From: Floyd L. Davidson on
"jaf" <me(a)here.com> wrote:
>"David J Taylor" <david-taylor(a)blueyonder.not-this-part.nor-this.co.uk.invalid> wrote in message
>news:4Auom.75842$OO7.73399(a)text.news.virginmedia.com...
>>
>> "jaf" <me(a)here.com> wrote in message news:VamdnW5SiuBUwz_XnZ2dnUVZ_sOdnZ2d(a)giganews.com...
>> []
>>> Two 50mm lenses set at f8 are two 50mm lenses set at f8.
>>> Same focal length.
>>> Same aperture.
>>> Same amount of light.
>>>
>>>
>>> John
>>
>> Not so. f/8 is a mechanical measurement relating to
>> aperture and focal length. It actually says nothing
>> about the actual amount of light, simply the /maximum/
>> light possible. The less than 100% transmission of
>> the glass will reduce the actual amount of light
>> (albeit, perhaps only by a small amount).
>>
>> Two lenses with the same T-stop would pass the same amount of light. See:
>> http://photonotes.org/cgi-bin/photo-entry.pl?id=Tstop
>>
>> Also: consider a standard and a mirror lens both at
>> f/8. The mirror lens typically has a central
>> obstruction which reduces the light....
>>
>> Cheers,
>> David
>
>Post some data that proves you would need a different
>exposure time for 2 otherwise identical lenses.

Why didn't you bother to read the URL he cited above?
It provides the "data" you request.

Or stop and think about it... for example a pair of
500mm f/8 lenses, have an equal sized aperture.
However, if one of them is a mirror lens and the other
is not, the mirror lens will almost certainly have at
most only half the light transmission efficiency that
the other does. In effect it will f/9 or worse. That
mirror which blocks off a portion the front element is
where the loss occurs. Same aperture and focal length
though, so it has the same f/stop, but with less light
passing through.

Or another very simple experiment will also demonstrate
the fact. Take a single lens (virtually any lens that
you can mount a filter on), and compare it to
itself... with a neutral density filter added. In
either instance the fstop you select will be exactly the
same with or without the filter because the filter does
not change the aperture or the focal length. But it
will change the amount of light going through the lens.

--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd(a)apaflo.com
From: David J Taylor on

"jaf" <me(a)here.com> wrote in message
news:usidnVRSKI-ZCz_XnZ2dnUVZ_qidnZ2d(a)giganews.com...
>
> "David J Taylor"
> <david-taylor(a)blueyonder.not-this-part.nor-this.co.uk.invalid> wrote in
> message news:4Auom.75842$OO7.73399(a)text.news.virginmedia.com...
>>
>> "jaf" <me(a)here.com> wrote in message
>> news:VamdnW5SiuBUwz_XnZ2dnUVZ_sOdnZ2d(a)giganews.com...
>> []
>>> Two 50mm lenses set at f8 are two 50mm lenses set at f8.
>>> Same focal length.
>>> Same aperture.
>>> Same amount of light.
>>>
>>>
>>> John
>>
>> Not so. f/8 is a mechanical measurement relating to aperture and focal
>> length. It actually says nothing about the actual amount of light,
>> simply the /maximum/ light possible. The less than 100% transmission
>> of the glass will reduce the actual amount of light (albeit, perhaps
>> only by a small amount).
>>
>> Two lenses with the same T-stop would pass the same amount of light.
>> See:
>> http://photonotes.org/cgi-bin/photo-entry.pl?id=Tstop
>>
>> Also: consider a standard and a mirror lens both at f/8. The mirror
>> lens typically has a central obstruction which reduces the light....
>>
>> Cheers,
>> David
>
> Post some data that proves you would need a different exposure time for
> 2 otherwise identical lenses.
>
>
> John

John,

Please look at the reference I gave, and other references to T-stops.
That T-stops were required at all proves the need. With today's optics I
don't think you would be talking about large changes - probably less than
1/3 f/stop - except perhaps in the case of the mirror lens.

This would only apply if you were not metering through the lens.

