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From: Martin Brown on 11 Jun 2010 03:59 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 11 Jun 2010 05:26 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
From: Wolfgang Weisselberg on 14 Jun 2010 12:45 DanP <dan.petre(a)gmail.com> wrote: > On Jun 9, 11:46 pm, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com> > wrote: >> DanP <dan.pe...(a)gmail.com> wrote: >> > 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. And I am trying to point out that a 200mm f/2.8 lens at f/2.8 will capture more light from a star than a 50mm f/2.8 lens at f/2.8. It's patently obvious that a 200mm f/2.8 lens has a larger diameter than a 50mm f/2.8 lens. -Wolfgang
From: Remmy Martin on 15 Jun 2010 00:08 On Mon, 14 Jun 2010 18:45:43 +0200, Wolfgang Weisselberg <ozcvgtt02(a)sneakemail.com> wrote: >DanP <dan.petre(a)gmail.com> wrote: >> On Jun 9, 11:46�pm, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com> >> wrote: >>> DanP <dan.pe...(a)gmail.com> wrote: >>> > 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. > >And I am trying to point out that a 200mm f/2.8 lens at f/2.8 will >capture more light from a star than a 50mm f/2.8 lens at >f/2.8. Not true. It depends on the duration that you are capturing photons through either optic as well as any AR coatings that may or may not exist on those optics' surfaces, including what wavelengths that you are trying to image. What is true, however, is that the 200mm lens can have more resolving power, but only if it is figured to diffraction-limited quality. Yes, I realize it is an exercise in futility in trying to educate you. About as much futility as you are experiencing in trying to educate DanP. But others reading this who have an IQ above 100 will be able to glean some knowledge from the above. > >It's patently obvious that a 200mm f/2.8 lens has a larger >diameter than a 50mm f/2.8 lens. > >-Wolfgang
From: David J Taylor on 15 Jun 2010 02:10
> And I am trying to point out that a 200mm f/2.8 lens at f/2.8 will > capture more light from a star than a 50mm f/2.8 lens at > f/2.8. > > It's patently obvious that a 200mm f/2.8 lens has a larger > diameter than a 50mm f/2.8 lens. > > -Wolfgang More light (photons) in a given period, when other things are similar. Cheers, David |