Constant F-stop Zoom - how?

[hartz]

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

 

[Infidel]

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.

 

[KmH]

......... 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.

 

[Big Mike]

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.

 

[prodigy2k7]

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.

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?

 

[Derrel]

"Science!"

 

[prodigy2k7]

"Science!"

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

 

[supraman215]

...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?

 

[table1349]

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

 

[GeneralBenson]

......... 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.

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?

 

[prodigy2k7]

......... 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.

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?

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

 

[RobWyse]

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.

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.

 

[prodigy2k7]

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.

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.

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."

 

[Garbz]

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.

 

[KmH]

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.