From: Wolfgang Weisselberg on
DanP <dan.petre(a)gmail.com> wrote:
> On 17 May, 19:48, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
>> DanP <dan.pe...(a)hotmail.com> wrote:

>> > To close the subject, after some reading and thinking lens diameter
>> > does not affect the amount of light captured.

>> But it does.  Take a distant star --- all lightrays are for all
>> purposes of a lens or telescope completely parallel here on Earth.
>> Obviously a larger lens diameter means a larger area and thus more
>> rays i.e. more light is captured.  (I understand that's one of
>> the reasons Canon's 200mm f/1.8 are popular for some computerized
>> skywatching tasks: comparatively large front lens at a manageable
>> pricepoint.)

> That is what I thought initially because is valid for telescopes and
> binoculars.
> Bigger binoculars lenses take in more light and have a smaller DOF
> (not a problem, manual focus on target).

> But camera lenses have internal apertures which come into play.

You can artifically restrict the effective front lens size.
This doesn't mean that the 200mm f/1.8 lens wide open will collect
the same amount of light as a 200mm f/5.6 lens wide open.

Does a 200mm lens collect the same amount of light from that star
as a 30mm even at the same aperture?

> I have heard here about the sunny f16 rule which say on a sunny day
> using f/16 the exposure time be the inverse of ISO number (if ISO is
> 400 then use 1/400 sec).

Irrelevant.

> So if change the lenses with a bigger diameter one everything else is
> the same but the size of the aperture measured in mm/inch will be
> smaller (but f number is the same, f/16).
> This is 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.

Huh.
EF 70-300mm f/4.5-5.6 DO IS USM: 82.4mm x 99.9mm
EF 70-300mm f/4 -5.6 IS USM: 76.5mm x 142.8mm

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?

-Wolfgang
From: DanP on
On May 30, 5:29 am, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
wrote:
> DanP <dan.pe...(a)gmail.com> wrote:

> Does a 200mm lens collect the same amount of light from that star
> as a 30mm even at the same aperture?

Yes. At 30mm all the light is concentrated in one overexposed spot.
At 200mm it is spread over a larger area and is fainter.

> > So if change the lenses with a bigger diameter one everything else is
> > the same but the size of the aperture measured in mm/inch will be
> > smaller (but f number is the same, f/16).
> > This is 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.
>
> Huh.
> EF 70-300mm f/4.5-5.6 DO IS USM:  82.4mm x  99.9mm
> EF 70-300mm f/4  -5.6 IS USM:     76.5mm x 142.8mm
>
> 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?

Both lenses achieve a focal length of 300mm with considerably shorter
overall lengths, 99.9mm and 142.8mm.
Internal optics make it possible and frankly I do not know how they
work but when I seeing a shorter overall length I would ask where is
the catch.

Let me explain why I cannot comment on your example.
The theoretical f number for the DO f/4.5 one is 300mm/82.4mm=3.64
(missing 1 stop here).
The other one with f/4 should have a f number of 300mm/76.5=3.92
(close enough).

So the DO is less brighter that what it should be (internal optics
should explain why).


DanP
From: Wolfgang Weisselberg on
DanP <dan.petre(a)hotmail.com> wrote:
> On May 30, 5:29 am, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
>> DanP <dan.pe...(a)gmail.com> wrote:

>> Does a 200mm lens collect the same amount of light from that star
>> as a 30mm even at the same aperture?

> Yes. At 30mm all the light is concentrated in one overexposed spot.
> At 200mm it is spread over a larger area and is fainter.

Interesting. What theory leads you to such a prediction?

>> > So if change the lenses with a bigger diameter one everything else is
>> > the same but the size of the aperture measured in mm/inch will be
>> > smaller (but f number is the same, f/16).
>> > This is 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.

>> Huh.
>> EF 70-300mm f/4.5-5.6 DO IS USM:  82.4mm x  99.9mm
>> EF 70-300mm f/4  -5.6 IS USM:     76.5mm x 142.8mm

>> 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?

> Both lenses achieve a focal length of 300mm with considerably shorter
> overall lengths, 99.9mm and 142.8mm.

That's the non-extended length. Many (but not all) zoom
lenses extend.

> Internal optics make it possible and frankly I do not know how they
> work but when I seeing a shorter overall length I would ask where is
> the catch.

