From: J. Clarke on
On 5/19/2010 5:38 AM, DanP wrote:
> 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.

The results will not necessarily be identical. The 72mm is still
passing light from all parts of the 72mm circle to the sensor, even when
stopped down to f/2, however there will be filtering of the paths taken
and some other effects.

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

Why would it be smaller on the 72mm than on the 32mm?

> 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: whisky-dave on

"Wolfgang Weisselberg" <ozcvgtt02(a)sneakemail.com> wrote in message
news:nlf8c7-faj.ln1(a)ID-52418.user.berlin.de...
> 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.)

http://en.wikipedia.org/wiki/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.

Is that true, I'm sure I have had lenes were this didn;t make any sense.
I remmebr whne peole tested lens and the faster the lens didn;t make
it better quaility, the quaslity was dependent on the manufacturing
technigues
rather than the size of the glass element.


From: Allen on
whisky-dave wrote:
> "Wolfgang Weisselberg" <ozcvgtt02(a)sneakemail.com> wrote in message
> news:nlf8c7-faj.ln1(a)ID-52418.user.berlin.de...
>> 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.)
>
> http://en.wikipedia.org/wiki/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.
>
> Is that true, I'm sure I have had lenes were this didn;t make any sense.
> I remmebr whne peole tested lens and the faster the lens didn;t make
> it better quaility, the quaslity was dependent on the manufacturing
> technigues
> rather than the size of the glass element.
>
>
The whisky is showing in your post. Don't you have a spellcheck program?
Allen
From: DanP on
On May 19, 12:37 pm, "J. Clarke" <jclarke.use...(a)cox.net> wrote:
> On 5/19/2010 5:38 AM, DanP wrote:

> > This is because although both are set to f/2 the aperture size
> > measured in inch/mm is smaller on the 72mm lens.
>
> Why would it be smaller on the 72mm than on the 32mm?

The same f number has to let in the same ammount of light, so bigger
surface captures more light, smaller aperture (measuren in inch/mm)
makes it right again.
Just remember the old days when exposure time used to be established
by the light meter, ISO and aperture f number.
And it worked for any combination of film (same ISO number), cameras
and lenses.

Being a bigger size the light is scattered more by the big lenses
because of the longer light path and thus smaller aperture (measured
in inch/mm) is required to have the same DOF.



DanP

From: Ray Fischer on
DanP <dan.petre(a)hotmail.com> wrote:
>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.
>
>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.

I referred to resolution. Are you having difficulty following along?
And "aperture diameter" refers to the size of the primary element.
It does not refer to "aperture" or f-stop.

--
Ray Fischer
rfischer(a)sonic.net

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