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Why "constant-aperture" zooms?

 
 
Ben Brugman
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      11-07-2006

>
> 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.
> --

Yes Nikon does that, but I do not consider that a feature in all
cercumstances. I often like to use an aperature as large as
possible but one stop down. You lose one stop, but gain a lot
of quality, this is not possible anymore with Nikon.

Offcourse there is an upside as well, taking flash pictures with
non TTL flashes it is a huge advantage that the aperature stays
constant over the complete zoomrange. (As Nikon does with
all aperatures except the 'largest' and probably the smallest).

Ben



> David Ruether
> http://www.velocityreviews.com/forums/(E-Mail Removed)
> (E-Mail Removed)
> http://www.ferrario.com/ruether
>
>



 
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Ben Brugman
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      11-07-2006

"Bryan Olson" <(E-Mail Removed)> schreef in bericht
news:hNW3h.6135$(E-Mail Removed) t...
>
> 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?
>


It could be that the lens design is such that the aperature
(the fysical blades) are in the back of the lens and that
all zooming/moving and focusing goes on in the front of the
lens, this would give you a fairly constant aperature.
This is conjecture of my side, but a setup like that could
have advantages. (And very probably disadvantages as well).

Ben






>
> --
> --Bryan



 
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David Ruether
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      11-08-2006



"Ben Brugman" <(E-Mail Removed)> wrote in message news:(E-Mail Removed)...
David Ruether wrote...

>> 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.


> Yes Nikon does that, but I do not consider that a feature in all
> cercumstances. I often like to use an aperature as large as
> possible but one stop down. You lose one stop, but gain a lot
> of quality, this is not possible anymore with Nikon.


Then you set the Nikon custom control the other way, and using
aperture priority and setting f5 on an f3.5-X lens at the short
end, the f-stop will slip smaller and the shutter speed slower with
zooming longer (assuming you have set the custom controls
for 1/3rd stop shift intervals). I prefer to select constant f-stop,
though, since most zooms perform better at their wider stops
toward the long end, with more stopping down required at the
shorter end for good performance. Also, it is useful to not have
the shutter speed slowing and becoming more difficult to hand
hold as the lens is zoomed longer (which is harder to hand hold
in any case).

> Offcourse there is an upside as well, taking flash pictures with
> non TTL flashes it is a huge advantage that the aperature stays
> constant over the complete zoomrange. (As Nikon does with
> all aperatures except the 'largest' and probably the smallest).
>
> Ben


Yes - it can make us realize again how useful "Auto" flash mode
can be now that some of our fancy TTL flashes no longer work
in TTL mode with our fancy new digital SLR bodies...;-(
--
David Ruether
(E-Mail Removed)
(E-Mail Removed)
http://www.ferrario.com/ruether


 
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Adrian Boliston
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      11-08-2006
"David Ruether" <(E-Mail Removed)> wrote in message
news:Ecn4h.9698$(E-Mail Removed)...

> Then you set the Nikon custom control the other way, and using
> aperture priority and setting f5 on an f3.5-X lens at the short
> end, the f-stop will slip smaller and the shutter speed slower with
> zooming longer (assuming you have set the custom controls
> for 1/3rd stop shift intervals). I prefer to select constant f-stop,
> though, since most zooms perform better at their wider stops
> toward the long end...


I used to get a bit annoyed when I set aperture at f3.5 on my 18-70 kit lens
and it would not return to that setting when I zoomed out then back in
again, so I was never really sure which f stop I was using without having to
contantly check the reading in the viewfinder! I'd much rather have a
constant f4 than a 3.5-4.5 lens, but I expect this would push the price up a
fair bit.


 
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Ed Ruf (REPLY to E-MAIL IN SIG!)
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Posts: n/a
 
      11-08-2006
On Wed, 8 Nov 2006 19:32:05 -0000, in rec.photo.digital "Adrian Boliston"
<(E-Mail Removed)> wrote:

>I used to get a bit annoyed when I set aperture at f3.5 on my 18-70 kit lens
>and it would not return to that setting when I zoomed out then back in
>again, so I was never really sure which f stop I was using without having to
>contantly check the reading in the viewfinder! I'd much rather have a
>constant f4 than a 3.5-4.5 lens, but I expect this would push the price up a
>fair bit.


FWIW, I have no such issues on my either my D70 or D200 with two different
kit lenses.

--
Ed Ruf ((E-Mail Removed))
http://edwardgruf.com/Digital_Photog...ral/index.html
 
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jpc
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Posts: n/a
 
      11-09-2006
On Tue, 07 Nov 2006 07:42:05 GMT, 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?



There is nothing artifical about the limiting aperture or field stop
in zoom lens. It's there to control the aberations. IF there were
another cost effective way to get around the aberation problems I
assure you that lens mamufactures would be using it.

