From: DanP on
On 31 May, 02:47, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
wrote:
> We are talking about stars ... you know, as good as
> infinitely far away point sources as there are?  With
> perfectly parallel rays?

To talk about sorces of perfecly parallel rays is to talk about points
size zero.
For that the focal length has no meaning. a point of size 0 is focused
by all lens to a size zero image.

You have brought focal lengths to discussion though you did not
express that clearly.
I was talking about lens diameters.

>
> > And that means the amount of light let in depends only of exposure
> > time and f number. If bigger lenses would let more light in at the
> > same f number then the film would have been over exposed.
>
> We are not talking about areas, we are talking point sources.

But the above is my answer to the other question. You have snipped "In
case you were referring to lens diameter this is my answer: "

>
> >> >> EF 70-300mm f/4.5-5.6 DO IS USM:  82.4mm x  99.9mm
> >> >> EF 70-300mm f/4.5-5.6 DO IS USM: 82.4mm x 99.9mm
> >> >> Care to explain why the bigger (larger diameter) DO lens is shorter
> >> >> and thus obviously less "light gets astray"?  If your theory was
> >> >> right, shouldn't the DO lens be brighter (it's actually darker
> >> >> at the short end!) and/or longer?
>
> Still waiting for your explanation.

You have started this by questioned my statement "To close the
subject, after some reading and thinking lens diameter does not affect
the amount of light captured. "
Then I said the only thing that matters is the f number.

I think your comparision between EF 70-300mm f/4.5-5.6 DO IS USM:
82.4mm x 99.9mm and EF 70-300mm f/4.5-5.6 DO IS USM: 82.4mm x
99.9mm proves my point.
And the answer to that is internal optics, the image gets cropped by
internal optics at the lower end of zoom range.

> > Which now that you have pointed out my mistake (I should have looked
> > for 70mm) now have to be compared to 5.6 at 300mm
> > And to come back with the proper results the DO at 70 has a
> > theoretical maximum f number of  .85 vs .91 for the non DO.
> > Formula used is f=Focal length/Diameter of entrance pupil
> > Seehttp://en.wikipedia.org/wiki/F-number#Notation
>
> Nice formula, misplaced identification of the entrance pupil.

I can accept your objection if you to tell me what is the entrance
pupil for lenses.
I say it is the size of the diaphragm. For widest aperture you can
take the diaphragm out of the lenses if you want, it won't matter.
What matters then is only the diameter of the lenses.

> >> > So the DO is less brighter that what it should be (internal optics
> >> > should explain why).
> >> Come on, same focal length, you replace a thinner, longer lens
> >> with a fatter shorter one and all you can say is "internal optics
> >> should explain why"?  If that's the case your theory of light
> >> going astray is bogus.
> > If you do that then you lose aperture.
>
> Explain why.  "you replace a thinner, longer lens with a
> fatter shorter one" and "then you lose aperture."?

Because a smaller lense will not be able to use a wide diaphragm.

> > See the Sigma 120mm-300mm f/2.8http://www.sigmaphoto.com/shop/120-300mm-f28-ex-dg-apo-hsm-sigma
> > It has a size of 112.8 x 268.5 mm giving a theoretical f number of
> > 2.65. Good fast lens but expensive.
>
> So what's your point?

If you want low f numbers you must have big lenses. But big lenses in
itself does not guarantee more light.
Lenses are advertised by their f number, not lens diameter.

> > To prove me wrong show me a 300mm f/2.8 with a lens size smaller than
> > 107mm or 300mm f/5.6 with a lens size smaller than 53.5mm.
> > Or any size that has a theoretical f number bigger than the real one.
> > Anyone can make a big diameter slow lens.
>
> And that proves that a "fatter shorter" lens loses aperture?

I give up. Can YOU prove anything? So far I have seen no arguments,
only questions.

> >> And ... if you change the non-DO lens for the fatter DO lens,
> >> "the size of the aperture measured in mm/inch will be smaller
> >> (but f number is the same, f/16) ... because bigger lenses will
> >> be further away from the sensor and more light gets astray,
> >> therefore requiring a smaller aperture size (in mm or inch)
> >> for same f number" is completely wrong.
> > You are right. The diaphragm size is the same for all lenses a given
> > focal length and f number so I was completely wrong about that.
>
> Actually, you are wrong again: the diaphragm is not next to
> the single element of your theoretical lens.  It is internal.

I did not say is external.

A 35mm lens at f/8 will have a aperture of 35mm/8=4.375mm for any lens
diameter.
See the photo on the right http://en.wikipedia.org/wiki/Diaphragm_(optics).

I have just said you were right. What is the point in arguig with
THAT?