It's a tele design.

> Let me explain why I cannot comment on your example.
> The theoretical f number for the DO f/4.5 one is 300mm/82.4mm=3.64
> (missing 1 stop here).
> The other one with f/4 should have a f number of 300mm/76.5=3.92
> (close enough).

Actually, both lenses are f/5.6 at 300mm, so your calculation
is twice bogus.

> 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.

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.

-Wolfgang
From: DanP on
On May 30, 4:45 pm, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
wrote:
> DanP <dan.pe...(a)hotmail.com> wrote:
> > On May 30, 5:29 am, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
> >> DanP <dan.pe...(a)gmail.com> wrote:
> >> Does a 200mm lens collect the same amount of light from that star
> >> as a 30mm even at the same aperture?
> > Yes. At 30mm all the light is concentrated in one overexposed spot.
> > At 200mm it is spread over a larger area and is fainter.
>
> Interesting.  What theory leads you to such a prediction?

Sorry, I have just realised your question can refer to either lens
diameter or focal length. My first answer was for focal length and it
makes sense because it either concentrates all light to a bright spot
or disperse it over a large area.

In case you were referring to lens diameter this is my answer:
A 200mm diameter lens having the same focal length with a 30mm smaller
one at the same f number will let in the same amount of light.
In the old days exposure time used to be set by reading a light meter
and using a chart with ISO and f numbers. Nothing to do with lens
diameters or focal lengths. This means that whatever lens you use if
you set it to the same f number then the exposure time is the same.

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.

> >> > So if change the lenses with a bigger diameter one everything else is
> >> > the same but the size of the aperture measured in mm/inch will be
> >> > smaller (but f number is the same, f/16).
> >> > This is 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.
> >> Huh.
> >> EF 70-300mm f/4.5-5.6 DO IS USM:  82.4mm x  99.9mm
> >> EF 70-300mm f/4  -5.6 IS USM:     76.5mm x 142.8mm
> >> 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?
> > Both lenses achieve a focal length of 300mm with considerably shorter
> > overall lengths, 99.9mm and 142.8mm.
>
> That's the non-extended length.  Many (but not all) zoom
> lenses extend.

True but I do not expect the full extended length to be 3 times the
length of the collapsed size (from 99.9mm to 300mm).

> > Let me explain why I cannot comment on your example.
> > The theoretical f number for the DO f/4.5 one is 300mm/82.4mm=3.64
> > (missing 1 stop here).
> > The other one with f/4 should have a f number of 300mm/76.5=3.92
> > (close enough).
>
> Actually, both lenses are f/5.6 at 300mm, so your calculation
> is twice bogus.

No, my calculations are correct, they are the theoretical f numbers
for 300mm.
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
See http://en.wikipedia.org/wiki/F-number#Notation

> > 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.

See the Sigma 120mm-300mm f/2.8 http://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.

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 ... 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.
I have learned something today.


DanP
From: Wolfgang Weisselberg on
DanP <dan.petre(a)hotmail.com> wrote:
> On May 30, 4:45 pm, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
>> DanP <dan.pe...(a)hotmail.com> wrote:
>> > On May 30, 5:29 am, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>

>> >> Does a 200mm lens collect the same amount of light from that star
>> >> as a 30mm even at the same aperture?

>> > Yes. At 30mm all the light is concentrated in one overexposed spot.
>> > At 200mm it is spread over a larger area and is fainter.

>> Interesting.  What theory leads you to such a prediction?

> Sorry, I have just realised your question can refer to either lens
> diameter or focal length.

Focal length.

> My first answer was for focal length and it
> makes sense because it either concentrates all light to a bright spot
> or disperse it over a large area.

We are talking about stars ... you know, as good as
infinitely far away point sources as there are? With
perfectly parallel rays?

> 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.

>> >> EF 70-300mm f/4.5-5.6 DO IS USM:  82.4mm x  99.9mm
>> >> EF 70-300mm f/4  -5.6 IS USM:     76.5mm x 142.8mm
>> >> 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.

> 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
> See http://en.wikipedia.org/wiki/F-number#Notation

Nice formula, misplaced identification of the entrance pupil.

>> > 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."?

> See the Sigma 120mm-300mm f/2.8 http://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?

> 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?

>> 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.

-Wolfgang