# Constant F-stop Zoom - how?



## hartz (Oct 5, 2010)

Hello,

How can constant F-stop zoom lenses be possible?  I've been googling the topic and fortunately I see that many others have asked the same question.

The answers they usually get make my head hurt.  I've read a few ... seems to have something to do with the fact that "Apterture" is not directly related to the size of the front lens element?  And sometimes it can be larger than the front lense physically is?  Maybe it is a virtual aperture?  

I can not grok this.  So maybe I need pictures!  Or at least a very concise and simple explanation!

Thank you,
  _Joha


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## Infidel (Oct 5, 2010)

If found this to be helpful: Digital Grin Photography Forum - View Single Post - Constant vs. Variable Aperture zooms

Also, remember that lens aperture is ultimately determined by the apparent size of the diaphragm opening, not just the size of the front element.


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## KmH (Oct 5, 2010)

hartz said:


> ......... I've read a few ... seems to have something to do with the fact that "Apterture" is not directly related to the size of the front lens element? And sometimes it can be larger than the front lense physically is? Maybe it is a virtual aperture?...


Aperture is a function of the lens focal length, and can never be larger than the front lens element.

Lets consider a lens that can zoom from 50 mm to 100 mm. Lets also assume the maximum aperture this lens can achieve is f/4.

F/4 means the maximum aperture diameter will be 1/4th of the focal length the lens is set to. At 50 mm the diameter of the aperture will be 50 mm / 4 = 12.5 mm. At 100 mm the diameter of the aperture will be 100 mm / 4 = 25 mm.

Lets look at a real world example: a Nikon 80-200 f/2.8 zoom lens. The lens has a front element that has about 73 mm of it exposed.

At 80 mm the maximum lens aperture is 28.57 mm, nowhere close to 73 mm. At 200 mm the maximum lens aperture is 71.4 mm, not quite the 73mm of the front lens element. By the way, that lens uses 77 mm screw-in filters.


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## Big Mike (Oct 5, 2010)

It's a bit of a misnomer, because when we see a zoom lens with a 'constant' aperture of F2.8, the actual max size of the aperture varies as you zoom...to keep the 'ratio' consistent.  Remember that the F number is a ratio between the focal length and the size of the entrance pupil.  

But when you have a cheaper lens, that has a 'variable' max aperture...something like F3.5 to F5.6, it's actually because the size of the aperture doesn't change as you zoom...but because the focal length does change, the ratio changes, and thus, the F number has to change.


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## prodigy2k7 (Oct 5, 2010)

Big Mike said:


> It's a bit of a misnomer, because when we see a zoom lens with a 'constant' aperture of F2.8, the actual max size of the aperture varies as you zoom...to keep the 'ratio' consistent.  Remember that the F number is a ratio between the focal length and the size of the entrance pupil.
> 
> But when you have a cheaper lens, that has a 'variable' max aperture...something like F3.5 to F5.6, it's actually because the size of the aperture doesn't change as you zoom...but because the focal length does change, the ratio changes, and thus, the F number has to change.


Its all confusing to me haha...
Checkout my EF 75-300 F/4-5.6
at 300mm I select F/5.6 and hold the DoF preview button, (its wide open, cant see the blades), then you start to zoom out to 75mm, at F/5.6 and is no longer "wide open" and you can see the aperture blades start to come out.

If I look into my 70-200 F/2.8, I set aperture wide open at 2.8 and zoom, and the aperture blades dont change physical size or move.

Which is pretty cool but still dont understand the difference. I know there is something optically different in design.



Big Mike said:


> It's a bit of a misnomer, because when we see a  zoom lens with a 'constant' aperture of F2.8, *the actual max size of the  aperture varies as you zoom*....



In what way? physically? or just "as seen through the front of the lens" such as magnification?


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## Derrel (Oct 5, 2010)

"Science!"


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## prodigy2k7 (Oct 5, 2010)

Derrel said:


> "Science!"


LOL, are you just tired of talking about lenses and optics? I know you know the answer!


