From: Miles Bader on
"jaf" <me(a)here.com> writes:
> I read it.
> "(in a sense, T-stops are absolute and f-stops are relative)"
>
> Absolute to what?
>
> Relative to what?

I think the meaning is that t-stops have a fixed relationship to an
external reference -- e.g., using t-stops, you can calculate exposure
using an external meter, and directly use the indicated settings on your
camera. That can't always be done using f-stops, but if you calculate
the correct exposure at a single f-stop, then other f-stops using the
same lens etc will have a fixed relationship to that (so f2 always lets
through twice as much light as f2.8).

-miles

--
Any man who is a triangle, has thee right, when in Cartesian Space,
to have angles, which when summed, come to know more, nor no less,
than nine score degrees, should he so wish. [TEMPLE OV THEE LEMUR]
From: jaf on

"Miles Bader" <miles(a)gnu.org> wrote in message news:874orgcg1q.fsf(a)catnip.gol.com...
> "jaf" <me(a)here.com> writes:
>> I read it.
>> "(in a sense, T-stops are absolute and f-stops are relative)"
>>
>> Absolute to what?
>>
>> Relative to what?
>
> I think the meaning is that t-stops have a fixed relationship to an
> external reference -- e.g., using t-stops, you can calculate exposure
> using an external meter, and directly use the indicated settings on your
> camera. That can't always be done using f-stops, but if you calculate
> the correct exposure at a single f-stop, then other f-stops using the
> same lens etc will have a fixed relationship to that (so f2 always lets
> through twice as much light as f2.8).
>
> -miles
>

You missed the point.
The questions were rhetorical.
A lens may have both T & f scales but it only has one aperture.
It only has one aperture ring.

The light getting to the film plane is the same regardless of which scale you look at.

John


From: jaf on

"Floyd L. Davidson" <floyd(a)apaflo.com> wrote in message news:877hwc40ol.fld(a)apaflo.com...
> "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.
>

Nikon 500mm
http://www.nikonusa.com/Find-Your-Nikon/Product/Camera-Lenses/2172/AF-S-NIKKOR-500mm-f%252F4G-ED-VR.html

Nikon 500mm mirror
http://www.kenrockwell.com/nikon/5008.htm

Take a camera.
Mount each lens, shoot a picture of a Color Chart http://figitalrevolution.files.wordpress.com/2009/04/macbeth-color-checker.jpg

100 ASA film speed. f8 aperture. 500mm focal length.

What shutter speed is required to get a correct exposure for each lens?

No photoshopping allowed.

Post back with the results.


John








From: Floyd L. Davidson on
"jaf" <me(a)here.com> wrote:
>
>Nikon 500mm
>http://www.nikonusa.com/Find-Your-Nikon/Product/Camera-Lenses/2172/AF-S-NIKKOR-500mm-f%252F4G-ED-VR.html
>
>Nikon 500mm mirror
>http://www.kenrockwell.com/nikon/5008.htm

You've got a very good example of exactly what I've been
talking about! Indeed, you've cited a (generally less
than reliable, but in this case acceptable) web page
that tells you exactly what you are then asking me.

>Take a camera.
>Mount each lens, shoot a picture of a Color Chart http://figitalrevolution.files.wordpress.com/2009/04/macbeth-color-checker.jpg
>
>100 ASA film speed. f8 aperture. 500mm focal length.
>
>What shutter speed is required to get a correct exposure for each lens?

The mirror lens will require a shutter speed that is
twice as long.

The reason, as Rockwell explains in the cited URL, is
because the mirror lens only passes half as much light
as the regular 500mm lens when they are both set to f/8.
The mirror lens passes the same light at f/8 as the
other lens does at about f/11.

>No photoshopping allowed.
>
>Post back with the results.

Stop and *think* about this. Do more research.

Post back with the results.

--
Floyd L. Davidson <http://www.apaflo.com/floyd_davidson>
Ukpeagvik (Barrow, Alaska) floyd(a)apaflo.com
From: Miles Bader on
"jaf" <me(a)here.com> writes:

> You missed the point.
> The questions were rhetorical.
> A lens may have both T & f scales but it only has one aperture.
> It only has one aperture ring.
>
> The light getting to the film plane is the same regardless of which scale you look at.

Obviously, but the scales are different on some lenses -- the
relationship between the scales is only fixed for a given lens, not
generally, whereas the relationship between the t-stop numbers and the
"amount of light" passed through is fixed for all lenses.

Frankly, I'm not sure _what_ you're trying to argue...

The meaning of t-stop and f-stop seems obvious, and the difference is
clearly useful in some applications. Your tone gives the impression
that you somehow disagree, but it's not clear how or why.

-Miles

--
"She looks like the wax version of herself."
[Comment under a Paris Hilton fashion pic]