DanP
From: J. Caldwell on
On Tue, 1 Jun 2010 05:04:52 -0700 (PDT), DanP <dan.petre(a)gmail.com> wrote:

>
>If you want low f numbers you must have big lenses. But big lenses in
>itself does not guarantee more light.

Study the design of microscope objectives. Many of them having f-ratios
(NA: numerical apertures) far exceeding any large camera lenses.

You really don't know enough about optics to even be arguing these things.
You can't even formulate a proper question concerning the issues that you
are attempting to preach about.

IOW: Stop making a fool of yourself on a worldwide platform.



From: DanP on
On 1 June, 13:24, J. Caldwell <nos...(a)anyserver.net> wrote:
> On Tue, 1 Jun 2010 05:04:52 -0700 (PDT), DanP <dan.pe...(a)gmail.com> wrote:
>
> >If you want low f numbers you must have big lenses. But big lenses in
> >itself does not guarantee more light.
>
> Study the design of microscope objectives. Many of them having f-ratios
> (NA: numerical apertures) far exceeding any large camera lenses.

I have no interest in microscopes and do not know anything about them.
How will that help me to understand camera optics?

Microscope lenses are huge compared to the size of the image being
observed.

Do you find anything wrong with my statement "If you want low f
numbers you must have big lenses. But big lenses in
itself does not guarantee more light." to which you are replying?

> You really don't know enough about optics to even be arguing these things.
> You can't even formulate a proper question concerning the issues that you
> are attempting to preach about.

Where did you get this idea I am preaching something?
I have started with one question, now that Sony has managed to make
such a small camera, can they miniaturise the lenses as well?
And my conclusion is if they do they will have slower lenses.

> IOW: Stop making a fool of yourself on a worldwide platform.

I was never at any time rude or pompous or patronising, I have
listened to other arguments and answered questions.
A lot better that to tell people to go away.


DanP
From: Wolfgang Weisselberg on
DanP <dan.petre(a)gmail.com> wrote:
> On 31 May, 02:47, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
> wrote:
>> We are talking about stars ... you know, as good as
>> infinitely far away point sources as there are?  With
>> perfectly parallel rays?

> To talk about sorces of perfecly parallel rays is to talk about points
> size zero.
> For that the focal length has no meaning. a point of size 0 is focused
> by all lens to a size zero image.

Lenses are imperfect, there's diffraction (and air for us down
here) and stars are definitively larger than 0, just very far away.
Stop waffling and try it.

> You have brought focal lengths to discussion though you did not
> express that clearly.
> I was talking about lens diameters.

So, talk about lens diameter. Will a lens diameter of 10.000km
result in a darker star image than one of 0.0001mm?

Speak!

>> > And that means the amount of light let in depends only of exposure
>> > time and f number. If bigger lenses would let more light in at the
>> > same f number then the film would have been over exposed.

>> We are not talking about areas, we are talking point sources.

> But the above is my answer to the other question.

It's wrong. That's what I tried to point out, but you must
do the thinking.

>> >> >> EF 70-300mm f/4.5-5.6 DO IS USM:  82.4mm x  99.9mm
>> >> >> EF 70-300mm f/4.5-5.6 DO IS USM: 82.4mm x 99.9mm
>> >> >> Care to explain why the bigger (larger diameter) DO lens is shorter
>> >> >> and thus obviously less "light gets astray"?  If your theory was
>> >> >> right, shouldn't the DO lens be brighter (it's actually darker
>> >> >> at the short end!) and/or longer?

>> Still waiting for your explanation.

> You have started this by questioned my statement "To close the
> subject, after some reading and thinking lens diameter does not affect
> the amount of light captured. "
> Then I said the only thing that matters is the f number.

And you are wrong. T-stop and stars.

> I think your comparision between EF 70-300mm f/4.5-5.6 DO IS USM:
> 82.4mm x 99.9mm and EF 70-300mm f/4.5-5.6 DO IS USM: 82.4mm x
> 99.9mm proves my point.

That light gets astray? Suuure.

> And the answer to that is internal optics, the image gets cropped by
> internal optics at the lower end of zoom range.

The answer to that is that you abandoned your theory.

>> > Which now that you have pointed out my mistake (I should have looked
>> > for 70mm) now have to be compared to 5.6 at 300mm
>> > And to come back with the proper results the DO at 70 has a
>> > theoretical maximum f number of  .85 vs .91 for the non DO.
>> > Formula used is f=Focal length/Diameter of entrance pupil
>> > Seehttp://en.wikipedia.org/wiki/F-number#Notation

>> Nice formula, misplaced identification of the entrance pupil.

> I can accept your objection if you to tell me what is the entrance
> pupil for lenses.

That depends on the optical formula and isn't necessarily
inside the lens.

> I say it is the size of the diaphragm.

You are approximately right for single element lenses.

