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From: Wolfgang Weisselberg on 30 May 2010 00:29 DanP <dan.petre(a)gmail.com> wrote: > On 17 May, 19:48, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com> >> 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. You can artifically restrict the effective front lens size. This doesn't mean that the 200mm f/1.8 lens wide open will collect the same amount of light as a 200mm f/5.6 lens wide open. Does a 200mm lens collect the same amount of light from that star as a 30mm even at the same aperture? > 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). Irrelevant. > 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. Huh. 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? -Wolfgang
From: DanP on 30 May 2010 04:42 On May 30, 5:29 am, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com> wrote: > DanP <dan.pe...(a)gmail.com> wrote: > 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. > > 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. > > Huh. > 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? Both lenses achieve a focal length of 300mm with considerably shorter overall lengths, 99.9mm and 142.8mm. Internal optics make it possible and frankly I do not know how they work but when I seeing a shorter overall length I would ask where is the catch. Let me explain why I cannot comment on your example. The theoretical f number for the DO f/4.5 one is 300mm/82.4mm=3.64 (missing 1 stop here). The other one with f/4 should have a f number of 300mm/76.5=3.92 (close enough). So the DO is less brighter that what it should be (internal optics should explain why). DanP
From: Wolfgang Weisselberg on 30 May 2010 11:45 DanP <dan.petre(a)hotmail.com> wrote: > On May 30, 5:29 am, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com> >> DanP <dan.pe...(a)gmail.com> wrote: >> 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? >> > 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. >> Huh. >> 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? > Both lenses achieve a focal length of 300mm with considerably shorter > overall lengths, 99.9mm and 142.8mm. That's the non-extended length. Many (but not all) zoom lenses extend. > Internal optics make it possible and frankly I do not know how they > work but when I seeing a shorter overall length I would ask where is > the catch. It's a tele design. > Let me explain why I cannot comment on your example. > The theoretical f number for the DO f/4.5 one is 300mm/82.4mm=3.64 > (missing 1 stop here). > The other one with f/4 should have a f number of 300mm/76.5=3.92 > (close enough). Actually, both lenses are f/5.6 at 300mm, so your calculation is twice bogus. > 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. 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. -Wolfgang
From: DanP on 30 May 2010 19:40 On May 30, 4:45 pm, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com> wrote: > DanP <dan.pe...(a)hotmail.com> wrote: > > On May 30, 5:29 am, Wolfgang Weisselberg <ozcvgt...(a)sneakemail.com> > >> DanP <dan.pe...(a)gmail.com> wrote: > >> 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. 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. In case you were referring to lens diameter this is my answer: A 200mm diameter lens having the same focal length with a 30mm smaller one at the same f number will let in the same amount of light. In the old days exposure time used to be set by reading a light meter and using a chart with ISO and f numbers. Nothing to do with lens diameters or focal lengths. This means that whatever lens you use if you set it to the same f number then the exposure time is the same. 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. > >> > 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. > >> Huh. > >> 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? > > Both lenses achieve a focal length of 300mm with considerably shorter > > overall lengths, 99.9mm and 142.8mm. > > That's the non-extended length. Many (but not all) zoom > lenses extend. True but I do not expect the full extended length to be 3 times the length of the collapsed size (from 99.9mm to 300mm). > > Let me explain why I cannot comment on your example. > > The theoretical f number for the DO f/4.5 one is 300mm/82.4mm=3.64 > > (missing 1 stop here). > > The other one with f/4 should have a f number of 300mm/76.5=3.92 > > (close enough). > > Actually, both lenses are f/5.6 at 300mm, so your calculation > is twice bogus. No, my calculations are correct, they are the theoretical f numbers for 300mm. 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 > > 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. 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. 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 ... 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. I have learned something today. DanP
From: Better Info on 31 May 2010 07:51
On Mon, 31 May 2010 05:57:27 -0500, 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." > >Laymen often overstate the importance of magnification, but...if the >object in question isn't seen, to begin with, it >can't< be magnified, >anyway. Higher resolution is the result of larger diameters. Greater light gathering ability of that larger diameter is just a welcome benefit for naked-eye visual observations by humans. Resolution does not depend on the amount of light collected. If you put a very strong neutral density filter in front of a large 10m dia. telescope, to effectively limit the amount of light entering to no more than 1 photon per second, and used a high-resolution detector array to measure those photons, in time you will still have higher resolution image than could be obtained from a 1m dia. telescope with no filter at all. The Hubble Telescope was trained on a patch of sky where nobody thought anything existed, as pure black as the sky can be. Named "The Hubble Deep Field" experiment. From a period of 10 days of collecting photons from that black region they imaged about 1500 whole galaxies in that empty spot of the sky with just as much resolution as they get on bright objects. Image brightness has nothing to do with resolution. http://hubblesite.org/newscenter/archive/releases/1996/01/ |