From: Wolfgang Weisselberg on
DanP <dan.petre(a)gmail.com> wrote:
> On 8 June, 02:53, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
> wrote:
>> DanP <dan.pe...(a)hotmail.com> wrote:
>> >> I guess you could do a thought experiment.
>> >> If 2 lenses gather the same amount of parallel rays along
>> >> their lens axis, they must have the same diameter.  Go from
>> >> there.
>> > Right, at maximum lens aperture bigger lenses gather more light.
>> > Close the bigger lenses aperture to match the small ones and you get
>> > the same amount of light.
>>
>> Please clarify, what exactly do you mean "to match the small
>> ones"?  f/stop?  Diameter?  Area?  What exactly?
>>
>> -Wolfgang

> f/stop.

> Big lens: 42mm diameter, 50mm focal length, f/1.2 aperture.
> Small lens: 18mm diameter, 50mm focal length, f/2.8 aperture.

> Set them both to f/2.8 or smaller and you get the same light.

So use a 200mm as the big lens and a 50mm as the small lens
and answer again.

-Wolfgang
From: DanP on
On Jun 9, 11:46 pm, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
wrote:
> DanP <dan.pe...(a)gmail.com> wrote:
> > On 8 June, 02:53, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
> > wrote:
> >> DanP <dan.pe...(a)hotmail.com> wrote:
> >> >> I guess you could do a thought experiment.
> >> >> If 2 lenses gather the same amount of parallel rays along
> >> >> their lens axis, they must have the same diameter.  Go from
> >> >> there.
> >> > Right, at maximum lens aperture bigger lenses gather more light.
> >> > Close the bigger lenses aperture to match the small ones and you get
> >> > the same amount of light.
>
> >> Please clarify, what exactly do you mean "to match the small
> >> ones"?  f/stop?  Diameter?  Area?  What exactly?
>
> >> -Wolfgang
> > f/stop.
> > Big lens: 42mm diameter, 50mm focal length, f/1.2 aperture.
> > Small lens: 18mm diameter, 50mm focal length, f/2.8 aperture.
> > Set them both to f/2.8 or smaller and you get the same light.
>
> So use a 200mm as the big lens and a 50mm as the small lens
> and answer again.
>
> -Wolfgang

200mm?
I am talking about lens diameter not focal length.


DanP
From: Paul Furman on
On 6/10/2010 12:25 PM, DanP wrote:
> On Jun 9, 11:46 pm, Wolfgang Weisselberg<ozcvgt...(a)sneakemail.com>
> wrote:
>> DanP<dan.pe...(a)gmail.com> wrote:
>>> On 8 June, 02:53, Wolfgang Weisselberg<ozcvgt...(a)sneakemail.com>
>>> wrote:
>>>> DanP<dan.pe...(a)hotmail.com> wrote:
>>>>>> I guess you could do a thought experiment.
>>>>>> If 2 lenses gather the same amount of parallel rays along
>>>>>> their lens axis, they must have the same diameter. Go from
>>>>>> there.
>>>>> Right, at maximum lens aperture bigger lenses gather more light.
>>>>> Close the bigger lenses aperture to match the small ones and you get
>>>>> the same amount of light.
>>
>>>> Please clarify, what exactly do you mean "to match the small
>>>> ones"? f/stop? Diameter? Area? What exactly?
>>
>>>> -Wolfgang
>>> f/stop.
>>> Big lens: 42mm diameter, 50mm focal length, f/1.2 aperture.
>>> Small lens: 18mm diameter, 50mm focal length, f/2.8 aperture.
>>> Set them both to f/2.8 or smaller and you get the same light.
>>
>> So use a 200mm as the big lens and a 50mm as the small lens
>> and answer again.
>
> 200mm?
> I am talking about lens diameter not focal length.

He's talking about both.