> For widest aperture you can
> take the diaphragm out of the lenses if you want, it won't matter.
> What matters then is only the diameter of the lenses.

But not necessarily directly.

>> >> > So the DO is less brighter that what it should be (internal optics
>> >> > should explain why).
>> >> Come on, same focal length, you replace a thinner, longer lens
>> >> with a fatter shorter one and all you can say is "internal optics
>> >> should explain why"?  If that's the case your theory of light
>> >> going astray is bogus.
>> > If you do that then you lose aperture.

>> Explain why.  "you replace a thinner, longer lens with a
>> fatter shorter one" and "then you lose aperture."?

> Because a smaller lense will not be able to use a wide diaphragm.

But the fatter shorter lens is WIDER. Hello?

>> > See the Sigma 120mm-300mm f/2.8http://www.sigmaphoto.com/shop/120-300mm-f28-ex-dg-apo-hsm-sigma
>> > It has a size of 112.8 x 268.5 mm giving a theoretical f number of
>> > 2.65. Good fast lens but expensive.

>> So what's your point?

> If you want low f numbers you must have big lenses.

Nope. Simple 1-element 10mm lens, just 1 cm across. Small
lens, low f/stop.

> But big lenses in
> itself does not guarantee more light.
> Lenses are advertised by their f number, not lens diameter.

Lower "f numbers" do not necessarily mean more light. Take a
f/2.8 lens and add a ND-filter to the design.

>> > To prove me wrong show me a 300mm f/2.8 with a lens size smaller than
>> > 107mm or 300mm f/5.6 with a lens size smaller than 53.5mm.
>> > Or any size that has a theoretical f number bigger than the real one.
>> > Anyone can make a big diameter slow lens.

>> And that proves that a "fatter shorter" lens loses aperture?

> I give up. Can YOU prove anything? So far I have seen no arguments,
> only questions.

I can prove lots of things, but I let YOU do the thinking.
It works better that way.

>> >> And ... if you change the non-DO lens for the fatter DO lens,
>> >> "the size of the aperture measured in mm/inch will be smaller
>> >> (but f number is the same, f/16) ... because bigger lenses will
>> >> be further away from the sensor and more light gets astray,
>> >> therefore requiring a smaller aperture size (in mm or inch)
>> >> for same f number" is completely wrong.
>> > You are right. The diaphragm size is the same for all lenses a given
>> > focal length and f number so I was completely wrong about that.

>> Actually, you are wrong again: the diaphragm is not next to
>> the single element of your theoretical lens.  It is internal.

> I did not say is external.

> A 35mm lens at f/8 will have a aperture of 35mm/8=4.375mm for any lens
> diameter.

But it will not have a diaphragm opening of 4.x mm in *all*
cases. That depends on the placement of the diaphragm.

-Wolfgang
From: DanP on
On 2 June, 02:00, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
wrote:
> DanP <dan.pe...(a)gmail.com> wrote:
> > On 31 May, 02:47, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
> > wrote:
> >> We are talking about stars ... you know, as good as
> >> infinitely far away point sources as there are?  With
> >> perfectly parallel rays?
> > To talk about sorces of perfecly parallel rays is to talk about points
> > size zero.
> > For that the focal length has no meaning. a point of size 0 is focused
> > by all lens to a size zero image.
>
> Lenses are imperfect, there's diffraction (and air for us down
> here) and stars are definitively larger than 0, just very far away.
> Stop waffling and try it.

So there is no source of light with perfect parallel rays.

> > You have brought focal lengths to discussion though you did not
> > express that clearly.
> > I was talking about lens diameters.
>
> So, talk about lens diameter.  Will a lens diameter of 10.000km
> result in a darker star image than one of 0.0001mm?

At maximum aperture the 10.000 km will give a brighter picture.
But set at the maximum aperurte of the .00001mm the results will be
the same.
To do that the diaphragm used for the 10.000km has to be set really
small.

I would like to have the big one because it can give me a lower f
number.
But do not think that bigger lenses give lower f numbers all the time,
your 2 examples show that.

> Speak!
>
> >> > And that means the amount of light let in depends only of exposure
> >> > time and f number. If bigger lenses would let more light in at the
> >> > same f number then the film would have been over exposed.
> >> We are not talking about areas, we are talking point sources.
> > But the above is my answer to the other question.
>
> It's wrong.  That's what I tried to point out, but you must
> do the thinking.

As far as getting the right exposure time (that depends on the amount
of light hitting the sensor) no one takes notice of lens diameter or
focal length, only f number.
Modern DSLR and SLR have a light sensor incorporated but before that
it was a separate universal light meter which worked with all camera
formats and lens sizes.


