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#1
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Why "constant-aperture" zooms?
The top-quality, high-priced, "professional" zoom lenses from the major manufacturers offer constant maximum f-number over their focal length range. The property is called "constant speed", but also often incorrectly called "constant aperture" or "fixed aperture". So-called "variable aperture" zooms come closer to offering constant aperture over their focal length range. My question is: given the f-number they offer at their longest focal length, why are best zooms so slow at the short end of their focal-length range? Fast is an advantage. That advantage necessarily imposes a cost: reaching a desired speed at a given focal length forces lens element size to be at least a certain proportion of the focal length. I'm not suggesting lenses should get slower at their longest focus; I'm asking what stops professional zooms from getting faster at their shorter focal lengths. Is there some optical principle of image quality that limits the speed at the lower end to the speed at the longer end? If so, can anyone cite the tech stuff? If not, doesn't that mean that professional zoom lenses attain constant f-number by artificially and detrimentally reducing their speed at shorter focal lengths? -- --Bryan |
#2
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Why "constant-aperture" zooms?
Bryan Olson wrote: The top-quality, high-priced, "professional" zoom lenses from the major manufacturers offer constant maximum f-number over their focal length range. The property is called "constant speed", but also often incorrectly called "constant aperture" or "fixed aperture". So-called "variable aperture" zooms come closer to offering constant aperture over their focal length range. My question is: given the f-number they offer at their longest focal length, why are best zooms so slow at the short end of their focal-length range? If you do videorecording and vary the focal length, the illumination of the sensor will be kept constant . This provides the best-quality illuminated picture. |
#3
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Why "constant-aperture" zooms?
minnesøtti wrote:
Bryan Olson wrote: The top-quality, high-priced, "professional" zoom lenses from the major manufacturers offer constant maximum f-number over their focal length range. The property is called "constant speed", but also often incorrectly called "constant aperture" or "fixed aperture". So-called "variable aperture" zooms come closer to offering constant aperture over their focal length range. My question is: given the f-number they offer at their longest focal length, why are best zooms so slow at the short end of their focal-length range? If you do videorecording and vary the focal length, the illumination of the sensor will be kept constant . This provides the best-quality illuminated picture. Who's talking about video?? -- Images (Plus Snaps & Grabs) by Mark² at: www.pbase.com/markuson |
#4
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Why "constant-aperture" zooms?
Bryan Olson writes:
Is there some optical principle of image quality that limits the speed at the lower end to the speed at the longer end? If so, can anyone cite the tech stuff? If not, doesn't that mean that professional zoom lenses attain constant f-number by artificially and detrimentally reducing their speed at shorter focal lengths? Well, I think making lenses with extremely low f-numbers is difficult in general. For example, if an 80-200/2.8 lens instead stayed at constant aperture (rather than constant f-number), it would be f/2.8 at the 200mm end and f/1.12 at the 80mm end. An 80/1.4 prime is already a complicated lens, so an 80-200 f/1.12-2.8 zoom would be astonishing. There have been some variable f-number zooms that were pretty fast at both ends by current "consumer zoom" standards, for example the old Vivitar Series 1 28-90/2.8-4.0. There was also an Angenieux 35-70 f/2.5-3.5. |
#5
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Why "constant-aperture" zooms?
Paul Rubin wrote:
Bryan Olson writes: Is there some optical principle of image quality that limits the speed at the lower end to the speed at the longer end? If so, can anyone cite the tech stuff? If not, doesn't that mean that professional zoom lenses attain constant f-number by artificially and detrimentally reducing their speed at shorter focal lengths? Well, I think making lenses with extremely low f-numbers is difficult in general. For example, if an 80-200/2.8 lens instead stayed at constant aperture (rather than constant f-number), it would be f/2.8 at the 200mm end and f/1.12 at the 80mm end. An 80/1.4 prime is already a complicated lens, so an 80-200 f/1.12-2.8 zoom would be astonishing. That's a reasonable point. Still, it doesn't really resolve the issue. It suggests that constant aperture might be unreachable in many popular lenses, but does not indicate that the lenses are somehow limited to constant f-number. If the lens offer f/2.8 at 200mm, can it really be no faster than f/2.8 at 80mm? And what about the lenses with a similar focal-length range that offer constant f/4? What limits their speed to f/4 at the short end? -- --Bryan |
#6
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Why "constant-aperture" zooms?
