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From: David J Taylor on 11 Jun 2010 06:27 "DanP" <dan.petre (a)gmail.com> wrote in message news:d603312e-320c-4c88-b6c4-f0f448226c9d (a)j8g2000yqd.googlegroups.com...[] > 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) [] > DanP A 50mm lens with a claimed f/number of 1.8 must have an entrance pupil larger than 18mm diameter. A typo? Cheers, David
From: DanP on 11 Jun 2010 09:04 On Jun 11, 11:27 am, "David J Taylor" <david- tay... (a)blueyonder.co.uk.invalid> wrote:> A 50mm lens with a claimed f/number of 1.8 must have an entrance pupil > larger than 18mm diameter. A typo? Well spotted, is a mistake. I have double checked the diameters: EF-S 18-55mm f/3.5-5.6 42mm diameter EF 50mm f/1.8 32mm diameter EF-S 55-250mm f/4-5.6 45mm diameter DanP
From: Peter on 11 Jun 2010 11:15 "DanP" <dan.petre (a)gmail.com> wrote in message news:cc4df53e-0fbe-4cdf-93b6-40953f397a40 (a)j8g2000yqd.googlegroups.com...On Jun 11, 11:27 am, "David J Taylor" <david- tay... (a)blueyonder.co.uk.invalid> wrote:> A 50mm lens with a claimed f/number of 1.8 must have an entrance pupil > larger than 18mm diameter. A typo? Well spotted, is a mistake. I have double checked the diameters: EF-S 18-55mm f/3.5-5.6 42mm diameter EF 50mm f/1.8 32mm diameter EF-S 55-250mm f/4-5.6 45mm diameter Last time I checked my math tables, 32mm is larger than 18mm. <G> -- Peter
From: DanP on 11 Jun 2010 11:46 On 11 June, 16:15, "Peter" <peter... (a)nospamoptonline.net> wrote:> "DanP" <dan.pe... (a)gmail.com> wrote in message> > news:cc4df53e-0fbe-4cdf-93b6-40953f397a40 (a)j8g2000yqd.googlegroups.com...> On Jun 11, 11:27 am, "David J Taylor" <david- > > tay... (a)blueyonder.co.uk.invalid> wrote:> > A 50mm lens with a claimed f/number of 1.8 must have an entrance pupil > > larger than 18mm diameter. A typo? > > Well spotted, is a mistake. > > I have double checked the diameters: > EF-S 18-55mm f/3.5-5.6 42mm diameter > EF 50mm f/1.8 32mm diameter > EF-S 55-250mm f/4-5.6 45mm diameter > > Last time I checked my math tables, 32mm is larger than 18mm. <G> > > -- > Peter True, but I am comparing 32mm with 42mm, so the prime lenses are still smaller. 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 |