> > You have started this by questioned my statement "To close the
> > subject, after some reading and thinking lens diameter does not affect
> > the amount of light captured. "
> > Then I said the only thing that matters is the f number.
>
> And you are wrong.  T-stop and stars.
>
> > I think your comparision between EF 70-300mm f/4.5-5.6 DO IS USM:
> > 82.4mm x  99.9mm and EF 70-300mm f/4.5-5.6 DO IS USM:  82.4mm x
> > 99.9mm proves my point.
>
> That light gets astray?  Suuure.

It is OK and you notice it in the shallower DOF at low f numbers for
out of focus area.
But to get the same results having the same DOF you have to close the
aperture.

>
> > And the answer to that is internal optics, the image gets cropped by
> > internal optics at the lower end of zoom range.
>
> The answer to that is that you abandoned your theory.

I do not have a theory, it is common knowledge that when you buy a new
lens you don't care about the lens diameter, only f number.

> >> > Which now that you have pointed out my mistake (I should have looked
> >> > for 70mm) now have to be compared to 5.6 at 300mm
> >> > And to come back with the proper results the DO at 70 has a
> >> > theoretical maximum f number of  .85 vs .91 for the non DO.
> >> > Formula used is f=Focal length/Diameter of entrance pupil
> >> > Seehttp://en.wikipedia.org/wiki/F-number#Notation
> >> Nice formula, misplaced identification of the entrance pupil.
> > I can accept your objection if you to tell me what is the entrance
> > pupil for lenses.
>
> That depends on the optical formula and isn't necessarily
> inside the lens.
>
> > I say it is the size of the diaphragm.
>
> You are approximately right for single element lenses.

You talk like you know more yhan I do. Just tell me where is that the
entrance pupil and how to measure it.

> > For widest aperture you can
> > take the diaphragm out of the lenses if you want, it won't matter.
> > What matters then is only the diameter of the lenses.
>
> But not necessarily directly.
>
> >> >> > So the DO is less brighter that what it should be (internal optics
> >> >> > should explain why).
> >> >> Come on, same focal length, you replace a thinner, longer lens
> >> >> with a fatter shorter one and all you can say is "internal optics
> >> >> should explain why"?  If that's the case your theory of light
> >> >> going astray is bogus.
> >> > If you do that then you lose aperture.
> >> Explain why.  "you replace a thinner, longer lens with a
> >> fatter shorter one" and "then you lose aperture."?
> > Because a smaller lense will not be able to use a wide diaphragm.
>
> But the fatter shorter lens is WIDER.  Hello?

You are mixing things too much, keep focal length the same to compare
lenses.

> >> > See the Sigma 120mm-300mm f/2.8http://www.sigmaphoto.com/shop/120-300mm-f28-ex-dg-apo-hsm-sigma
> >> > It has a size of 112.8 x 268.5 mm giving a theoretical f number of
> >> > 2.65. Good fast lens but expensive.
> >> So what's your point?
> > If you want low f numbers you must have big lenses.
>
> Nope.  Simple 1-element 10mm lens, just 1 cm across.  Small
> lens, low f/stop.

Yeah, that is for a small sensor. To compare lenses keep things equal,
same sensor size.

> > But big lenses in
> > itself does not guarantee more light.
> > Lenses are advertised by their f number, not lens diameter.
>
> Lower "f numbers" do not necessarily mean more light.  Take a
> f/2.8 lens and add a ND-filter to the design.

Fine, put your hand in front of the lenses and you get even less
light.

> >> > To prove me wrong show me a 300mm f/2.8 with a lens size smaller than
> >> > 107mm or 300mm f/5.6 with a lens size smaller than 53.5mm.
> >> > Or any size that has a theoretical f number bigger than the real one..
> >> > Anyone can make a big diameter slow lens.
> >> And that proves that a "fatter shorter" lens loses aperture?
> > I give up. Can YOU prove anything? So far I have seen no arguments,
> > only questions.
>
> I can prove lots of things, but I let YOU do the thinking.
> It works better that way.
>
> >> >> And ... if you change the non-DO lens for the fatter DO lens,
> >> >> "the size of the aperture measured in mm/inch will be smaller
> >> >> (but f number is the same, f/16) ... because bigger lenses will
> >> >> be further away from the sensor and more light gets astray,
> >> >> therefore requiring a smaller aperture size (in mm or inch)
> >> >> for same f number" is completely wrong.
> >> > You are right. The diaphragm size is the same for all lenses a given
> >> > focal length and f number so I was completely wrong about that.
> >> Actually, you are wrong again: the diaphragm is not next to
> >> the single element of your theoretical lens.  It is internal.
> > I did not say is external.
> > A 35mm lens at f/8 will have a aperture of 35mm/8=4.375mm for any lens
> > diameter.
>
> But it will not have a diaphragm opening of 4.x mm in *all*
> cases.  That depends on the placement of the diaphragm.

Can you find me an example?


DanP