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Are primes brighter and sharper than wide open zooms



 
 
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  #1  
Old September 28th 05, 03:33 PM
Siddhartha Jain
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Default Are primes brighter and sharper than wide open zooms

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?

- Siddhartha

  #2  
Old September 28th 05, 04:01 PM
Joseph Meehan
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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?

- Siddhartha


In a word NO. At least it is no if you are talking about real life
situations.

However ---- In general primes will be sharper unless the zoom is a
much better quality lens. Of course that is possible and a really good zoom
can outperform a poor prime any day.

--
Joseph Meehan

Dia duit


  #3  
Old September 28th 05, 04:15 PM
Digital Photography Now
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A good prime should be sharper and exhibit fewer optical aberrations
compared to a zoom, with the aperture set wide open. Stopping the zoom down
should improve sharpness, but then you may lose the desirable effect of
limited depth of field, if that's what you wanted. Zooms typically have
different geometric distortion throughout the zoom range, usually barrell at
the wide end and pincushion at the tele end, being neutral somewhere in the
middle. A good prime should be able to combine better sharpness, contrast
and distortion characteristics compared to a zoom.

But there are some outstanding zooms out there these days and the advantage
of primes has been lessened as a result.

As the apertures are the same, there should be no difference in brightness.
The glass doesn't lose enough light in the way you fear to be a major
factor.

Ian

Digital Photography Now
http://dpnow.com
Visit our discussion forum at http://dpnow.com/Forums.html


"Siddhartha Jain" wrote in message
ups.com...
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?

- Siddhartha



  #4  
Old September 28th 05, 04:15 PM
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Default

The prime may be slightly brighter than the zoom, something which may
be more important in the motion-picture industry
They use a term called the "t stop" of a lens and it is a measure of
the light loss through a lens.
http://artsci-ccwin.concordia.ca/comm/lighting.htm
The difference could be as little as a third or as great as (or greater
than) a 2 stop difference between what the f-stop is and the t-stop,
while the DoF will be the same and sharpnes is likely to be less with a
zoom than a prime.
In real life still photography I don't believe it would be noticable
(the difference in brightness), because of a variety of factors.
I personally would not lose any sleep over it.

  #6  
Old September 29th 05, 12:21 AM
BC
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"I would very much like to hear which particular lenses for DSLRs or
35mm SLRs produce a 2 stop reduction in illumination from that
expected at any given aperture."

2 stops is an awful lot, although I suppose some ancient zoom lenses
with lousy coatings might be that bad. One of the most complex zooms
I'm personally familiar with has about 40 elements, but nevertheless
suffers less than a 1-stop illumination reduction.

Brian

  #7  
Old September 28th 05, 04:42 PM
David Littlewood
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In article . com,
Siddhartha Jain writes
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?

- Siddhartha

Other things being equal, yes to both, though with well-designed modern
lenses the difference may be small.

Brightness:

The conventional f-stop designation of aperture is a purely geometric
measure and takes no account of the actual transmission properties of
the lens. In reality, not all of the light going into a lens will come
out at the other end, and some of the light that does come out will be
scattered, and hence will reduce image quality.

Attenuation takes two forms: absorption and reflection/scattering.
Absorption is purely proportional to the depth of glass; a typical
figure for normal optical glass would be 10% for a total glass path
length of 100mm. Most photographic lenses would fall far short of this,
though some big lenses may get there. This attenuation is entirely
proportional to the length of the light path through glass; thus a zoom
with 12-15 elements is likely to experience more absorption than a fixed
focal length lens with 5-10 elements.

The other form is reflection from glass-air interfaces. This is
unavoidable, but can be reduced very greatly by coating. The percentage
of reflection depends on the refractive index of the glass, but for
typical n=1.50 optical glass (uncoated) the percentage is about 4%.
This, remember, is at each glass air interface, two per lens element.
Thus a compound lens with 15 elements will have 30 interfaces, and will
only transmit (0.96)^30 or about 20% of the light. (In fact another
20-40% will get to the film or sensor as scattered light - giving an
image of appallingly bad contrast). A single layer coating will reduce
the reflection to about 1-1.5%, and modern multi-coating reduces it to
around 0.3-0.5%.

This still gives a transmission factor of about 83.5% for a 20-surface
(10-element) system, against 91.4% for a 10-surface (5-element) system.

