Patent: Optical Formula for New EF 24-70mm f/2.8L

[Canon Rumors Guy]

<a href="http://www.northlight-images.co.uk/canon-ef-lenses-rumours-and-news/">Northlight</a> has uncovered a patent relating to a zoom lens design with two aperture stops.</p>
<p>Within the patent are two different optical formulas, one for an EF 16-35mm f/2.8 and another for an EF 24-70 f/2.8.</p>
<p><strong>According to Northlight:</strong></p>
<blockquote><p>The aim is to produce a zoom lens with constant minimum aperture, but having small variations in lens aberrations when focusing near/far. The second aperture stop is varied as the lens zooms.</p></blockquote>

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[KirkD]

If that 16-35 f2.8L is as good optically as the Mark III, but has IS, I'll be getting one. I presently use Canon's 16-35 f4L IS.

 

[SV]

Yawn..Yet another Canon patent for a 24-70mm with nothing to show for it.

We should count how many Canon 24-70mm patents have been published since their version II f/2.8 has been out...

 

[melgross]

Is anybody else confused by the description, of constant minimum aperture, but variable 2nd stop? What am I missing?

 

[BeenThere]

Is anybody else confused by the description, of constant minimum aperture, but variable 2nd stop? What am I missing?

Trying to do something to control lens aberrations caused by focal length changes. Sounds like there is a second iris not at the aperature plane that changes slightly with focus etting. That is just a guess, so take with a grain of salt. I think one would have to be into optics design to understand how or why this works.
Probably just protecting the IP with a patent at this point.

 

[keithcooper]

The description here is obviously a short precis of the patent itself - like all of them, they are probably not written for the average CR punter ;-) ;-) I found the original and had to read through several times to see how this was something distinctly new in EF lens design. Go to the source before dismissing it as nothing much?
--
From looking at the details...

The second aperture stop only covers a limited range of operation - it is linked to focus and zoom. The graph gives one example of how it changes with lens settings.

The second aperture stop works differently to the normal one in that it's entirely automatic.

The perpetual design problem of any zoom is that it's a collection of compromises for focus and focal length.

If you want a constant aperture as well as good performance over the range then it gets even more tricky.

The additional aperture is an interesting design variation

 

[neuroanatomist]

Trying to do something to control lens aberrations caused by focal length changes. Sounds like there is a second iris not at the aperature plane that changes slightly with focus etting.

Close, the idea is the aperture changes with focal length (i.e. zooming) and focusing. Typical constant aperture zoom lenses have the size of the iris diaphragm fixed, and as the focal length is increased the elements in front of the aperture move to increase magnification, which maintains a constant size of the entrance pupil (which is really the basis for the f/number, not the actual iris diaphragm diameter). The result of the greater magnification at the long end of a zoom lens is that some optical aberrations (particularly astigmatism) are more evident (screenshot from the patent below, 7A is the wide end, 7B is the tele end). Close focus has a similar effect of increasing the impact of optical aberrations (particularly spherical aberration). The patent indicates that adding a secondary iris diaphragm that varies in diameter with zooming and focusing will reduce the effects of zooming/focusing on the aberrations.

 

[Daan Stam]

Canon CEO right now: "why not just add another shade of black?''

 

[FramerMCB]

Canon CEO right now: "why not just add another shade of black?''

;D

 

[RGF]

Trying to do something to control lens aberrations caused by focal length changes. Sounds like there is a second iris not at the aperature plane that changes slightly with focus etting.

Close, the idea is the aperture changes with focal length (i.e. zooming) and focusing. Typical constant aperture zoom lenses have the size of the iris diaphragm fixed, and as the focal length is increased the elements in front of the aperture move to increase magnification, which maintains a constant size of the entrance pupil (which is really the basis for the f/number, not the actual iris diaphragm diameter). The result of the greater magnification at the long end of a zoom lens is that some optical aberrations (particularly astigmatism) are more evident (screenshot from the patent below, 7A is the wide end, 7B is the tele end). Close focus has a similar effect of increasing the impact of optical aberrations (particularly spherical aberration). The patent indicates that adding a secondary iris diaphragm that varies in diameter with zooming and focusing will reduce the effects of zooming/focusing on the aberrations.

Thanks for the clear explanation.

At least this update is more than optical coatings.

 

[Talys]

Sounds cool, exciting stuff. This is a FL where people who need/want it will break out their wallets to get precisely what they want, I think.

 

[melgross]

Is anybody else confused by the description, of constant minimum aperture, but variable 2nd stop? What am I missing?

