of light somewhere, doesn't it?

[DaveG]

Whoops... sorry. I had a misleading conclusion. I wrote:



What I meant was:
It's IMPOSSIBLE from the point of view of optics to get that 70/70= f/1 aperture in the 70-200 2.8 lens that satisfies the above design requirements... at least not with current technology and required image quality.


I'm not sure of the point of the statement. Need for ... ?

"No "tricks in optical design" will allow your front element be smaller in diameter than the focal length/aperture. Or did you find a lens like that somewhere? HEHE :D"

In fact, tricks are needed to keep the front element from being too big. IE: Canon's 50mm f/1.0 has a front element of around 70mm...

Where do you suggest the light goes after entering the lens? If all of the light that came in the 70mm front element was put on the sensor, it would have an aperture of f/0.7... Somehow it loses that one stop of light somewhere, doesn't it?

"If all of the light that came in the 70mm front element was put on the sensor, it would have an aperture of f/0.7... Somehow it loses that one stop of light somewhere, doesn't it?"

If the exit pupil was 70mm and the focal length of the lens was 70mm then it would be f1 not f0.7, yes?

"Where do you suggest the light goes after entering the lens?"

I think that the diaphragm is stopping down and that's where the light goes.

No "tricks in optical design" will allow your front element be smaller in diameter than the focal length/aperture. Or did you find a lens like that somewhere? HEHE :D


I had a Nikkor 50mm f1.4 lens and a 50mm f1.2. They both took 52 mm filters, had roughly the same size front elements and yet the exit pupil must have been bigger on the 50 f1.2 Nikkor. My 16-35 Canon lens is an f2.8 and yet the wider 10-22mm f3.5-4.5 share EXACTLY the same size front element The front elements are 51mm in diameter, and I know this because I just measured them. Yet the 10-22mm lens is 2/3 to 1 & 2/3's of a stop slower. Although the front element size does have some effect on lens speed it's obviously not everything.

[DocFrankenstein]

Where did I say it's everything? I said that the minimum is focal length/aperture.

Please try to understand my post before replying. Where did I say that aperture = focal length/d of front element? Nowhere! Where did I say that the front element can't be bigger than focal lenght/aperture? Nowhere!

Then why are you giving me these examples to my statement that:

MINIMAL AND THEORETICALLY IDEAL D OF FRONT ELEMENT = (FOCAL LENGHT/DIAMETER)

Let's see:

I had a Nikkor 50mm f1.4 lens and a 50mm f1.2. They both took 52 mm filters, had roughly the same size front elements and yet the exit pupil must have been bigger on the 50 f1.2 Nikkor.
Ok... I know... I agree. I don't get why you're quoting it to me. I didn't say anything about it not being possible.

My 16-35 Canon lens is an f2.8 and yet the wider 10-22mm f3.5-4.5 share EXACTLY the same size front element The front elements are 51mm in diameter, and I know this because I just measured them.
o....k

Minimal front element for 16-35/2.8 is 12.5 mm

12.5 mm is less than 51 mm... Why quote it?

Although the front element size does have some effect on lens speed it's obviously not everything.
Where did I say it's everything?

You said that:
I also don't see where the light is getting "stuck" inside of the lens barrel. My formula for aperture is focal length/exit pupil = fstop number.

This is just arithmatic and has nothing to do with the glass. F-Stops are inexact enough that professional movie lenses are tested individually and then marked not in F-Stops but in T (transmission) stops!
My lenses for some reason have something to do with the glass and depend on the laws of physics. Your lenses are arithmetic. :D

You seem to be very sure of what you are talking about. Therefore, if the aperture has nothing to do with the glass, I challenge you to give me an example of a lens whose front element is LESS than the focal length/aperture. Not more... less! :D

I am anxoisly waiting for that example.

[DocFrankenstein]

Still waiting....

[kawter2]

so far DocFrankenstein & DaveG are tied at 15ft 3 inches


Way to piss guys, I haven't seen a pissing match this good in a LOOOONG TIME!!!!!1!1

[DocFrankenstein]

WTH does that mean? :D

[Jon]

My formula for aperture is focal length/exit pupil = fstop number.

Your formula is wrong. It's the entry pupil not the exit pupil. The Entry pupil is what collects the light, and the lenses in front of the diaphragm are able to concentrate the light going through the diaphragm, so the effective aperture can be larger, and variable in size, than the physical diaphragm opening. As I've said several times before on this thread - look into the front of your lens as you zoom it. You'll see the apparent aperture size change. But nothing is happening to the diaphragm; it's all the glass moving around.

[DaveG]

I did misinterpret your post. I thought that you were saying that the front element was what defined the aperture. Big front element = faster lens.

My point about the aperture formula not having anything to do with glass is still appropriate I think. All f-stop formula IS arithmatic in the sense that it's theoretical. Why would the movie lenses be in T-stops if it was not? Of course you need glass too, but - based on real world movie lenses - theoretical f8 may not be the same as tested t8.

[DocFrankenstein]

My point about the aperture formula not having anything to do with glass is still appropriate I think.
:D:D:D:D
Fine! Be that way! :D ;)

[Jon]

My point about the aperture formula not having anything to do with glass is still appropriate I think. All f-stop formula IS arithmatic in the sense that it's theoretical. Why would the movie lenses be in T-stops if it was not? Of course you need glass too, but - based on real world movie lenses - theoretical f8 may not be the same as tested t8. T-stops also include light transmission losses due to reflection at lens interfaces, scatter and absorption within elements, and such. They were initially used in the movie industry because the lenses there tended to be more complicated than those used for still work, and because motion picture exposure is more critical (because of the need to match shots from different lenses and from different locations, and because it's a lot harder to correct exposure differences on 50 feet of 24 frames/second than it is on a single negative). As optical science has improved, the need for t-stops may have diminished, but old habits die hard (look at the number of people around here who'll still respond to ASA ratings), among other things. But the effective aperture, the projection of the diaphragm opening through the front of the lens, as influenced by the refractive properties of the lenses in front of it, is a calculable value, and can be empirically shown to work. Just take any 5 lenses with a given f/stop, meter a subject for the proper exposure with any one of them at the chosen f/stop, and you can use the same settings for all of the others to obtain a correct exposure. But, getting back to TQ's original question, the only way of getting a different, and faster, aperture value from one of these would be to dramatically alter the optical design of the lens. There's no simple override you can apply to any single component that will make the lens "faster" at a particular focal length (if you don't want to shred the image-making qualities).