Cheers,
David

From: jaf on

"Floyd L. Davidson" <floyd(a)apaflo.com> wrote in message news:87my594882.fld(a)apaflo.com...
> "jaf" <me(a)here.com> wrote:
>>"David J Taylor" <david-taylor(a)blueyonder.not-this-part.nor-this.co.uk.invalid> wrote in message
>>news:4Auom.75842$OO7.73399(a)text.news.virginmedia.com...
>>>
>>> "jaf" <me(a)here.com> wrote in message news:VamdnW5SiuBUwz_XnZ2dnUVZ_sOdnZ2d(a)giganews.com...
>>> []
>>>> Two 50mm lenses set at f8 are two 50mm lenses set at f8.
>>>> Same focal length.
>>>> Same aperture.
>>>> Same amount of light.
>>>>
>>>>
>>>> John
>>>
>>> Not so. f/8 is a mechanical measurement relating to
>>> aperture and focal length. It actually says nothing
>>> about the actual amount of light, simply the /maximum/
>>> light possible. The less than 100% transmission of
>>> the glass will reduce the actual amount of light
>>> (albeit, perhaps only by a small amount).
>>>
>>> Two lenses with the same T-stop would pass the same amount of light. See:
>>> http://photonotes.org/cgi-bin/photo-entry.pl?id=Tstop
>>>
>>> Also: consider a standard and a mirror lens both at
>>> f/8. The mirror lens typically has a central
>>> obstruction which reduces the light....
>>>
>>> Cheers,
>>> David
>>
>>Post some data that proves you would need a different
>>exposure time for 2 otherwise identical lenses.
>
> Why didn't you bother to read the URL he cited above?
> It provides the "data" you request.
>
> Or stop and think about it... for example a pair of
> 500mm f/8 lenses, have an equal sized aperture.
> However, if one of them is a mirror lens and the other
> is not, the mirror lens will almost certainly have at
> most only half the light transmission efficiency that
> the other does. In effect it will f/9 or worse. That
> mirror which blocks off a portion the front element is
> where the loss occurs.


The "mirror which blocks off a portion the front element " is in a blind spot.



> Same aperture and focal length
> though, so it has the same f/stop, but with less light
> passing through.
>
> Or another very simple experiment will also demonstrate
> the fact. Take a single lens (virtually any lens that
> you can mount a filter on), and compare it to
> itself... with a neutral density filter added. In
> either instance the fstop you select will be exactly the
> same with or without the filter because the filter does
> not change the aperture or the focal length. But it
> will change the amount of light going through the lens.

Ya! It blocks some of the light REQUIRING a change in the exposure time!

If you measure the light falling on an object with a meter, and you know what your doing, there is ONE aperture setting and ONE
shutter speed that can be used to get a properly exposed picture.
You will get the same image regardless of the "lens glass" used. The coloration may be different, but the exposure is the same.

John



From: jaf on

"David J Taylor" <david-taylor(a)blueyonder.not-this-part.nor-this.co.uk.invalid> wrote in message
news:5axom.75922$OO7.1347(a)text.news.virginmedia.com...
>
> "jaf" <me(a)here.com> wrote in message news:usidnVRSKI-ZCz_XnZ2dnUVZ_qidnZ2d(a)giganews.com...
>>
>> "David J Taylor" <david-taylor(a)blueyonder.not-this-part.nor-this.co.uk.invalid> wrote in message
>> news:4Auom.75842$OO7.73399(a)text.news.virginmedia.com...
>>>
>>> "jaf" <me(a)here.com> wrote in message news:VamdnW5SiuBUwz_XnZ2dnUVZ_sOdnZ2d(a)giganews.com...
>>> []
>>>> Two 50mm lenses set at f8 are two 50mm lenses set at f8.
>>>> Same focal length.
>>>> Same aperture.
>>>> Same amount of light.
>>>>
>>>>
>>>> John
>>>
>>> Not so. f/8 is a mechanical measurement relating to aperture and focal length. It actually says nothing about the actual
>>> amount of light, simply the /maximum/ light possible. The less than 100% transmission of the glass will reduce the actual
>>> amount of light (albeit, perhaps only by a small amount).
>>>
>>> Two lenses with the same T-stop would pass the same amount of light. See:
>>> http://photonotes.org/cgi-bin/photo-entry.pl?id=Tstop
>>>
>>> Also: consider a standard and a mirror lens both at f/8. The mirror lens typically has a central obstruction which reduces the
>>> light....
>>>
>>> Cheers,
>>> David
>>
>> Post some data that proves you would need a different exposure time for 2 otherwise identical lenses.
>>
>>
>> John
>
> John,
>
> Please look at the reference I gave, and other references to T-stops. That T-stops were required at all proves the need. With
> today's optics I don't think you would be talking about large changes - probably less than 1/3 f/stop - except perhaps in the case
> of the mirror lens.
>
> This would only apply if you were not metering through the lens.
>
> Cheers,
> David

I read it.
"(in a sense, T-stops are absolute and f-stops are relative)"

Absolute to what?