Bryan Olson writes:
And what about the lenses with a similar focal-length range that offer constant f/4? What limits their speed to f/4 at the short end? I think there may have been some 80-200/2.8-4 lenses but my memory is hazy. It also seems to me that maybe users didn't care very much. In the heyday of those zooms, the users also had primes. The Nikon 75-150/3.5E was a favorite among fashion photographers. They liked to move around and shoot quickly, zooming without wanting to have to readjust exposure (especially with studio strbes), so they wanted constant f-number, and if they wanted speed at the short end they tended to have 85mm or 105mm primes. |
#7
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Why "constant-aperture" zooms?
Bryan Olson wrote:
The top-quality, high-priced, "professional" zoom lenses from the major manufacturers offer constant maximum f-number over their focal length range. The property is called "constant speed", but also often incorrectly called "constant aperture" or "fixed aperture". So-called "variable aperture" zooms come closer to offering constant aperture over their focal length range. .... As I recall, and my memory is fuzzy, there are some zoom designs that by their nature have or require a constant maximum f number. -- Joseph Meehan Dia 's Muire duit |
#8
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Why "constant-aperture" zooms?
In article , Bryan
Olson wrote: My question is: given the f-number they offer at their longest focal length, why are best zooms so slow at the short end of their focal-length range? Glass half full: Wow! they took my f2.8-4.5 lens and made it f2.8-2.8! Glass half empty: Then why didn't they make it f1.8-2.8? I've always looked at the half full side and been happy. Now you're gonna make me think of the half empty side and be bitter. heh. The answer may lie in that it is easier to move a many-element complex zoom lens from 4.5 to 2.8 with increases in diameter and other internal mods than it is to move a similarly complex lens from 2.8 to 1.8. At wide angle, it sees more light, but it still has all that internal complexity. |
#9
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Why "constant-aperture" zooms?
On Nov 7, 2:42 am, Bryan Olson wrote: The top-quality, high-priced, "professional" zoom lenses from the major manufacturers offer constant maximum f-number over their focal length range. The property is called "constant speed", but also often incorrectly called "constant aperture" or "fixed aperture". So-called "variable aperture" zooms come closer to offering constant aperture over their focal length range. My question is: given the f-number they offer at their longest focal length, why are best zooms so slow at the short end of their focal-length range? Fast is an advantage. That advantage necessarily imposes a cost: reaching a desired speed at a given focal length forces lens element size to be at least a certain proportion of the focal length. I'm not suggesting lenses should get slower at their longest focus; I'm asking what stops professional zooms from getting faster at their shorter focal lengths. Is there some optical principle of image quality that limits the speed at the lower end to the speed at the longer end? If so, can anyone cite the tech stuff? If not, doesn't that mean that professional zoom lenses attain constant f-number by artificially and detrimentally reducing their speed at shorter focal lengths? -- --Bryan It is a photography thing, if you are shooting manual, you want to know 4 clicks is f5.6 on a f2.8 lens. Good photographers can get a light reading and work from there is the light doesn't change substantially. This is called knowing your equipment. Now with a variable focal length the focal lenngth muddies the exposure. So if you are using automatic the variable fstop doesn't mean squat, if you are working manually it really helps. Haven't decided yet since fstops are controled by the front wheel on my camera, makes my life a little difficult and I'm not as facile as I am with my Leicas (just about 40 years of practice). |
#10
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Why "constant-aperture" zooms?
"tomm42" wrote in message ups.com... It is a photography thing, if you are shooting manual, you want to know 4 clicks is f5.6 on a f2.8 lens. Good photographers can get a light reading and work from there is the light doesn't change substantially. This is called knowing your equipment. Now with a variable focal length the focal lenngth muddies the exposure. So if you are using automatic the variable fstop doesn't mean squat, if you are working manually it really helps. Haven't decided yet since fstops are controled by the front wheel on my camera, makes my life a little difficult and I'm not as facile as I am with my Leicas (just about 40 years of practice). Many cameras permit using variable f-stop lenses as if they were constant f-stop (it is a custom setting on my Nikons) - so, for instance, an f3.5-4.5 zoom can be used (with constant resultant shutter speed with zooming) as if it were an f4.5 lens. -- David Ruether http://www.ferrario.com/ruether |
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