200mm with 42mm aperture f/4.5
50mm with 18mm aperture f/2.8

or

200mm with 18mm aperture f/11
50mm with 18mm aperture f/2.8
From: Martin Brown on
On 09/06/2010 23:46, Wolfgang Weisselberg wrote:
> DanP<dan.petre(a)gmail.com> wrote:
>> On 8 June, 02:53, Wolfgang Weisselberg<ozcvgt...(a)sneakemail.com>
>> wrote:
>>> DanP<dan.pe...(a)hotmail.com> wrote:
>>>>> I guess you could do a thought experiment.
>>>>> If 2 lenses gather the same amount of parallel rays along
>>>>> their lens axis, they must have the same diameter. Go from
>>>>> there.
>>>> Right, at maximum lens aperture bigger lenses gather more light.
>>>> Close the bigger lenses aperture to match the small ones and you get
>>>> the same amount of light.
>>>
>>> Please clarify, what exactly do you mean "to match the small
>>> ones"? f/stop? Diameter? Area? What exactly?
>>>
>>> -Wolfgang
>
>> f/stop.
>
>> Big lens: 42mm diameter, 50mm focal length, f/1.2 aperture.
>> Small lens: 18mm diameter, 50mm focal length, f/2.8 aperture.
>
>> Set them both to f/2.8 or smaller and you get the same light.

Only exactly on axis or very near to the axis.

Actually in practice that may not be entirely true. A small lens with
maximum aperture f2.8 might well vignette at the extreme corners of the
field of view whereas the f1.2 stopped down to f2.8 will be almost
perfect uniform film plane illumination for a well designed lens.

Most lenses designed down to a price need to be stopped down around half
a stop to avoid some corner vignetting. Vignetting at the corners of an
image can be a nuisance when you don't want it.

Regards,
Martin Brown
From: DanP on
On 11 June, 08:23, Paul Furman <pa...@-edgehill.net> wrote:
> On 6/10/2010 12:25 PM, DanP wrote:
>
>
>
>
>
> > On Jun 9, 11:46 pm, Wolfgang Weisselberg<ozcvgt...(a)sneakemail.com>
> > wrote:
> >> DanP<dan.pe...(a)gmail.com>  wrote:
> >>> On 8 June, 02:53, Wolfgang Weisselberg<ozcvgt...(a)sneakemail.com>
> >>> wrote:
> >>>> DanP<dan.pe...(a)hotmail.com>  wrote:
> >>>>>> I guess you could do a thought experiment.
> >>>>>> If 2 lenses gather the same amount of parallel rays along
> >>>>>> their lens axis, they must have the same diameter.  Go from
> >>>>>> there.
> >>>>> Right, at maximum lens aperture bigger lenses gather more light.
> >>>>> Close the bigger lenses aperture to match the small ones and you get
> >>>>> the same amount of light.
>
> >>>> Please clarify, what exactly do you mean "to match the small
> >>>> ones"?  f/stop?  Diameter?  Area?  What exactly?
>
> >>>> -Wolfgang
> >>> f/stop.
> >>> Big lens: 42mm diameter, 50mm focal length, f/1.2 aperture.
> >>> Small lens: 18mm diameter, 50mm focal length, f/2.8 aperture.
> >>> Set them both to f/2.8 or smaller and you get the same light.
>
> >> So use a 200mm as the big lens and a 50mm as the small lens
> >> and answer again.
>
> > 200mm?
> > I am talking about lens diameter not focal length.
>
> He's talking about both.
>
> 200mm with 42mm aperture f/4.5
>   50mm with 18mm aperture f/2.8
>
> or
>
> 200mm with 18mm aperture f/11
>   50mm with 18mm aperture f/2.8- Hide quoted text -
>
> - Show quoted text -

I only have 3 lenses:
EF-S 18-55mm f/3.5-5.6 42mm diameter
EF 50mm f/1.8 18mm diameter
EF-S 55-250mm f/4-5.6 (did not measure lens diameter but close to
42mm)

I wanted to take some pictures last night as it was clear but I live
in Glasgow and at half past midnight I could not see any stars from my
window.

Any tests that I will do will have fixed aperture (f number) on all
lenses and focal lengths, my point being is only the aperture (f
number) that matters in terms of gathering light.


DanP
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