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#41
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Dr. Joel M. Hoffman wrote:
Given two lenses, one a prime (say 28mm) and the other a zoom (say 28-75mm) and both with an aperture of f2.8 - - Will the prime be brighter than the zoom because it has fewer lens elements? No. f/2.8 tells you exactly how bright the lens is. (It's almost like the old question about a pound of lead and a pound of feathers - they do both weigh the same.) However, a pound of gold and a pound of feathers do not. |
#42
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"Randall Ainsworth" wrote in message ... An f/2.8 lens is going to allow a specific amount of light to go through regardless of what kind of glass it's made of. Strictly speaking you are on the wrong track. I think you may be confusing transmission with focal ratio. F2.8 simply speaks to the focal ratio.......slower materials would result in less light being transmitted. |
#43
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Randall Ainsworth wrote: In article , Brion K. Lienhart wrote: Nope. F:2.8 is solely based on the size of the lens. Obsidian is glass, you can grind it into a lens shape, but if you use it as an element in a lens, you're going to get some reallllllly slow shutter speeds. Of course that's an extreme case, the difference between plain old glass glass, and exotic flouro-silicates is only a few fractions of a percent. An f/2.8 lens is going to allow a specific amount of light to go through regardless of what kind of glass it's made of. Actually no, but in most cases the difference is unimportant. One case where the difference generally is important is with a mirror lens. A mirror lens is generally only about 60% efficient compared with around 90%+ efficiency for a glass lens with ten multicoated air-glass surfaces. A 500mm f/6.3 mirror lens may let in about the same light as a 500mm f/8 coated glass lens. This can be a significant factor when deciding what lens to buy. A mirror lens does not gather quite as much light as you would expect from the aperture. Peter. -- |
#44
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In article , Randall
Ainsworth wrote: An f/2.8 lens is going to allow a specific amount of light to go through regardless of what kind of glass it's made of. Ahem. Notice the previous 50+ responses in this thread? If not, read some - you're dead wrong. What you're thinking of is a T-stop. Lenses of the same mathematical aperture (f/stop) can and do wary widely in transmission. Period. |
#45
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Brion K. Lienhart wrote: Peter wrote: You know, sometimes words have two meanings. Most of us can live with slang terminology and standard technical terminology without getting particularly confused. Slang terminology can be very handy: I'm not going to stop saying "Hypo" when I know that fixer is actually thiosulphate. It isn't very likely that someone Not to be pedantic, but I think you mean "Jargon" not "Slang". Actually you are being pedantic, saying "not to be pedantic" in front of a sentence doesn't make it so. I looked up "slang" and "jargon" in several dictionaries, and at least some of the meanings are nearly interchangable. I chose "slang" because I wanted to emphasize the non-standard nature of the vocabulary rather than any lack of intelligibility to outsiders. Peter. -- |
#46
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In article , Tony Polson
writes Eugene wrote: Perhaps my comments were a bit harsh. I just took offense to the suggestion that it was ignorant to use the widely accepted and understood term "prime lens". But "prime lens" is NOT a "widely accepted and understood term". Because of the history of its use, it is essentially ambiguous. It has only been used as a descriptor for all fixed focal length lenses in recent times, and only by people who are unaware of its prior usage, which was as a descriptor for a "high quality" subset of fixed focal length lenses. You might be too young to recall this. There was no accepted definition of which fixed focal length lenses were "prime" and which were not. It was merely an attempt by marketing people to promote some lenses as being "better" than others, the implication being that one brand was "better" than another because of the "high quality" of their "prime" lenses. The terms "high quality", "better" and "prime" are all essentially meaningless unless backed up with something more objective, repeatable and reliable. As we all (should) know, comparisons between lenses are qualitative at best. The most commonly used "objective" comparator - MTF - was never intended for making such comparisons. It was intended as an aid to lens designers and nothing more. As a result it misleads far more than it ever informs, and those who claim it is of any real value when comparing photographic lenses are suffering from delusions. I agree with the first part of the above. The only point in which my understanding differs is that the traditional use of the term "prime" was in the sense of "primary" as opposed to secondary or auxiliary optical components such as tele-converters, wide angle attachments, close up lenses and the like. Thus prime as in the Latin "primus", first or primary. This is the interpretation given in the more rigorous works on photography I consulted on this issue when the point was debated here (ad nauseam) several years ago. (Anyone remember Neil Harrington?) David -- David Littlewood |
#47
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In article , no_name
writes Tony Polson wrote: Eugene wrote: Perhaps my comments were a bit harsh. I just took offense to the suggestion that it was ignorant to use the widely accepted and understood term "prime lens". But "prime lens" is NOT a "widely accepted and understood term". Who here does not understand what is meant when the term "prime lens" is used? A show of hands please? As it is an ambiguous usage, i.e. one which is in conflict with the traditional meaning of the word, then I personally avoid using the word altogether. I agree that when others use the word it is usually apparent from the context what they mean, but IMO it is mildly rude to one's readers to deliberately choose to make them work out meaning from the context. David -- David Littlewood |
#48
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"Randall Ainsworth" wrote in message
... An f/2.8 lens is going to allow a specific amount of light to go through regardless of what kind of glass it's made of. I believe that you are incorrect. the F-stops are geometric measurements of the aperture opening. If a lens has a large number of elements, like a zoom lens, less light may ultimately reach the film plane (or the chip) than would be the case if a lens with a lesser number of elements were substituted, even if both lenses were set to the same aperture opening. My SMC Takumar 50mm f/1.4 is an interesting example. As is well known, the rear element on those lenses had Thorium mixed into the optical glass formula, and the decaying atomic particles have yellowed the lens over the decades. That lens, set at f/1.4, probably transmits only an amount of light equivalent to another (non-yellowed) lens at f/1.8. I admit, however, that most lenses, at any given f-stop, probably transmit about the same amount of light to the film or chip. But it is not absolutely guaranteed. And these days, with TTL metering, the exposure values can be adjusted to compensate. |
#49
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no_name writes:
(It's almost like the old question about a pound of lead and a pound of feathers - they do both weigh the same.) However, a pound of gold and a pound of feathers do not. Due to the archaic system of weights&measures that uses a different unit with the same name for measuring precious metals. But a gram of gold and a gram of feathers *are* the same mass, and have the same weight in the same gravity. Dave |
#50
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"no_name" wrote in message om... John A. Stovall wrote: On 28 Sep 2005 07:33:55 -0700, "Siddhartha Jain" wrote: Hi, Given two lenses, one a prime (say 28mm) and the other a zoom (say 28-75mm) and both with an aperture of f2.8 - - Will the prime be brighter than the zoom because it has fewer lens elements? - Will the prime be sharper wide open than the zoom at 28mm? Ofcourse, a lot will depend on the particular makes and models being compared but is there a rule of thumb? Rule of Thumb: A prime at any focal length and wide open is better than a zoom at any focal length wide open. Except when it's not ... like a really well made fast zoom lens being compared to a really poorly made slow prime lens. So a Ferrari is faster than a Goggomobile, who'd of thought it?? |
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