Trying to do something to control lens aberrations caused by focal length changes. Sounds like there is a second iris not at the aperature plane that changes slightly with focus etting. That is just a guess, so take with a grain of salt. I think one would have to be into optics design to understand how or why this works.
Probably just protecting the IP with a patent at this point.

I had a year of optics, but the description is odd to me.

 

[melgross]

Trying to do something to control lens aberrations caused by focal length changes. Sounds like there is a second iris not at the aperature plane that changes slightly with focus etting.

Close, the idea is the aperture changes with focal length (i.e. zooming) and focusing. Typical constant aperture zoom lenses have the size of the iris diaphragm fixed, and as the focal length is increased the elements in front of the aperture move to increase magnification, which maintains a constant size of the entrance pupil (which is really the basis for the f/number, not the actual iris diaphragm diameter). The result of the greater magnification at the long end of a zoom lens is that some optical aberrations (particularly astigmatism) are more evident (screenshot from the patent below, 7A is the wide end, 7B is the tele end). Close focus has a similar effect of increasing the impact of optical aberrations (particularly spherical aberration). The patent indicates that adding a secondary iris diaphragm that varies in diameter with zooming and focusing will reduce the effects of zooming/focusing on the aberrations.

Ok, reading that, and the illustration clears it up. But it’s a complex way of doing it. It could also have been accomplished with an additional internal focusing group.

 

[neuroanatomist]

Ok, reading that, and the illustration clears it up. But it’s a complex way of doing it. It could also have been accomplished with an additional internal focusing group.

Partially, but a floating focus group wouldn't help with the astigmatism at the long end of the zoom range.

Incidentally, your earlier confusion over the 'constant minimum aperture' may have resulted from Keith/Northlight's paraphrasing. The actual patent refers to a 'constant minimum f-number', and since the f-number is lowest when the aperture is widest, that's just another way of stating constant maximum aperture.

 

[melgross]

Ok, reading that, and the illustration clears it up. But it’s a complex way of doing it. It could also have been accomplished with an additional internal focusing group.

Partially, but a floating focus group wouldn't help with the astigmatism at the long end of the zoom range.

Incidentally, your earlier confusion over the 'constant minimum aperture' may have resulted from Keith/Northlight's paraphrasing. The actual patent refers to a 'constant minimum f-number', and since the f-number is lowest when the aperture is widest, that's just another way of stating constant maximum aperture.

Yes. That wording was confusing. And extra diaphragm might not help that either, and could result in interference. The rays will be lassi g through two diaphragms, and depending on the setting ratio, there could be a lot of diffraction.

 

[keithcooper]

> Yes. That wording was confusing. And extra diaphragm might not help that either, and could result in interference.
> The rays will be lassi g through two diaphragms, and depending on the setting ratio, there could be a lot of diffraction.

Perhaps not the issue you think, since the secondary aperture has a lot of leaves, has a very limited range and is not necessarily in a place to cause any more diffraction problems than the normal edge of a lens element.

Complex yes, but there are people who will pay for that extra bit of performance ;-)

Unfortunately this one also goes well beyond my own limits of optical design theory :-(
I check the patents regularly, but have to be in the right frame of mind to plough through all the patent speak...

 

[melgross]

> Yes. That wording was confusing. And extra diaphragm might not help that either, and could result in interference.
> The rays will be lassi g through two diaphragms, and depending on the setting ratio, there could be a lot of diffraction.

Perhaps not the issue you think, since the secondary aperture has a lot of leaves, has a very limited range and is not necessarily in a place to cause any more diffraction problems than the normal edge of a lens element.

Complex yes, but there are people who will pay for that extra bit of performance ;-)

Unfortunately this one also goes well beyond my own limits of optical design theory :-(
I check the patents regularly, but have to be in the right frame of mind to plough through all the patent speak...

I’m not doing my usual due diligence. I was in the hospital for close to three weeks, and have to do a fun meditation routine three times a day, so I’m just too tired to pursue detail right now. I’m not trolling for sympathy, but just saying that I’m not thinking as straight as normal.

 

[djack41]

Watching Canon is like watching a glacier.........imperceptibly slow.

 

[neuroanatomist]

Watching Canon is like watching a glacier.........imperceptibly slow.

And yet in the end, glaciers crush everything in their path and change the face of the landscape forever.

Nice analogy, even if in a way you didn’t plan...

 

[jolyonralph]

And yet in the end, glaciers crush everything in their path and change the face of the landscape forever.

Nice analogy, even if in a way you didn’t plan...

Unless a change in climate makes them melt