Resolution:

There is not the same direct relationship between complexity and
resolution as that above between complexity and transmission. However,
the compromises required to balance zoom ratio, overall size, mechanical
complexity and cost at the same time as controlling the seven distinct
varieties of lens aberrations mean that in almost every case the zoom
lens will have lower resolution than the fixed focal length lens of
similar quality of design and manufacture. You can see this from the MTF
function curves published by most lens manufacturers. Having just
checked some of these myself to answer your question, I am actually
quite impressed by how small these differences are; a couple of decades
ago the differences would have been much greater. (Be aware when
comparing MTF curves that they usually show wide open and f/8 data; as a
zoom will usually have a smaller maximum aperture than the comparable
fixed focal length lenses, you should avoid comparing these - best look
at the f/8 curves for a fair comparison.)

David
--
David Littlewood
  #8  
Old September 28th 05, 04:57 PM
Nostrobino
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Zoom lenses ARE prime lenses, notwithstanding the now-popular misusage of
"prime."

A prime lens is the camera lens as distinct from some other lens or
lenticular device (close-up lens, tele converter, etc.) used with it. It has
meant that since long before zoom lenses became commonplace, and therefore
no need to use another term to mean "non-zoom."

"Prime" is properly used in the sense of primary, main, chief or
original--all dictionary definitions for "prime."

There is NO dictionary definition for "prime" which means fixed focal length
or single focal length, or fixed or single anything else.

It would be nice if this nonsensical misusage, which obviously is based on
someone's misunderstanding of the term some years ago (and then spread like
cancer through the power of the Internet) could be stamped out. Surely "FFL"
is at least as easy to type as "prime" anyway, and there never was any
reason other than shortness to replace "fixed focal length" with the
incorrect term.

N.


  #9  
Old September 28th 05, 05:25 PM
Siddhartha Jain
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Default

Nostrobino wrote:
Zoom lenses ARE prime lenses, notwithstanding the now-popular misusage of
"prime."

A prime lens is the camera lens as distinct from some other lens or
lenticular device (close-up lens, tele converter, etc.) used with it. It has
meant that since long before zoom lenses became commonplace, and therefore
no need to use another term to mean "non-zoom."

"Prime" is properly used in the sense of primary, main, chief or
original--all dictionary definitions for "prime."

There is NO dictionary definition for "prime" which means fixed focal length
or single focal length, or fixed or single anything else.

It would be nice if this nonsensical misusage, which obviously is based on
someone's misunderstanding of the term some years ago (and then spread like
cancer through the power of the Internet) could be stamped out. Surely "FFL"
is at least as easy to type as "prime" anyway, and there never was any
reason other than shortness to replace "fixed focal length" with the
incorrect term.


I am aware of the mis-usage of the term *prime* and so guilty of
propogating the mis-usage but I feel its time the FFL camp realised
that there is no turning back.

- Siddhartha

  #10  
Old September 28th 05, 06:35 PM
Nostrobino
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Default


"Siddhartha Jain" wrote in message
ups.com...
Nostrobino wrote:
Zoom lenses ARE prime lenses, notwithstanding the now-popular misusage of
"prime."

A prime lens is the camera lens as distinct from some other lens or
lenticular device (close-up lens, tele converter, etc.) used with it. It
has
meant that since long before zoom lenses became commonplace, and
therefore
no need to use another term to mean "non-zoom."

"Prime" is properly used in the sense of primary, main, chief or
original--all dictionary definitions for "prime."

There is NO dictionary definition for "prime" which means fixed focal
length
or single focal length, or fixed or single anything else.

It would be nice if this nonsensical misusage, which obviously is based
on
someone's misunderstanding of the term some years ago (and then spread
like
cancer through the power of the Internet) could be stamped out. Surely
"FFL"
is at least as easy to type as "prime" anyway, and there never was any
reason other than shortness to replace "fixed focal length" with the
incorrect term.


I am aware of the mis-usage of the term *prime* and so guilty of
propogating the mis-usage but I feel its time the FFL camp realised
that there is no turning back.


Well, not necessarily, though of course the more people who misuse the term,
the harder it will be to correct it.

Most people do not want to use wrong terminology since it makes them look
ignorant. In the case of "prime" being used to mean FFL, this has only
spread because readers who have not seen the term before, and then see it
used by people they assume are knowledgeable, naturally adopt it themselves.
Thus newbies are caught up in the misusage and (perhaps partly because they
feel using jargon will make them look knowledgeable too), contribute to the
spread.

Some will continue to use it anyway, but others will drop it (and some have
dropped it) when the error is pointed out to them.

N.


 




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