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## supraman215 (Oct 5, 2010)

hartz said:


> ...at least a very concise and simple explanation!
> 
> Thank you,
> _Joha



With higher quality, larger glass you can make a lens have a continuous fstop through the range. 

LOL 

Wouldn't my variable max-fstop, 18-105, f3.5-5.6 be equivalent to a 18-105 f/5.6 continuous?


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## table1349 (Oct 5, 2010)

Don't they teach physics in schools these days?? 

More than you will ever want to know.

Amazon.com: Lens Design Fundamentals (9780124086500): Rudolf Kingslake: Books

Amazon.com: Lens Design, Third Edition, (Optical Science and Engineering) (9780824705077): Milton Laikin: Books


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## GeneralBenson (Oct 7, 2010)

KmH said:


> hartz said:
> 
> 
> > ......... I've read a few ... seems to have something to do with the fact that "Apterture" is not directly related to the size of the front lens element? And sometimes it can be larger than the front lense physically is? Maybe it is a virtual aperture?...
> ...



So here is something I've always wondered...

If said 80-200/2.8 can open up to 71.4 mm to be f/2.8 at 200mm FL, then why can't it open up to 71.4 mm at 80mm FL to be f/1.2 or whatever that would work out to?


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## prodigy2k7 (Oct 7, 2010)

GeneralBenson said:


> KmH said:
> 
> 
> > hartz said:
> ...



Cuz the physical size of the aperture doesn't change. It's size of the magnification of the image of the aperture that changes. I think lol


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## RobWyse (Oct 7, 2010)

GeneralBenson said:


> So here is something I've always wondered...
> 
> If said 80-200/2.8 can open up to 71.4 mm to be f/2.8 at 200mm FL, then why can't it open up to 71.4 mm at 80mm FL to be f/1.2 or whatever that would work out to?


Because then we'd all be clamouring for a constant 1.2 zoom . Probably doesn't help that having 2.8 constant looks more professional than variable length, even if it was 1.2-2.8. 



prodigy2k7 said:


> Cuz the physical size of the aperture doesn't change. It's size of the  magnification of the image of the aperture that changes. I think  lol


With a constant aperture zoom the physical size does change. The aperture will be physically smaller at 70mm than it will at 200mm, even though the aperture is still 2.8 for both.

The physical size won't change with a variable aperture zoom, the physical size will be the same at 70mm and 300mm, so the aperture changes from 4-5.6.

The aperture is determined by the focal length of the lens divided by the physical size of the aperture opening. So a 50mm f/1 has a 50mm aperture opening, but if stepped down to f/2, that physical opening size becomes 25mm.


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## prodigy2k7 (Oct 7, 2010)

RobWyse said:


> GeneralBenson said:
> 
> 
> > So here is something I've always wondered...
> ...



 There is a lot of mis-information on the Internet about how "constant aperture" zooms work, but the most lucid explanation comes from Bob Shell:

Comments from Bob Shell (January 8, 2003):     "An f-stop is the ratio between the focal length of the lens and the *apparent* size of the lens opening as viewed through the front. It must take into account the magnification factor of all lens elements in front of the diaphragm, because it is the size of the opening that the light "sees" as it passes through the lens, not the actual physical diameter of the diaphragm opening.     It is this fact that allows companies to make constant aperture zoom lenses which maintain a constant f-stop when the focal length changes, because such lenses are designed so that the magnification factor (diopter value) of all elements in front of the diaphragm changes as focal length is changed to hold the aperture value constant."


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## Garbz (Oct 8, 2010)

I can't believe that no one mentioned that. It's not the physical size of the aperture blades. It's the apparent size of them that changes when you zoom.

Just look in the front of your lens and start zooming.


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## KmH (Oct 8, 2010)

GeneralBenson said:


> So here is something I've always wondered...
> 
> If said 80-200/2.8 can open up to 71.4 mm to be f/2.8 at 200mm FL, then why can't it open up to 71.4 mm at 80mm FL to be f/1.2 or whatever that would work out to?


Do the math. :lmao: The front lens element diameter is determined by the maximum aperture at the longest focal length.

