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From: Wolfgang Weisselberg on 17 May 2010 14:48 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 18 May 2010 12:37 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 18 May 2010 13:21 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 19 May 2010 05:38 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 19 May 2010 07:23
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. |