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From: Paul Furman on 23 May 2010 03:19 DanP wrote: > On May 23, 7:31 am, Paul Furman <pa...@-edgehill.net> wrote: >> DanP wrote: >>> the bigger the size of the lens the smaller the internal aperture >>> has to be to keep the same f number. >> The opposite. > > Can you argue why? If you mean longer, the aperture has to be bigger given the math. If you mean larger front element on a wide angle, that doesn't matter for shorter lenses, the diameter of the front element isn't the aperture; it depends on the design. I can't explain better as it's over my head but measure some lenses and you'll see. I just checked a 28mm f/2 and it's front element is about 42mm. 28/42= f/0.7 (not f/2). A 40mm f/2 measures 22mm which is f/1.8, pretty close since it's a simple, normal pancake lens but a small 20mm f/2.8 measures 45mm which would be f/0.4. Long lenses come pretty close to the rated aperture by measuring the front element but wider lenses make no sense. > If you are right then it means for the same f number more light is let > in by bigger lenses, first by the lenses then by the aperture. Longer lenses crop a smaller window of the scene, so less light makes it to the sensor. They need a larger opening to gather the same light. > And another consequence will be that DOF for bigger size lenses will > be even smaller, bigger lenses will scatter more light and bigger > aperture will make it even less sharp. > > By aperture I mean the size of the diaphragm measured in inch/mm. For > the purpose of comparing apples to apples f number has to be kept the > same. > > > DanP
From: DanP on 23 May 2010 04:08 On May 23, 8:19 am, Paul Furman <pa...@-edgehill.net> wrote: > DanP wrote: > > On May 23, 7:31 am, Paul Furman <pa...@-edgehill.net> wrote: > >> DanP wrote: > >>> the bigger the size of the lens the smaller the internal aperture > >>> has to be to keep the same f number. > >> The opposite. > > > Can you argue why? > > If you mean longer, the aperture has to be bigger given the math. > > If you mean larger front element on a wide angle, that doesn't matter > for shorter lenses, the diameter of the front element isn't the > aperture; it depends on the design. I can't explain better as it's over > my head but measure some lenses and you'll see. I just checked a 28mm > f/2 and it's front element is about 42mm. 28/42= f/0.7 (not f/2). A 40mm > f/2 measures 22mm which is f/1.8, pretty close since it's a simple, > normal pancake lens but a small 20mm f/2.8 measures 45mm which would be > f/0.4. Long lenses come pretty close to the rated aperture by measuring > the front element but wider lenses make no sense. I agree with your ratios but I was taking about the size of the internal aperture (diaphragm measured in inch). DanP
From: Rich on 23 May 2010 12:00 On May 23, 3:19 am, Paul Furman <pa...@-edgehill.net> wrote: > DanP wrote: > > On May 23, 7:31 am, Paul Furman <pa...@-edgehill.net> wrote: > >> DanP wrote: > >>> the bigger the size of the lens the smaller the internal aperture > >>> has to be to keep the same f number. > >> The opposite. > > > Can you argue why? > > If you mean longer, the aperture has to be bigger given the math. > > If you mean larger front element on a wide angle, that doesn't matter > for shorter lenses, the diameter of the front element isn't the > aperture; it depends on the design. I can't explain better as it's over > my head but measure some lenses and you'll see. I just checked a 28mm > f/2 and it's front element is about 42mm. 28/42= f/0.7 (not f/2). A 40mm > f/2 measures 22mm which is f/1.8, pretty close since it's a simple, > normal pancake lens but a small 20mm f/2.8 measures 45mm which would be > f/0.4. Long lenses come pretty close to the rated aperture by measuring > the front element but wider lenses make no sense. There are optical design reasons for this. Check out the short f.l. macros out from Olympus, Pentax, etc. They have very small elements and could be mounted in bodies 1/4 the size they are now.
From: Ray Fischer on 23 May 2010 13:50 DanP <dan.petre(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? -- Ray Fischer rfischer(a)sonic.net
From: DanP on 23 May 2010 16:03
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 and you want the biggest lens/mirror you can get. Binoculars have focus and if you want a bigger DOF you pick smaller lenses. The less light you let through the longer the DOF. In cameras DOF is a relation of the f number which in turn depends of the size of the lens and the size of the internal diaphragm. DanP |