With the focal length at 80 mm and the lens opening at 71.4 mm, the lens is not longer at f/2.8, it's now f/1.1 and to be a constant aperture zoom, at 200 mm the lens opening would be 181.8 mm. That's a lot bigger than 71.4 mm.


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## Helen B (Oct 8, 2010)

Garbz said:


> I can't believe that no one mentioned that. It's not the physical size of the aperture blades. It's the apparent size of them that changes when you zoom.



It was mentioned in posts #2, #4 and a few others.

It's one of the reasons why it is important to explain that the f-number is calculated from the diameter of the entrance pupil, and not from the physical diameter of the iris.

One could say that there are four classes of zoom lens, as far as aperture goes:

Variable aperture, in which the iris does not change physical size when the lens is zoomed. The movement of the front elements does change the magnification of the iris (ie the diameter of the entrance pupil changes), therefore the ratio of f-numbers at the extremities of the zoom range does not match (is not inversely proportional to) the corresponding change in focal length.

Variable maximum aperture, constant thereafter. For example, a fast movie zoom might be f/1.6 max aperture when wide, f/2.2 when zoomed in, but be constant aperture beyond f/2.2. These used to be quite common, and they help when using an external lightmeter. The iris does not move when the lens is wide open, and therefore there is a change in the maximum aperture from one end of the zoom range to another. After that either the lens design (magnification of the iris) or that coupled with iris movement keeps the aperture constant.

Constant maximum aperture at all apertures, using a combination of iris movement and variable magnification of the iris.

Constant maximum aperture at all apertures, only using variable magnification of the iris.

Best,
Helen


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## Infidel (Oct 8, 2010)

prodigy2k7 said:


> There is a lot of mis-information on the Internet about how "constant aperture" zooms work, but the most lucid explanation comes from Bob Shell:
> 
> Comments from Bob Shell (January 8, 2003):     "An f-stop is the ratio between the focal length of the lens and the *apparent* size of the lens opening as viewed through the front. It must take into account the magnification factor of all lens elements in front of the diaphragm, because it is the size of the opening that the light "sees" as it passes through the lens, not the actual physical diameter of the diaphragm opening.     It is this fact that allows companies to make constant aperture zoom lenses which maintain a constant f-stop when the focal length changes, because such lenses are designed so that the magnification factor (diopter value) of all elements in front of the diaphragm changes as focal length is changed to hold the aperture value constant."



Just for the record, this is the _exact_ content from the link I posted earlier in the thread.


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## prodigy2k7 (Oct 8, 2010)

Infidel said:


> prodigy2k7 said:
> 
> 
> > There is a lot of mis-information on the Internet about how "constant aperture" zooms work, but the most lucid explanation comes from Bob Shell:
> ...


So it is. Too bad nobody read it (including me) haha...
I actually found it through a google search. Same forum post.


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## Helen B (Oct 8, 2010)

It's not a complete answer, however. As I mentioned, not all constant aperture zooms achieve constant aperture solely by changing the magnification of the iris.

Best,
Helen


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## prodigy2k7 (Oct 8, 2010)

Helen B said:


> It's not a complete answer, however. As I mentioned, not all constant aperture zooms achieve constant aperture solely by changing the magnification of the iris.
> 
> Best,
> Helen


Nice to see you on the forums! I love reading your posts.

Isn't another way is to move the physical location of the aperture forward or backward?


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## epatsellis (Oct 8, 2010)

not likely, a better way would be to cam the actual iris opening to the zoom setting, varying the actual iris diameter.


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## prodigy2k7 (Oct 8, 2010)

epatsellis said:


> not likely, a better way would be to cam the actual iris opening to the zoom setting, varying the actual iris diameter.



Can you explain this further please? I would like to know more.


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## Garbz (Oct 9, 2010)

I think he's referring to physically making the iris open wider for a given focal length. 

I'm not quite sure how that would work though. Size restrictions aside if you can do that it sounds like you'd be actively limiting your max aperture at the telephoto side if you do. Kind of like turning an 18-55 f/3.5-4.5 into an 18-55 f/4.5. If I'm envisioning this correctly.


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