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On 08/11/2012 00:27, Rich wrote:
> $1800 for an 85mm f1.4 from Nikon. That's about 2x what the old one cost.
> Is the lens $1000 better or should it be as good at f/1.4 as f/4? I'm not
> sure. I know that some optics made as f/4.0 have been diffraction limited.
> Pentax had some, but they weren't camera lenses. Some have claimed certain
> telephotos in the pro bracket have been diffraction-limited at f/2.8, but
> I've never seen it demonstrated. So, the question is, is it possible to
> make say a 35mm, 85mm diffraction-limited at f/1.4 and if so, at what
> price? Likely it is, but they haven't done it.

f1.6 and full achromatic mirror telescope has been done by Cambridge
University.

http://www.ast.cam.ac.uk/about/three-mirror.telescope

20" aperture prototype was built. That is one of the fastest diffraction
limited wide field instruments I know of.

You are hampered in real cameras by simultaneously wanting diffraction
limited and a flat film plane when the lens is fast and the small angle
approximations no longer hold. There is always a trade off.

Anything can be done in principle but the cost to manufacture it and
difficulties in assembly make it prohibitive. You could get the on axis
sharpness truly diffraction limited by sacrificing edge resolution but
never all at the same time and a flat film plane. Something has to give.

At around f4 or f5 things are a lot easier. Most real lenses tend to
have their resolution sweet spot at about that working aperture.

--
Regards,
Martin Brown

On Nov 8, 5:49*am, Martin Brown <|||newspam...@nezumi.demon.co.uk>
wrote:
> On 08/11/2012 00:27, Rich wrote:
>
> > $1800 for an 85mm f1.4 from Nikon. *That's about 2x what the old one cost.
> > Is the lens $1000 better or should it be as good at f/1.4 as f/4? *I'm not
> > sure. *I know that some optics made as f/4.0 have been diffraction limited.
> > Pentax had some, but they weren't camera lenses. *Some have claimed certain
> > telephotos in the pro bracket have been diffraction-limited at f/2.8, but
> > I've never seen it demonstrated. So, the question is, is it possible to
> > make say a 35mm, 85mm diffraction-limited at f/1.4 and if so, at what
> > price? *Likely it is, but they haven't done it.
>
> f1.6 and full achromatic mirror telescope has been done by Cambridge
> University.
>
> http://www.ast.cam.ac.uk/about/three-mirror.telescope
>
> 20" aperture prototype was built. That is one of the fastest diffraction
> limited wide field instruments I know of.
>
> You are hampered in real cameras by simultaneously wanting diffraction
> limited and a flat film plane when the lens is fast and the small angle
> approximations no longer hold. There is always a trade off.
>
> Anything can be done in principle but the cost to manufacture it and
> difficulties in assembly make it prohibitive. You could get the on axis
> sharpness truly diffraction limited by sacrificing edge resolution but
> never all at the same time and a flat film plane. Something has to give.
>
> At around f4 or f5 things are a lot easier. Most real lenses tend to
> have their resolution sweet spot at about that working aperture.
>
> --
> Regards,
> Martin Brown

Only problem, with a central obstruction like it has, contrast would
suffer.

RichA <> wrote:
> On Nov 8, 5:49*am, Martin Brown <|||newspam...@nezumi.demon.co.uk>
> wrote:
>> On 08/11/2012 00:27, Rich wrote:
>>
>> > $1800 for an 85mm f1.4 from Nikon. *That's about 2x what the old one cost.
>> > Is the lens $1000 better or should it be as good at f/1.4 as f/4? *I'm not
>> > sure. *I know that some optics made as f/4.0 have been diffraction limited.
>> > Pentax had some, but they weren't camera lenses. *Some have claimed certain
>> > telephotos in the pro bracket have been diffraction-limited at f/2.8, but
>> > I've never seen it demonstrated. So, the question is, is it possible to
>> > make say a 35mm, 85mm diffraction-limited at f/1.4 and if so, at what
>> > price? *Likely it is, but they haven't done it.
>>
>> f1.6 and full achromatic mirror telescope has been done by Cambridge
>> University.
>>
>> http://www.ast.cam.ac.uk/about/three-mirror.telescope
>>
>> 20" aperture prototype was built. That is one of the fastest diffraction
>> limited wide field instruments I know of.
>>
>> You are hampered in real cameras by simultaneously wanting diffraction
>> limited and a flat film plane when the lens is fast and the small angle
>> approximations no longer hold. There is always a trade off.
>>
>> Anything can be done in principle but the cost to manufacture it and
>> difficulties in assembly make it prohibitive. You could get the on axis
>> sharpness truly diffraction limited by sacrificing edge resolution but
>> never all at the same time and a flat film plane. Something has to give.
>>
>> At around f4 or f5 things are a lot easier. Most real lenses tend to
>> have their resolution sweet spot at about that working aperture.
>>
>> --
>> Regards,
>> Martin Brown

> Only problem, with a central obstruction like it has, contrast would
> suffer.

It should be possible with today's lens and mirror making technology
to devise a mirror which instead of reflecting straight back, offset
the folded beam offset to one side, thus avoiding the need for the
obstruction.

--
Chris Malcolm

On 09/11/2012 23:39, Chris Malcolm wrote:
> RichA <> wrote:
>> On Nov 8, 5:49 am, Martin Brown <|||newspam...@nezumi.demon.co.uk>
>> wrote:
>>> On 08/11/2012 00:27, Rich wrote:
>>>
>>>> $1800 for an 85mm f1.4 from Nikon. That's about 2x what the old one cost.
>>>> Is the lens $1000 better or should it be as good at f/1.4 as f/4? I'm not
>>>> sure. I know that some optics made as f/4.0 have been diffraction limited.
>>>> Pentax had some, but they weren't camera lenses. Some have claimed certain
>>>> telephotos in the pro bracket have been diffraction-limited at f/2.8, but
>>>> I've never seen it demonstrated. So, the question is, is it possible to
>>>> make say a 35mm, 85mm diffraction-limited at f/1.4 and if so, at what
>>>> price? Likely it is, but they haven't done it.
>>>
>>> f1.6 and full achromatic mirror telescope has been done by Cambridge
>>> University.
>>>
>>> http://www.ast.cam.ac.uk/about/three-mirror.telescope
>>>
>>> 20" aperture prototype was built. That is one of the fastest diffraction
>>> limited wide field instruments I know of.
>>>
>>> You are hampered in real cameras by simultaneously wanting diffraction
>>> limited and a flat film plane when the lens is fast and the small angle
>>> approximations no longer hold. There is always a trade off.
>>>
>>> Anything can be done in principle but the cost to manufacture it and
>>> difficulties in assembly make it prohibitive. You could get the on axis
>>> sharpness truly diffraction limited by sacrificing edge resolution but
>>> never all at the same time and a flat film plane. Something has to give.
>>>
>>> At around f4 or f5 things are a lot easier. Most real lenses tend to
>>> have their resolution sweet spot at about that working aperture.
>
>> Only problem, with a central obstruction like it has, contrast would
>> suffer.
>
> It should be possible with today's lens and mirror making technology
> to devise a mirror which instead of reflecting straight back, offset
> the folded beam offset to one side, thus avoiding the need for the
> obstruction.

It can be done but the folded mirror designs are a bit exotic and only
valid for slow focal ratios like f10. An example:

http://bhs.broo.k12.wv.us/homepage/a...tevick/fsp.htm


--
Regards,
Martin Brown