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F2.8 not equal F2.8 ?

 
 
Alfred Molon
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      01-17-2004
Is it true that different zoom lenses will let through different amounts
of light because "...glass is not 100% transmissive, and that zooms use
lots of elements, and that cheap prosumer glass isn't very high quality,
and that coatings and finish and materials changes transmissivity..." ?

In other would a cheap zoom lens opened at F2.8 let through the same
amount of light as a better zoom lens at F3.5 for instance ?
--

Alfred Molon
------------------------------
http://groups.yahoo.com/group/Olympus_405060/
Olympus 5050 resource - http://www.molon.de/5050.html
Olympus 5060 resource - http://www.molon.de/5060.html
 
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?
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      01-17-2004

"Alfred Molon" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) news.com...
> Is it true that different zoom lenses will let through different amounts
> of light because "...glass is not 100% transmissive, and that zooms use
> lots of elements, and that cheap prosumer glass isn't very high quality,
> and that coatings and finish and materials changes transmissivity..." ?
>
> In other would a cheap zoom lens opened at F2.8 let through the same
> amount of light as a better zoom lens at F3.5 for instance ?
> --
>
> Alfred Molon
> ------------------------------
> http://groups.yahoo.com/group/Olympus_405060/
> Olympus 5050 resource - http://www.molon.de/5050.html
> Olympus 5060 resource - http://www.molon.de/5060.html


All things being equal, and no missleading going on on the part of the lens
maker f2.8 should represent f2.8 regardless of the transmissivity of the
elements in the lens. In many cases with thru the lens metering there would
not be a problem. But say and there are few examples any more aside from
view cameras f x.x should be f x.x as such on each lens.
But then take Sigma for example as well as Fuji with the pixel count on the
imaging devices. I guess it all boils down like Clinton said it depends on
what " if " means.
The same can be said for shutters. If the exposure is calculated for a
camera with a slow or fast shutter, or even the old mercury battery
replacement being slightly under voltage the negative should come out OK.
But in the case of extreemly slow or fast shutter you would be hard pressed
to explain to someone how you froze the speeding auto at 1/30th.

Just my thoughts about your question.


 
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David J Taylor
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      01-17-2004
> Is it true that different zoom lenses will let through different amounts
> of light because "...glass is not 100% transmissive, and that zooms use
> lots of elements, and that cheap prosumer glass isn't very high quality,
> and that coatings and finish and materials changes transmissivity..." ?
>
> In other would a cheap zoom lens opened at F2.8 let through the same
> amount of light as a better zoom lens at F3.5 for instance ?


In principle, yes. The F number only says something about the physical
dimensions of the lens, not its actual optical transmission.

Dvid


 
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Povl H. Pedersen
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Posts: n/a
 
      01-17-2004
On 2004-01-17, David J Taylor <(E-Mail Removed)-this-bit> wrote:
>> Is it true that different zoom lenses will let through different amounts
>> of light because "...glass is not 100% transmissive, and that zooms use
>> lots of elements, and that cheap prosumer glass isn't very high quality,
>> and that coatings and finish and materials changes transmissivity..." ?
>>
>> In other would a cheap zoom lens opened at F2.8 let through the same
>> amount of light as a better zoom lens at F3.5 for instance ?

>
> In principle, yes. The F number only says something about the physical
> dimensions of the lens, not its actual optical transmission.


I have been told that for camera equipment, the formulas for calculating
the f-stop from dimensions is not valid, as f-stop in cameras are only
related to light-transmission these days.

Otherwise a lens with sunglass glass would always underexposure a picture.

 
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George Preddy
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      01-17-2004

"?" <?@?.?> wrote in message
news:Tb8Ob.25163$(E-Mail Removed)...
>
> "Alfred Molon" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed) news.com...
> > Is it true that different zoom lenses will let through different amounts
> > of light because "...glass is not 100% transmissive, and that zooms use
> > lots of elements, and that cheap prosumer glass isn't very high quality,
> > and that coatings and finish and materials changes transmissivity..." ?
> >
> > In other would a cheap zoom lens opened at F2.8 let through the same
> > amount of light as a better zoom lens at F3.5 for instance ?
> > --
> >
> > Alfred Molon
> > ------------------------------
> > http://groups.yahoo.com/group/Olympus_405060/
> > Olympus 5050 resource - http://www.molon.de/5050.html
> > Olympus 5060 resource - http://www.molon.de/5060.html

>
> All things being equal, and no missleading going on on the part of the

lens
> maker f2.8 should represent f2.8 regardless of the transmissivity of the
> elements in the lens.


Incorrect.

All lenses vary in brightness at the same f-stop, from a marginal
difference, to more than a stop. There's no way around it, rated f-stop is
only the size of the opening, it has nothing to do with number or quality of
lens elements, or basic lens design.

Sigma's DG lenses (see http://sigma-photo.com/html/zoom_intro.htm) are a
great example. They DG lenses are (quite intentionally) designed to be
brighter at the same f-stop with digital sensors, because not only is the
glassof extremely high quality, but the basic design is such that the rays
are more parallel when exiting the rear element. Same basic thing
microlenses are used for, to capture more of the angled rays at the sensor's
periphery, but Sigma's DG line builds this characteristic into the lenses
themselves. Put a the 24-70 EX DG and 28-70 EX (non-DG) on the same camera
and set the same f-stop, the DG will be about a third stop brighter at the s
ame aperture and focal length than the otherwise fabulous non-DG lens.

The 24-70 EX DG is about a half stop brighter than the 24-70 HF at the same
aperture and focal length - a reflection of the superb value HF's $85
pricetag- even though the EX has 5 more elements, 14 vs. 9.

