From: Wolfgang Weisselberg on
DanP <dan.petre(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.)

> But a smaller lens needs to be polished more precise than a big one to
> have the same IQ.

.... since a smaller sensor needs more enlargement.

> In real life better quality lenses have a bigger diameter.

In real life better quality lenses are faster, too.

-Wolfgang
From: DanP on
On 17 May, 19:48, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com>
wrote:
> 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.
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).
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.

> > But a smaller lens needs to be polished more precise than a big one to
> > have the same IQ.
>
> ... since a smaller sensor needs more enlargement.
>
> > In real life better quality lenses have a bigger diameter.
>
> In real life better quality lenses are faster, too.
>
> -Wolfgang

Fully agree.


DanP
From: Ray Fischer on
DanP <dan.petre(a)hotmail.com> wrote:
>To close the subject, after some reading and thinking lens diameter
>does not affect the amount of light captured.
>But a smaller lens needs to be polished more precise than a big one to
>have the same IQ.
>In real life better quality lenses have a bigger diameter.

When lenses are (effectively) perfect the only way to improve
resolution is to increase the aperture diameter.

--
Ray Fischer
rfischer(a)sonic.net

From: DanP on
On May 18, 6:21 pm, rfisc...(a)sonic.net (Ray Fischer) wrote:
> 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 a smaller lens needs to be polished more precise than a big one to
> >have the same IQ.
> >In real life better quality lenses have a bigger diameter.
>
> When lenses are (effectively) perfect the only way to improve
> resolution is to increase the aperture diameter.
>
> --
> Ray Fischer        
> rfisc...(a)sonic.net  

Say you have a f/2 50mm prime lens with an outside lens diameter of
32mm and another one with a diameter of 72mm and for the sake of the
argument both perfect.

The 72mm can have a lower f number but if both set at f/2 it will
produce identical results as both will let in the same ammount of
light.

This is because although both are set to f/2 the aperture size
measured in inch/mm is smaller on the 72mm lens.

If you want to set the exposure time manualy on an old film camera you
will read a light meter, consider the film ISO and the lens aperture
(f number).
So the size of the lenses is irrelevant.

Am I missing something?


DanP
From: J. Clarke on
On 5/16/2010 5:29 AM, Chris Malcolm wrote:
> In rec.photo.digital DanP<dan.petre(a)hotmail.com> wrote:
>> On 15 May, 16:34, Bruce<docnews2...(a)gmail.com> wrote:
>>> On Sat, 15 May 2010 06:59:27 -0700 (PDT), DanP<dan.pe...(a)hotmail.com>
>>> wrote:
>>>> No, I do not think of camera design at all.
>>>> Instead I think of binoculars and telescopes where the bigger lens
>>>> diameter gives a better IQ.
>>>
>>> Does it really? ?Do binoculars and telescopes really offer better IQ
>>> than camera lenses?
>
>> I am not saying that. I said the bigger the lens diameter the better
>> IQ is.
>
> I can't imagine why. In binoculars and terrestial telescopes the
> aperture is in effect provided by the iris of the human eye using it,
> and optically the larger lens diameters are used to accomodate larger
> iris openings (exit pupils) to accommodate lower light levels.

In principle, an ideally designed and ideally made large diameter
optical system will have a smaller Airy disc than one with a smaller
diameter and thus be capable of greater sharpness. For the reasons, you
really need to consult an optics text.







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