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From: J. Clarke on 15 Jun 2010 08:34 On 6/15/2010 2:10 AM, David J Taylor wrote: >> 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. I don't see Wolfgang's post, but it contains the usual misconception about photographic vs astronomical imaging. The 200 will "capture more light" in the sense that more light will enter the lens. However for a given sensor size, a smaller proportion of that light will strike the sensor. If you make adjustments so that both cover the same field of view, for example by adding two 2x teleconverters to the 50, _then_ the 200 will show its "greater light gathering", and this will be reflected in the photographic formulae, as at that point the 50/2.8 will be functioning as a 200/11.2. Astronomers generally work with a magnified field to begin with--in other words they almost always use some kind of teleconverter--so that they are working at effective focal lengths longer than that of their primary lens or mirror. In that situation, larger aperture always results in a brighter image. Photographers, on the other hand, hardly ever use a teleconverter these days, so effect of the diameter of their lens is accurately reflected in the f/ratio.
From: Wolfgang Weisselberg on 15 Jun 2010 17:53 Remmy Martin <remmymartin(a)gooddrinksnotspam.net> wrote: > Yes, I realize it is an exercise in futility in trying to educate you. > But others reading this who have an IQ above 100 will be able to glean some > knowledge from the above. You belong to the IQ-below-70 group. Plonk. -Wolfgang
From: whisky-dave on 16 Jun 2010 08:49 "David J Taylor" <david-taylor(a)blueyonder.co.uk.invalid> wrote in message news:hv75gj$aac$1(a)news.eternal-september.org... >> And I am trying to point out that a 200mm f/2.8 lens at f/2.8 will >> capture more light from a star than a 50mm f/2.8 lens at >> f/2.8. >> >> It's patently obvious that a 200mm f/2.8 lens has a larger >> diameter than a 50mm f/2.8 lens. >> >> -Wolfgang > > More light (photons) in a given period, when other things are similar. Well that makes sense, but I'd have thought it obvious..... that 200mm f/2.8 lens would have a larger diameter than a 50mm f/2.8 lens. But what would happen in a parallel universe where the laws of physics are the same ? would the 200mm taste of strawberries while the 50mm taste like chocolate.
From: Pete on 16 Jun 2010 09:15 On 2010-06-16 13:49:07 +0100, whisky-dave said: > "David J Taylor" <david-taylor(a)blueyonder.co.uk.invalid> wrote in > message news:hv75gj$aac$1(a)news.eternal-september.org... >>> And I am trying to point out that a 200mm f/2.8 lens at f/2.8 will >>> capture more light from a star than a 50mm f/2.8 lens at >>> f/2.8. >>> >>> It's patently obvious that a 200mm f/2.8 lens has a larger >>> diameter than a 50mm f/2.8 lens. >>> >>> -Wolfgang >> >> More light (photons) in a given period, when other things are similar. > > Well that makes sense, but I'd have thought it obvious..... > that 200mm f/2.8 lens would have a larger diameter than a 50mm f/2.8 lens. > > But what would happen in a parallel universe where the laws of physics > are the same ? would the 200mm taste of strawberries while the 50mm > taste like chocolate. Other way around. Can't figure out what a tripod would taste like, don't think I've eaten anything with three legs. Packaging in my supermarket suggest some chickens have 12 legs, which is strange: if they had then how come they got caught? -- Pete
From: David J Taylor on 16 Jun 2010 09:28
"whisky-dave" <whisky-dave(a)final.front.ear> wrote in message news:hvah80$fuj$1(a)qmul... [] >> More light (photons) in a given period, when other things are similar. > > Well that makes sense, but I'd have thought it obvious..... > that 200mm f/2.8 lens would have a larger diameter than a 50mm f/2.8 > lens. Indeed, by definition of f/number. What some people seem to be having difficulty with is that - given nominally identical lenses but with everything scaled by four - is that the light collected would be spread out over a larger area with the 200mm lens, even though the number of photons per unit area would be the same, and the implications of that. Cheers, David |