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Are primes brighter and sharper than wide open zooms



 
 
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  #41  
Old September 29th 05, 02:01 PM
no_name
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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  
Old September 29th 05, 02:46 PM
W.E. O'Neil
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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  
Old September 29th 05, 02:50 PM
Peter
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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  
Old September 29th 05, 03:03 PM
Scott Schuckert
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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  
Old September 29th 05, 03:17 PM
Peter
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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  
Old September 29th 05, 05:09 PM
David Littlewood
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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  
Old September 29th 05, 05:12 PM
David Littlewood
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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  
Old September 29th 05, 05:32 PM
Jeremy
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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  
Old September 29th 05, 09:52 PM
Dave Martindale
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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  
Old September 30th 05, 04:38 AM
Pete D
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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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