The other problems are cost, size, weight and the need to have a low
aberation image big enough to cover a large area sensor .For example I
own a 35-350 zoom lens for a studio size orthicon TV camera. The
front lens element is 7 inches in diameter and the complete lens
weighs 19 pounds.

jpc
 
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ben brugman
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Posts: n/a
 
      11-09-2006

"Adrian Boliston" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> "David Ruether" <(E-Mail Removed)> wrote in message
> news:Ecn4h.9698$(E-Mail Removed)...
>
>> Then you set the Nikon custom control the other way, and using
>> aperture priority and setting f5 on an f3.5-X lens at the short
>> end, the f-stop will slip smaller and the shutter speed slower with
>> zooming longer (assuming you have set the custom controls
>> for 1/3rd stop shift intervals). I prefer to select constant f-stop,
>> though, since most zooms perform better at their wider stops
>> toward the long end...

>
> I used to get a bit annoyed when I set aperture at f3.5 on my 18-70 kit
> lens and it would not return to that setting when I zoomed out then back
> in again, so I was never really sure which f stop I was using without
> having to contantly check the reading in the viewfinder! I'd much rather
> have a constant f4 than a 3.5-4.5 lens, but I expect this would push the
> price up a fair bit.

To prevent this from hapening, zoom in to the 70 mm setting. (Or any other
zoom to the highest setting). Change your aperature to the setting you
prefere. For the 3.5-4.5 lens the largest aperature you can set will be 4.5,
so set it to this value and when zooming in or out it will remain on this
setting.

(On the other end of the scale aperature on 22 or 28, this does work
differently, only in extreme conditions that will matter).

ben brugman





>



 
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ben brugman
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      11-09-2006

"Ed Ruf (REPLY to E-MAIL IN SIG!)" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Wed, 8 Nov 2006 19:32:05 -0000, in rec.photo.digital "Adrian Boliston"
> <(E-Mail Removed)> wrote:
>
>>I used to get a bit annoyed when I set aperture at f3.5 on my 18-70 kit
>>lens
>>and it would not return to that setting when I zoomed out then back in
>>again, so I was never really sure which f stop I was using without having
>>to
>>contantly check the reading in the viewfinder! I'd much rather have a
>>constant f4 than a 3.5-4.5 lens, but I expect this would push the price up
>>a
>>fair bit.

>
> FWIW, I have no such issues on my either my D70 or D200 with two different
> kit lenses.


If you do not have these issues, how do you keep the D70 constant on 1 stop
from the largest aperature. (This was my issue).

The issue described above can be solved to set the Aperature when completely
zoomed in, then it does not change when zooming. (Not for the smallest
aperature).

Ben


>
> --
> Ed Ruf ((E-Mail Removed))
> http://edwardgruf.com/Digital_Photog...ral/index.html



 
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Ed Ruf
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Posts: n/a
 
      11-09-2006
On Thu, 9 Nov 2006 14:39:22 +0100, in rec.photo.digital "ben brugman"
<(E-Mail Removed)> wrote:

>
>"Ed Ruf (REPLY to E-MAIL IN SIG!)" <(E-Mail Removed)> wrote in message
>news:(E-Mail Removed).. .
>> On Wed, 8 Nov 2006 19:32:05 -0000, in rec.photo.digital "Adrian Boliston"
>> <(E-Mail Removed)> wrote:
>>
>>>I used to get a bit annoyed when I set aperture at f3.5 on my 18-70 kit
>>>lens
>>>and it would not return to that setting when I zoomed out then back in
>>>again, so I was never really sure which f stop I was using without having
>>>to
>>>contantly check the reading in the viewfinder! I'd much rather have a
>>>constant f4 than a 3.5-4.5 lens, but I expect this would push the price up
>>>a
>>>fair bit.

>>
>> FWIW, I have no such issues on my either my D70 or D200 with two different
>> kit lenses.

>
>If you do not have these issues, how do you keep the D70 constant on 1 stop
>from the largest aperature. (This was my issue).


That's not what you originally said. You said "I used to get a bit
annoyed when I set aperture at f3.5 on my 18-70 kit lens and it would
not return to that setting when I zoomed out then back in again."

If you set the lens to 18mm, set the camera to f/3.5, then zoom to
70mm the camera changes to f/4.5. Then when you zoom back to 18mm the
camera changes back to f/3.5. Actually with the D200 you can set any
aperture wider than f/4.5 while at 18mm and it will return to that
again at 18mm.

-
Ed Ruf ((E-Mail Removed))
http://edwardgruf.com/Digital_Photog...ral/index.html
 
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ben brugman
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Posts: n/a
 
      11-09-2006

>
> Then you set the Nikon custom control the other way, and using
> aperture priority and setting f5 on an f3.5-X lens at the short
> end, the f-stop will slip smaller and the shutter speed slower with
> zooming longer (assuming you have set the custom controls


I am not aware of a Nikon custom control on the D70 to do this.


>> Ben

>
> Yes - it can make us realize again how useful "Auto" flash mode
> can be now that some of our fancy TTL flashes no longer work
> in TTL mode with our fancy new digital SLR bodies...;-(


Yes exactly my sentiments.

Ben

> --
> David Ruether
> (E-Mail Removed)
> (E-Mail Removed)
> http://www.ferrario.com/ruether
>
>



 
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