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Why "constant-aperture" zooms?

 
 
Bryan Olson
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      11-07-2006

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
 
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=?iso-8859-1?B?bWlubmVz+HR0aQ==?=
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      11-07-2006

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.

 
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Mark²
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      11-07-2006
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


 
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Paul Rubin
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      11-07-2006
Bryan Olson <(E-Mail Removed)> 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.
 
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Bryan Olson
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      11-07-2006
Paul Rubin wrote:
> Bryan Olson <(E-Mail Removed)> 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
 
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Paul Rubin
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      11-07-2006
Bryan Olson <(E-Mail Removed)> 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.
 
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Joseph Meehan
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      11-07-2006
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



 
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Steve Cutchen
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      11-07-2006
In article <hNW3h.6135$(E-Mail Removed)> , Bryan
Olson <(E-Mail Removed)> 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.
 
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tomm42
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      11-07-2006


On Nov 7, 2:42 am, Bryan Olson <(E-Mail Removed)> 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).

 
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David Ruether
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      11-07-2006



"tomm42" <(E-Mail Removed)> wrote in message news:(E-Mail Removed) 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.velocityreviews.com/forums/(E-Mail Removed)
(E-Mail Removed)
http://www.ferrario.com/ruether


 
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