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From: Ray Fischer on 31 May 2010 14:37 John Turco <jtur(a)concentric.net> wrote: >Ray Fischer wrote: >> >> DanP <dan.petre(a)hotmail.com> wrote: >> >On 23 May, 18:50, rfisc...(a)sonic.net (Ray Fischer) wrote: >> >> DanP <dan.pe...(a)gmail.com> wrote: >> >> >On May 23, 3:31 am, rfisc...(a)sonic.net (Ray Fischer) wrote: >> >> >> >> >> Wrong. Bigger apertures allow higher resolution. That's why big >> >> >> telescopes are better than tiny ones. >> >> >> >> >Telescopes are focused at infinity so that is a different case. >> >> >> >> ?!? >> >> >> >> Why is that different? >> > >> >Because their optics are fixed >> >> Nope. >> >> >and you want the biggest lens/mirror>you can get. >> >> Because bigger means higher resolution. > >Where astronomical telescopes are concerned, greater light-gathering >capability "means higher resolution." They are separate concepts. It is possible to have a very wide aperture with high resolution but limited light gathering ability. -- Ray Fischer rfischer(a)sonic.net
From: Wolfgang Weisselberg on 30 May 2010 21:47 DanP <dan.petre(a)hotmail.com> wrote: > On May 30, 4:45 pm, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com> >> DanP <dan.pe...(a)hotmail.com> wrote: >> > On May 30, 5:29 am, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com> >> >> Does a 200mm lens collect the same amount of light from that star >> >> as a 30mm even at the same aperture? >> > Yes. At 30mm all the light is concentrated in one overexposed spot. >> > At 200mm it is spread over a larger area and is fainter. >> Interesting. What theory leads you to such a prediction? > Sorry, I have just realised your question can refer to either lens > diameter or focal length. Focal length. > My first answer was for focal length and it > makes sense because it either concentrates all light to a bright spot > or disperse it over a large area. We are talking about stars ... you know, as good as infinitely far away point sources as there are? With perfectly parallel rays? > And that means the amount of light let in depends only of exposure > time and f number. If bigger lenses would let more light in at the > same f number then the film would have been over exposed. We are not talking about areas, we are talking point sources. >> >> EF 70-300mm f/4.5-5.6 DO IS USM: 82.4mm x 99.9mm >> >> EF 70-300mm f/4 -5.6 IS USM: 76.5mm x 142.8mm >> >> Care to explain why the bigger (larger diameter) DO lens is shorter >> >> and thus obviously less "light gets astray"? If your theory was >> >> right, shouldn't the DO lens be brighter (it's actually darker >> >> at the short end!) and/or longer? Still waiting for your explanation. > Which now that you have pointed out my mistake (I should have looked > for 70mm) now have to be compared to 5.6 at 300mm > And to come back with the proper results the DO at 70 has a > theoretical maximum f number of .85 vs .91 for the non DO. > Formula used is f=Focal length/Diameter of entrance pupil > See http://en.wikipedia.org/wiki/F-number#Notation Nice formula, misplaced identification of the entrance pupil. >> > So the DO is less brighter that what it should be (internal optics >> > should explain why). >> Come on, same focal length, you replace a thinner, longer lens >> with a fatter shorter one and all you can say is "internal optics >> should explain why"? If that's the case your theory of light >> going astray is bogus. > If you do that then you lose aperture. Explain why. "you replace a thinner, longer lens with a fatter shorter one" and "then you lose aperture."? > See the Sigma 120mm-300mm f/2.8 http://www.sigmaphoto.com/shop/120-300mm-f28-ex-dg-apo-hsm-sigma > It has a size of 112.8 x 268.5 mm giving a theoretical f number of > 2.65. Good fast lens but expensive. So what's your point? > To prove me wrong show me a 300mm f/2.8 with a lens size smaller than > 107mm or 300mm f/5.6 with a lens size smaller than 53.5mm. > Or any size that has a theoretical f number bigger than the real one. > Anyone can make a big diameter slow lens. And that proves that a "fatter shorter" lens loses aperture? >> And ... if you change the non-DO lens for the fatter DO lens, >> "the size of the aperture measured in mm/inch will be smaller >> (but f number is the same, f/16) ... 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" is completely wrong. > You are right. The diaphragm size is the same for all lenses a given > focal length and f number so I was completely wrong about that. Actually, you are wrong again: the diaphragm is not next to the single element of your theoretical lens. It is internal. -Wolfgang
From: DanP on 1 Jun 2010 08:04 On 31 May, 02:47, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com> wrote: > We are talking about stars ... you know, as good as > infinitely far away point sources as there are? With > perfectly parallel rays? To talk about sorces of perfecly parallel rays is to talk about points size zero. For that the focal length has no meaning. a point of size 0 is focused by all lens to a size zero image. You have brought focal lengths to discussion though you did not express that clearly. I was talking about lens diameters. > > > And that means the amount of light let in depends only of exposure > > time and f number. If bigger lenses would let more light in at the > > same f number then the film would have been over exposed. > > We are not talking about areas, we are talking point sources. But the above is my answer to the other question. You have snipped "In case you were referring to lens diameter this is my answer: " > > >> >> EF 70-300mm f/4.5-5.6 DO IS USM: 82.4mm x 99.9mm > >> >> EF 70-300mm f/4.5-5.6 DO IS USM: 82.4mm x 99.9mm > >> >> Care to explain why the bigger (larger diameter) DO lens is shorter > >> >> and thus obviously less "light gets astray"? If your theory was > >> >> right, shouldn't the DO lens be brighter (it's actually darker > >> >> at the short end!) and/or longer? > > Still waiting for your explanation. You have started this by questioned my statement "To close the subject, after some reading and thinking lens diameter does not affect the amount of light captured. " Then I said the only thing that matters is the f number. I think your comparision between EF 70-300mm f/4.5-5.6 DO IS USM: 82.4mm x 99.9mm and EF 70-300mm f/4.5-5.6 DO IS USM: 82.4mm x 99.9mm proves my point. And the answer to that is internal optics, the image gets cropped by internal optics at the lower end of zoom range. > > Which now that you have pointed out my mistake (I should have looked > > for 70mm) now have to be compared to 5.6 at 300mm > > And to come back with the proper results the DO at 70 has a > > theoretical maximum f number of .85 vs .91 for the non DO. > > Formula used is f=Focal length/Diameter of entrance pupil > > Seehttp://en.wikipedia.org/wiki/F-number#Notation > > Nice formula, misplaced identification of the entrance pupil. I can accept your objection if you to tell me what is the entrance pupil for lenses. I say it is the size of the diaphragm. For widest aperture you can take the diaphragm out of the lenses if you want, it won't matter. What matters then is only the diameter of the lenses. > >> > So the DO is less brighter that what it should be (internal optics > >> > should explain why). > >> Come on, same focal length, you replace a thinner, longer lens > >> with a fatter shorter one and all you can say is "internal optics > >> should explain why"? If that's the case your theory of light > >> going astray is bogus. > > If you do that then you lose aperture. > > Explain why. "you replace a thinner, longer lens with a > fatter shorter one" and "then you lose aperture."? Because a smaller lense will not be able to use a wide diaphragm. > > See the Sigma 120mm-300mm f/2.8http://www.sigmaphoto.com/shop/120-300mm-f28-ex-dg-apo-hsm-sigma > > It has a size of 112.8 x 268.5 mm giving a theoretical f number of > > 2.65. Good fast lens but expensive. > > So what's your point? If you want low f numbers you must have big lenses. But big lenses in itself does not guarantee more light. Lenses are advertised by their f number, not lens diameter. > > To prove me wrong show me a 300mm f/2.8 with a lens size smaller than > > 107mm or 300mm f/5.6 with a lens size smaller than 53.5mm. > > Or any size that has a theoretical f number bigger than the real one. > > Anyone can make a big diameter slow lens. > > And that proves that a "fatter shorter" lens loses aperture? I give up. Can YOU prove anything? So far I have seen no arguments, only questions. > >> And ... if you change the non-DO lens for the fatter DO lens, > >> "the size of the aperture measured in mm/inch will be smaller > >> (but f number is the same, f/16) ... 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" is completely wrong. > > You are right. The diaphragm size is the same for all lenses a given > > focal length and f number so I was completely wrong about that. > > Actually, you are wrong again: the diaphragm is not next to > the single element of your theoretical lens. It is internal. I did not say is external. A 35mm lens at f/8 will have a aperture of 35mm/8=4.375mm for any lens diameter. See the photo on the right http://en.wikipedia.org/wiki/Diaphragm_(optics). I have just said you were right. What is the point in arguig with THAT? DanP
From: J. Caldwell on 1 Jun 2010 08:24 On Tue, 1 Jun 2010 05:04:52 -0700 (PDT), DanP <dan.petre(a)gmail.com> wrote: > >If you want low f numbers you must have big lenses. But big lenses in >itself does not guarantee more light. Study the design of microscope objectives. Many of them having f-ratios (NA: numerical apertures) far exceeding any large camera lenses. You really don't know enough about optics to even be arguing these things. You can't even formulate a proper question concerning the issues that you are attempting to preach about. IOW: Stop making a fool of yourself on a worldwide platform.
From: DanP on 1 Jun 2010 09:09
On 1 June, 13:24, J. Caldwell <nos...(a)anyserver.net> wrote: > On Tue, 1 Jun 2010 05:04:52 -0700 (PDT), DanP <dan.pe...(a)gmail.com> wrote: > > >If you want low f numbers you must have big lenses. But big lenses in > >itself does not guarantee more light. > > Study the design of microscope objectives. Many of them having f-ratios > (NA: numerical apertures) far exceeding any large camera lenses. I have no interest in microscopes and do not know anything about them. How will that help me to understand camera optics? Microscope lenses are huge compared to the size of the image being observed. Do you find anything wrong with my statement "If you want low f numbers you must have big lenses. But big lenses in itself does not guarantee more light." to which you are replying? > You really don't know enough about optics to even be arguing these things. > You can't even formulate a proper question concerning the issues that you > are attempting to preach about. Where did you get this idea I am preaching something? I have started with one question, now that Sony has managed to make such a small camera, can they miniaturise the lenses as well? And my conclusion is if they do they will have slower lenses. > IOW: Stop making a fool of yourself on a worldwide platform. I was never at any time rude or pompous or patronising, I have listened to other arguments and answered questions. A lot better that to tell people to go away. DanP |