In general, prosumer lenses are awful compared to even the cheapest D/SLR
glass. The F707/717 would be one exception, on par with low to mid level
D/SLR glass. But Fuji's f2.8 is not an exception, in fact the min ISO of
160 shows that it is particularly dim. Though dimness is the least of
prosumer lens problems, CA and barrel distortion are borderline absurd on
most prosumers. Take a peek at cracker jack box Fuji optics...
http://www.pbase.com/image/25203605/original

Notice anything about those perfectly straight columns as they rise? Not to
mention visible CA, even without backlighting. Then again, with all the
SuperCCD Bayer interpolation artifacts littering the image, the lens hardly
matters. It's hard to decide what ruins the image most.

That's prosumer Fuji optics at 35mm equivalent, have a look at superior 25mm
equivalent EX DG glass in front of 10.3MP...
http://www.pbase.com/image/24323811/original

Granted, the 15-30 EX DG alone costs about the same as the S602.

> In many cases with thru the lens metering there would
> not be a problem. But say and there are few examples any more aside from
> view cameras f x.x should be f x.x as such on each lens.
> But then take Sigma for example as well as Fuji with the pixel count on

the
> imaging devices. I guess it all boils down like Clinton said it depends on
> what " if " means.
> The same can be said for shutters. If the exposure is calculated for a
> camera with a slow or fast shutter, or even the old mercury battery
> replacement being slightly under voltage the negative should come out OK.
> But in the case of extreemly slow or fast shutter you would be hard

pressed
> to explain to someone how you froze the speeding auto at 1/30th.
>
> Just my thoughts about your question.
>
>



 
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Manfred Thaler
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Posts: n/a
 
      01-17-2004

"George Preddy" <(E-Mail Removed)> schrieb im Newsbeitrag
news:bubb3r$e6b$(E-Mail Removed)...
> In general, prosumer lenses are awful compared to even the cheapest D/SLR
> glass. The F707/717 would be one exception, on par with low to mid level
> D/SLR glass.


If understand you correctly, a Minolta DiMAGE A1's zoom with F2,8 is no F2,8
at least compared with a comparable 28-200/F2,8-3,5 zoom for a dSLR?


 
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George Preddy
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Posts: n/a
 
      01-17-2004

"Manfred Thaler" <(E-Mail Removed)> wrote in message
news:bube4q$7sk$01$(E-Mail Removed)-online.com...
>
> "George Preddy" <(E-Mail Removed)> schrieb im Newsbeitrag
> news:bubb3r$e6b$(E-Mail Removed)...
> > In general, prosumer lenses are awful compared to even the cheapest

D/SLR
> > glass. The F707/717 would be one exception, on par with low to mid

level
> > D/SLR glass.

>
> If understand you correctly, a Minolta DiMAGE A1's zoom with F2,8 is no

F2,8
> at least compared with a comparable 28-200/F2,8-3,5 zoom for a dSLR?


You don't.


 
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George Preddy
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Posts: n/a
 
      01-17-2004

"George Preddy" <(E-Mail Removed)> wrote in message
news:bubdpo$fcu$(E-Mail Removed)...
>
> "Manfred Thaler" <(E-Mail Removed)> wrote in message
> news:bube4q$7sk$01$(E-Mail Removed)-online.com...
> >
> > "George Preddy" <(E-Mail Removed)> schrieb im Newsbeitrag
> > news:bubb3r$e6b$(E-Mail Removed)...
> > > In general, prosumer lenses are awful compared to even the cheapest

> D/SLR
> > > glass. The F707/717 would be one exception, on par with low to mid

> level
> > > D/SLR glass.

> >
> > If understand you correctly, a Minolta DiMAGE A1's zoom with F2,8 is no

> F2,8
> > at least compared with a comparable 28-200/F2,8-3,5 zoom for a dSLR?

>
> You don't.


There is no way to know without comparing directly. A cheap lens might
actually be brighter but with terrible other qualities, while a very
expensive lens might be slightly dimmer with otherwise superb qualities. In
general, the better the lens, the brighter the glass, but you cannot apply
generalizations to specific lenses.

Here is a good example, conditions are absolutely identical between images.
Flip back and forth in medium thumbnail view, using the next/previous
hyperlink to compare brightness. The image parameters and lens used is
noted next at the bottom...

http://www.pbase.com/imageprocessing/lenscompare

SPP measures the difference at 0.3 stops.


 
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George Preddy
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      01-17-2004
Added another set... 24-70 EX DG vs. 24-70 HF

http://www.pbase.com/imageprocessing/lenscompare

Both shots at f5.6 and 0.25 secs = 0.4 EV difference.


 
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Tony Spadaro
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      01-17-2004
If the metering is done TTL - it matters not a whit. If you use a hand
meter - you are probably within 1/6th of a stop of the actual transmission.
In other words - don't sweat it. The only lenses that transmit seriously
less light than the aperture claims are mirror lenses.

--
http://www.chapelhillnoir.com
home of The Camera-ist's Manifesto
The Improved Links Pages are at
http://www.chapelhillnoir.com/links/mlinks00.html
A sample chapter from my novel "Haight-Ashbury" is at
http://www.chapelhillnoir.com/writ/hait/hatitl.html
"Alfred Molon" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed) news.com...
> Is it true that different zoom lenses will let through different amounts
> of light because "...glass is not 100% transmissive, and that zooms use
> lots of elements, and that cheap prosumer glass isn't very high quality,
> and that coatings and finish and materials changes transmissivity..." ?
>
> In other would a cheap zoom lens opened at F2.8 let through the same
> amount of light as a better zoom lens at F3.5 for instance ?
> --
>
> Alfred Molon
> ------------------------------
> http://groups.yahoo.com/group/Olympus_405060/
> Olympus 5050 resource - http://www.molon.de/5050.html
> Olympus 5060 resource - http://www.molon.de/5060.html



 
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