Relative to what?

Would you use a different EXPOSURE if you are looking a the T stop scale than if you are looking at the f stop scale on the lens?
T stops are just a different scale of measuring light.
EV steps/stops are another.


John






From: Floyd L. Davidson on
"jaf" <me(a)here.com> wrote:
>"Floyd L. Davidson" <floyd(a)apaflo.com> wrote in message news:87my594882.fld(a)apaflo.com...
>> "jaf" <me(a)here.com> wrote:
>>>"David J Taylor" <david-taylor(a)blueyonder.not-this-part.nor-this.co.uk.invalid> wrote in message
>>>news:4Auom.75842$OO7.73399(a)text.news.virginmedia.com...
>>>>
>>>> "jaf" <me(a)here.com> wrote in message news:VamdnW5SiuBUwz_XnZ2dnUVZ_sOdnZ2d(a)giganews.com...
>>>> []
>>>>> Two 50mm lenses set at f8 are two 50mm lenses set at f8.
>>>>> Same focal length.
>>>>> Same aperture.
>>>>> Same amount of light.
>>>>>
>>>>>
>>>>> John
>>>>
>>>> Not so. f/8 is a mechanical measurement relating to
>>>> aperture and focal length. It actually says nothing
>>>> about the actual amount of light, simply the /maximum/
>>>> light possible. The less than 100% transmission of
>>>> the glass will reduce the actual amount of light
>>>> (albeit, perhaps only by a small amount).
>>>>
>>>> Two lenses with the same T-stop would pass the same amount of light. See:
>>>> http://photonotes.org/cgi-bin/photo-entry.pl?id=Tstop
>>>>
>>>> Also: consider a standard and a mirror lens both at
>>>> f/8. The mirror lens typically has a central
>>>> obstruction which reduces the light....
>>>>
>>>> Cheers,
>>>> David
>>>
>>>Post some data that proves you would need a different
>>>exposure time for 2 otherwise identical lenses.
>>
>> Why didn't you bother to read the URL he cited above?
>> It provides the "data" you request.
>>
>> Or stop and think about it... for example a pair of
>> 500mm f/8 lenses, have an equal sized aperture.
>> However, if one of them is a mirror lens and the other
>> is not, the mirror lens will almost certainly have at
>> most only half the light transmission efficiency that
>> the other does. In effect it will f/9 or worse. That
>> mirror which blocks off a portion the front element is
>> where the loss occurs.
>
>The "mirror which blocks off a portion the front element " is in a blind spot.

No, the mirro *creates* a blind spot, and restricts the amount
of light that can pass to less than it would be if the mirror
were not there.

>> Same aperture and focal length
>> though, so it has the same f/stop, but with less light
>> passing through.
>>
>> Or another very simple experiment will also demonstrate
>> the fact. Take a single lens (virtually any lens that
>> you can mount a filter on), and compare it to
>> itself... with a neutral density filter added. In
>> either instance the fstop you select will be exactly the
>> same with or without the filter because the filter does
>> not change the aperture or the focal length. But it
>> will change the amount of light going through the lens.
>
>Ya! It blocks some of the light REQUIRING a change in the exposure time!

Exactly. Even though the lens is "identical" (one has
an extra element, but otherwise the construction is
exactly the same and both have exactly the same focal
length and exactly the same aperture) one requires a
different exposure time if the f/stop is set the same.

>If you measure the light falling on an object with a
>meter, and you know what your doing, there is ONE
>aperture setting and ONE shutter speed that can be used
>to get a properly exposed picture.

In the above examples that is simply not true.

The mirror lense will require more exposure time, and
the lense that has a neutral density element added will
also require more exposure time.

>You will get the same image regardless of the "lens
>glass" used. The coloration may be different, but the
>exposure is the same.

You'll need to rethink that, as it is clearly not the
case. Re-examine the above examples.

--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd(a)apaflo.com